Metal Finishing Industry
Pollution Prevention in the Plating Process
This chapter provides an overview of pollution prevention techniques that apply to plating lines within metal finishing operations. As described in Overview of the Metal Finishing Industry, the plating line is the part of the metal finishing process where metal is applied to a substrate.
The first section of this chapter describes general pollution prevention techniques for plating solutions and covers housekeeping, monitoring, additives, equipment modification, and on-site recycling and recovery. The next section covers general issues in pollution prevention for cyanide-based plating. The next seven sections cover pollution prevention options for plating specific metals such as brass, cadmium, chromium, copper, nickel, precious metals, and zinc. The sections that follow cover additional types of plating including electroless, aluminum, chemical and electrical conversion, and others.
Common Pollution Prevention Practices presented a number of general pollution prevention techniques for all types of metal finishing operations. These general techniques can apply in a variety ways to plating lines. The following are some specific applications of these techniques to plating baths.
Keeping the plating areas clean and preventing foreign material from entering or remaining can prolong the life of a bath. Companies can use a number of simple and inexpensive techniques to reduce contamination of the process bath. A part that falls off the rack into a bath should be removed quickly to reduce contamination. Operators should maintain racks so that they are clean and free of contaminants. Firms should avoid using broken or cracked racks because they can increase the amount of process solution that is dragged into the rinse process, increasing sludge generation. Other general housekeeping methods include protecting anode bars from corrosion, using corrosion-resistant tanks and equipment, and filtering incoming air to reduce airborne contaminants. A clean process area also makes detecting problems such as leaking tanks or pipes much easier. For more information on these techniques, refer to Common Pollution Prevention Practices.
Proper control of bath operating parameters can result in more consistent workpiece quality as well as longer bath life. This strategy is simple: determine critical operating parameters and maintain them within the acceptable limits. The first step in this process is to determine optimum operating parameters for the process. The next step is to ensure regular monitoring of bath chemistry, which is essential in determining the proper amount of chemicals to add to maintain efficient operating parameters. For many solutions, simple field test kits are available. Determining operating parameters on an individual plating line basis is important because suppliers sometimes set concentration specifications for levels higher than is required for effective operation. Higher concentrations mean increased dragout and waste generation. Many plating facilities rely heavily on suppliers to provide them with optimum operating parameters. In some cases, shops send samples on a monthly basis to their vendors in addition to the daily analyses performed at facilities. The following sections describe the operating parameters that a facility should establish and the ways to determine those values (IAMS 1995).
Process Bath Operating Temperature
Increased bath temperatures will reduce the viscosity of the plating solution, enabling faster drainage from the workpiece and reducing the amount of solution that is dragged into subsequent baths. Operators, however, should avoid using very high temperatures because many additives break down in high heat, and carbonate buildup increases in cyanide solutions. Excessive temperatures also can cause the process solution to dry onto the workpiece during removal, increasing dragout, water use, and labor costs (APPU 1995).
Higher operating temperatures also will increase the evaporation rate of the process solution. A facility can take advantage of the increased evaporation rate by using solution from the process line rinse tanks to replenish the process bath and to maintain the proper chemical equilibrium. This replenishment reduces wastewater and recovers dragout while maintaining a stable plating solution. A facility might consider using deionized water when operating plating solutions at higher temperatures since deionized water will minimize the natural contaminant buildup in the process bath. Increasing the operating temperature also can increase energy costs (EPA 1992).
Plating Solution Concentration
Facilities should determine the lowest concentration of chemicals that can be used to obtain a quality finish. If the process line is operated at higher temperatures, lower concentrations can be used to obtain results equivalent to higher concentrations at lower temperatures. Generally, the greater the concentration of chemicals in a solution, the greater the viscosity and dragout. As a result, the film that adheres to the workpiece during removal from the process bath is thicker and does not drain back into the process bath as quickly. Reducing the concentration of the plating solution can increase the ability of process solution to drain efficiently from a workpiece (EPA 1992).
Many chemical product manufacturers recommend an operating concentration that is higher than necessary. To determine the lowest possible process bath concentration, a facility should mix a new process bath at the median recommended concentration. As the process bath is replenished, operators can continue to reduce the chemical concentration until product quality begins to deteriorate. Alternatively, operators can mix the new bath at a low concentration and gradually increase the concentration until the bath cleans, etches, or plates the test pieces adequately. Facilities can operate fresh cleaning process baths at lower concentrations than used baths. Makeup chemicals can be added to the used bath increasing the concentration gradually to maintain effective operation (EPA 1992).
Platers commonly use several chemical additives to aid in the plating process and to reduce waste generation. Most of these chemicals are used to reduce dragout of solution into rinsewater. Some of the more common additives are described below.
Metal finishers have used wetting agents for years in process solutions to aid in plating. A wetting agent is a substance, usually a surfactant, that reduces surface tension. The addition of a very small amount of surfactant or wetting agents can reduce dragout by as much as 50 percent (EPA 1992). However, platers should be careful to use only non-ionic wetting agents. The use of certain ionic wetting agents can reduce plating quality and limit reclamation of metals in wastewater. If a shop is considering the use of wetting agents for dragout reduction, they should conduct experiments to determine the potential benefit and to ensure compatibility with bath chemistry, especially for hard chromium plating.
Wetting agents also can create foaming problems in process baths and might not be compatible with waste treatment systems. For these reasons, impacts on both the process bath and treatment system should be evaluated prior to use (Ford 1994).
Non-Chelated Process Chemicals
Firms use complexers, including chelators, in chemical process baths to control the concentration of free metal ions in the solution beyond their normal solubility limit. Chelators are usually found in baths used for metal etching, cleaning, and electroless plating. However, once chelating compounds enter the wastestream, they inhibit the precipitation of metals and additional treatment must be used. These treatment chemicals end up as sludge and contribute to the volume of hazardous waste. For example, when platers use ferrous sulfate, a popular precipitant, the volume of sludge increases significantly. For some applications, operators add ferrous sulfate at an 8:1 ratio. Also, many of the spent process baths containing chelators cannot be treated on site and are put into containers for off-site disposal, adding to waste disposal costs (EPA 1992).
Metal finishers use a variety of chelators in different processes. In general, mild complexors such as phosphates and silicates are used for most cleaning and etching processes. Electroless plating baths typically are chelated with stronger organic acid chelating compounds including citric acid, maleic acid, and oxalic acid. Some firms also use ethylenediaminetetraacetic acid (EDTA), but with less frequency than the other chelators (PRC 1989). However, EDTA is a common component in many cleaning solutions.
Operators must make a trade off between extending bath life and removing chelated process chemicals from wastewater to meet required discharge levels. Often, the pH of wastestreams must be adjusted to break down the metal complexes formed by the chelators. EDTA, for example, requires lowering the pH below 3.0 and adding treatment chemicals (PRC 1989). In some cases, even this form of treatment does not enable metals to precipitate.
Firms can use non-chelated process chemistries for processes (e.g., alkaline cleaning and etching) in which keeping the metals removed from the workpiece surfaces in solution for later treatment might not be necessary. An application of the above is dummy plating. In such cases, the metals can be allowed to precipitate and the process bath can be filtered to remove the solids. However, non-chelated chemicals are not used for electroless plating because the chelators play a significant role in allowing the plating bath to function (PRC 1989).
Non-chelated process cleaning baths usually require continuous filtration to remove the solids. These systems generally have a filter with pore sizes 1 to 5 microns thick with a pump that can filter the tank contents once or twice each hour (PRC 1989). The cost of a filter system ranges from approximately $400 to $1,000 for each tank. Operating costs include filter element replacement as well as disposal and maintenance costs. However, firms will realize savings in reduced waste treatment, sludge handling, and disposal costs for spent baths. Another important advantage of non-chelated process chemicals is that the metal removal capability of wastewater treatment usually is improved and the treated effluent is more likely to meet discharge limits (EPA 1992).
A facility can implement several modifications to reduce contamination of the process bath, extending its life and reducing waste generation. These techniques include using the proper anode care, purified water, and ventilation/exhaust systems.
Anodes: Purity, Bagging, and Placement
Anodes used in the plating process often contain impurities that can contaminate a process bath. Anodes with higher grades of purity do not contribute to bath contamination, however, their cost might be higher than less pure anodes. In addition, some contaminants are added to the anode to aid in the plating process. Therefore, properly matching the anode to the process is critical. One method for reducing contamination from anodes is placing cloth bags around them.
This technique can prevent insoluble impurities from entering a bath. However, the bags must be maintained and made of a material that is compatible with the process solution (EPA 1992). For some process solutions, such as copper cyanide, bagging is not a feasible option. Facilities also can experiment with the placement of the anode in the process bath. Proper placement of the anode can increase the quality of the plating process resulting in fewer rejects, and can reduce the need to rework workpieces.
Firms can use deionized, distilled, or reverse osmosis water to replace tap water for process bath makeup and rinsing operations. Natural contaminants such as calcium, iron, magnesium, manganese, chlorine, carbonates, and phosphates can reduce rinsewater efficiency, minimizing the potential for dragout recovery and increasing the frequency of process bath dumping. These contaminants also contribute to sludge volume when removed from wastewater during treatment (EPA 1992). Further information on issues related to purified water are included in Common Pollution Prevention Practices in the section on water quality monitoring.
Scrubbers, de-misters, and condensate traps remove entrained droplets and vapors from air passing through ventilation and exhaust systems. If segregated, operators can return some wastes from scrubbers to process baths after filtering. Updraft ventilation allows mist to collect in the duct work and flow back to the process tank. For example, hard chromium plating baths can benefit from an updraft ventilation system (EPA 1992).
Process baths that generate mist (e.g., hexavalent chromium plating baths and air-agitated nickel/copper baths) should be in tanks that have more freeboard in order to reduce the amount of mist in the ventilation system. The added space at the top of the tank (i.e., the freeboard) allows the mist to return to the bath before entrainment in the air entering the exhaust system. Platers also can use foam blankets or floating polypropylene balls in hard or decorative chromium baths to keep mists from reaching the exhaust system (EPA 1992).
A plater can use several chemical substitutes to reduce the amount of toxic materials. Detailed information on these substitutes is presented in Pre-Finishing Operations and in upcoming sections in this chapter. Substitutes are used most commonly for cyanide because of its toxicity.
Replace Cyanide-Based Plating with Non-Cyanide-Based Processes
Converting process baths to non-cyanide process chemistries can, in some cases, simplify wastewater treatment, reduce treatment costs, and decrease sludge generation. Alternatives are available for most cyanide-containing processes including silver, cadmium, zinc, gold, and copper plating. However, drawbacks often are associated with switching to non-cyanide process plating. For a more detailed description of cyanide alternatives, refer to the Pollution Prevention for Cyanide-Based Plating section in this chapter.
Several opportunities exist for platers to recycle or reuse solutions in baths either within the same tanks or in other processes. This section covers acid solution regeneration, reactive rinsing, and spent solution reuse.
Acid Solution Regeneration
Firms can regenerate acid solutions using several processes including distillation, acid sorption, membrane electrolysis, crystallization, and diffusion dialysis. Technologies such as membrane electrolysis and diffusion dialysis rely on the ability of a membrane to selectively diffuse anions and hydrogen while at the same time rejecting metals. Diffusion dialysis functions by passing water in a countercurrent flow to the spent acid stream. The two streams meet at a membrane where anions and hydrogen diffuse through the membrane into the water. Operators end with an acid solution at the approximate strength with which they started and a dilute acid waste that contains the metal component. The acid then is reused and the waste is treated or sent off site for disposal. Acid solution regeneration technologies are discussed in further detail in Pollution Prevention in Rinsing.
Spent Acid Bath Reuse (Reactive Rinsing)
Companies might have opportunities to reuse spent process baths in other facets of a metal finishing operation. Used acid and alkaline cleaners from the cleaning process are the most common example of this technique. For example, rinsewater from an acid dip process can be piped to the alkaline cleaning process for use as rinsewater (or vice versa). If both systems have the same flow rate, water use would be reduced by 50 percent. This system also can increase rinsing efficiency by reducing the viscosity of the alkaline dragout (EPA 1992). However, facilities should make sure that rinse tanks, pipes, plumbing, and bath chemistries are compatible with the rinse solution (EPA 1992).
Another use for spent acid cleaning rinsewater is as an influent for rinsing after a mild etch process. Furthermore, rinsewater from final or critical rinses, which tend to be less contaminated, can be used in rinsing operations where a high degree of rinsing efficiency is not required. Costs for implementing a system to reuse water can vary greatly. Simple systems can cost as little as a few hundred dollars while a complex system can cost hundreds of thousands. Figure 6 illustrates rinsewater reuse for an alkaline cleaning, mild acid etch, and acid cleaning line.
Spent Solution Bath Reuse
Process baths that have become too contaminated to be used for plating operations often are dumped. However, these solutions can have valuable uses in other metal finishing operations such as:
Platers can extend the life of process solutions by removing impurities from the bath. The following sections provide an overview of removal techniques including filtration, carbonate freezing, precipitation, electrolysis, and carbon treatment.
Removal of Solids via Filtration
Filtration is one of the most common techniques available for maintaining process bath purity. Most frequently, platers use cartridge filters as either in-tank or external units to remove suspended solids from the process solution. The majority of cartridges in use are disposable. However, reusable filters also are available. Filter systems also can be used on pre-finishing operations (mainly on larger tanks). The cost varies depending on the size and type of filter the shop uses (Cushnie 1994).
Removal of Salts via Carbonate Freezing
Cyanide baths are adversely affected by the formation of carbonate buildup during the breakdown of cyanide. An excessive carbonate concentration can affect the smoothness of deposits, plating efficiency, and plating range. Salt buildup can increase process solution dragout by as much as 50 percent.
Carbonate freezing can prevent the buildup of salts. The carbonate freezing process takes advantage of the low solubility of carbonate salts in the bath. Bath temperature is lowered to approximately 26 degrees Fahrenheit to crystallize the salts. This process also can remove sodium sulfate and sodium ferrocyanide. Carbonate freezing is used most often in cadmium cyanide plating, zinc cyanide plating, copper cyanide plating, and copper cyanide strike. Sodium cyanide baths can be treated by carbonate freezing or crystallization. However, potassium cyanide baths must be treated by precipitation rather than freezing (Cushnie 1994).
Removal of Metal Contaminants via Precipitation
Metal finishers use precipitation as an alternative to carbonate freezing for cyanide baths. Table 8 lists common bath contaminants and precipitators that platers can use to remove contaminants. The process generally is performed in a spare tank where the solution is chemically treated and filtered and then returned to the original tank. For example, in a zinc bath, zinc sulfide can be used to precipitate lead and cadmium; the precipitant then is removed via filtration. In addition, iron and chromium contamination is common in acidic nickel baths. In most formulations, these contaminants can be removed with peroxide combined with pH elevation and batch filtration. As with all chemical reactions, facilities must take care to ensure that the precipitation reagents are compatible with the bath constituents (Cushnie 1994).
Removal of Metal Contaminants via Low-Current Electrolysis (Dummy Plating)
A common problem with plating baths is the introduction of metal contaminants into the bath that reduce the effectiveness of the solution. Copper is a common metal contaminant that builds up in plating baths. Copper can be removed from zinc and nickel baths through a process called dummy plating. Dummy plating is an electrolytic treatment process in which metallic contaminants in a metal finishing solution are plated out using low current density electrolysis. The process is based on the electrolytic principle that copper can be plated at a low electrical current (Ford 1994).
When the copper concentration in a process bath becomes too high, an operator can place an electrolytic panel in the bath (the bath must be inoperative for 1 or 2 days). A trickle current then is run through the system, usually at a current density of 1 to 2 amperes per square foot. At this current, the copper in the plating bath solution will plate out onto the panel, but the plating bath additives are unaffected. Some of the plating metals also might be removed inadvertently, but the savings from extending the life of the bath usually justifies the metal loss. For more information on this process, refer to the recovery techniques section in Pollution Prevention in Rinsing.
Removal of Organics Using Carbon Treatment
Carbon treatment of plating baths is a common method of removing organic contaminants. The carbon absorbs organic impurities that are present as a result of introducing oil or breaking down bath constituents. Carbon treatment can be used on both a continuous and batch basis. Various filtration methods are available, including carbon filtration cartridges (restricted to use on small applications), carbon canisters, pre-coat filters, and bulk application/agitation/filtration. Typical dosages are 1 to 4 pounds of carbon per 100 gallons of solution (Cushnie 1994).
Perhaps the single most toxic chemical used in metal finishing on a weight-for-weight basis is cyanide. Electroplaters are most at risk for exposure to hydrogen cyanide (HCN) through ingestion and inhalation, either through a catastrophic event or low levels associated with processing. Skin contact with dissolved cyanide salts is somewhat less dangerous but will cause skin irritation and rashes (Mabbett 1993).
This section contains information on the available alternatives to cyanide plating. The first part discusses general information regarding the substitution of non-cyanide solutions for traditional cyanide-based baths. The next section addresses specific plating solutions (e.g., brass, cadmium, copper, precious metals, and zinc) and provides information on alternative bath chemistries and successful implementation of recycling and recovery technologies.
Substitution of cyanide can have profound effects on a metal finisher. Cyanide, in the form of sodium or potassium cyanide, has been a key component of plating solutions for many years, particularly in plating copper, zinc, and other metals. Cyanide is an excellent complexer and has a wide tolerance for impurities and variations in bath composition. Cyanide's principle disadvantages are toxicity and the high cost of wastewater treatment (Ford 1994).
For these reasons, EPA and many states severely limit the discharge of cyanide. Platers typically use an alkaline chlorination process requiring sodium hypochlorite or chlorine to treat wastestreams containing free cyanide. These chemicals can contribute substantially to sludge generation (Braun Intertec 1992). For complex cyanides, platers typically use ultraviolet (UV)/ozone or UV/peroxide treatment. This process is simple and cost effective (Gallerani 1996).
Many metal platers are seeking alternatives to traditional cyanide-based plating. Concerns over occupational health and safety, waste treatment costs, regulatory compliance requirements, and potential liability have encouraged process managers to investigate new, non-cyanide plating technologies. The earliest and most complete cyanide substitution that has taken place in the industry is the conversion from zinc cyanide to zinc chloride or zinc alkaline (TURI 1994).
Non-cyanide alternatives generally have proven to be base specific and, therefore, are not simple to substitute. Also, non-cyanide plating solutions are less forgiving than cyanide baths to soils left on parts for plating. Firms must maintain higher cleaning standards if they switch to non-cyanide solutions. Another disadvantage of non-cyanide substitutes is that they tend to cost more than conventional baths (Ford 1994). Also, some of the common recovery technologies are more difficult to use with non-cyanide substitutes.
Using non-cyanide process chemistries can reduce hazardous waste sludge by eliminating a treatment step. However, many non-cyanide processes are difficult to treat and produce more sludge than cyanide baths. Some platers also have found that they need to install more than one process line to replace a single cyanide line. Usually, no substitute will meet all the requirements for replacing the single cyanide line. Multiple substitutes must be used, and some applications have no available substitute (TURI 1994).
Non-cyanide-based alternatives are available for cyanide copper, zinc, and cadmium plating processes. These substitutes can reduce regulatory reporting requirements, lower risks to workers, decrease environmental impact, and decrease corporate liability. Platers should weigh the advantages and disadvantages of non-cyanide baths for specific applications (Braun Intertec 1992).
The following list describes the factors that technical assistance providers should consider when recommending changes to a non-cyanide solution:
Technical assistance providers should make sure that companies that are considering a conversion to a non-cyanide substitute understand the inherent dangers in converting a cyanide line. Many problems can be averted as long as companies develop a well thought-out plan. A majority of the accidents involving cyanide in metal finishing operations have occurred because of badly planned conversions of a plating line from cyanide to non-cyanide operations (Gallerani 1996).
The following sections provide detailed descriptions of commonly used cyanide plating processes (brass, cadmium, copper, precious metals, tin, and zinc) and the available alternatives.
Brass plating is one of the most common alloy plating processes in use today. Brass can be plated in many applications and in varying thicknesses. Another property of brass plating is its ability to provide good adhesion to steel and rubber. Brass is, therefore, commonly used in the manufacture of steel wire cord for use in tires. Other applications of brass plating include a variety of decorative and engineering finishes (Strow 1982).
Brass plate comes in variety of colors from yellow to various shades of bronze and brown. In some cases, platers use brass as a very thin plate over other bright plates. Nickel often is used under a brass plate to level the surface. A brass plate then is applied over the nickel to provide a bright brass surface. Yellow brass is the most common material used in brass plating. Gold-colored brass often is used as a decorative plate. The main problem in applying a brass finish is rapid tarnishing. The conventional solution to this problem is to apply a protective layer of clear transparent powder coat or lacquer (Strow 1982).
Common Bath Solutions
Typical brass plating solutions are cyanide-based. The basic ingredients of a cyanide brass plating solution are sodium cyanide, copper cyanide, and zinc cyanide. Other constituents include ammonia and carbonate. In some cases, platers also add sodium carbonate to provide a buffering action so that the plate color is consistent. The ratio of cyanide to zinc is the key element in controlling plate color and alloy composition (Strow 1982).
Plating efficiency is controlled by copper content (i.e., the higher the copper content, the higher the efficiency). Temperature also plays a key role in the efficiency of the bath solution. For example, plating at 95 degrees Fahrenheit is twice as efficient as plating at 75 degrees Fahrenheit. Process lines operated at higher than 95 degrees Fahrenheit require more frequent additions of ammonia; lines below 95 degrees require less frequent additions (Strow 1982).
Alternative Bath Solutions for Brass Plating
Various non-cyanide brass solutions have been developed in the past, however, cyanide brass solutions are still the most prevalent solutions used by metal finishers today. Some of the original non-cyanide solutions had some problems including insufficient color in the deposit, poor appearance, narrow operating ranges, or bath instability. One of the most critical disadvantages is the lack of uniformity of color or appearance of the non-cyanide brass deposit (Fujiwara 1993b). Currently, not much literature is available for alternatives to brass cyanide baths.
Among the non-cyanide brass plating baths, pyrophosphate appears to be one of the most promising. However, field reports have stated that additives are necessary to operate this application properly. Otherwise, problems develop with unalloyed zinc getting contained in the deposit. Metal finishers have used the additive histidine in a pyrophosphate solution successfully. The deposits have shown similar qualities to the traditional copper zinc alloy deposits (Fujiwara 1993a).
Tests have been completed on an alkaline pyrophosphate-tartare bath containing histidines as an alternative to brass cyanide solutions. Tests on these solutions have found that their alloy composition was almost constant over a wide range of current densities. Moreover, bright brass deposits having a uniform composition and color were obtained over almost the entire cathode area. The tests were performed on a bath solution that had a pH of 12.0 and a constant temperature of 30 degrees Celsius.
Zirconium nitride is a coating that has similar characteristics to brass and is applied using an alternative deposition process. This compound is much easier than brass to plate and does not tarnish. The surface has a metallic appearance and a brass color tone. The solution uses a deposition process call sputterion plating. Sputterion plating involves coating a thin film in an even layer on a material to form a strong atomic bond. The film provides good wear resistance without color variation that can result from tarnishing. In this process, all or some portions of the material to be deposited enter a gas phase and condensation of the material takes place under constant ion bombardment (Kopacz 1992). For additional information on sputterion plating, refer to Alternative Methods of Metal Deposition.
Alternative Deposition Methods
The electrocoating process has been used as an alternative to brass electroplating. This process places the metal coating on the substrate via electrocoating. It comes in a brass color and in clear and can be used for some decorative applications. It does not involve metal plating, however, the finished surface resembles a plated finish. This finish provides excellent resistance under salt spray tests. A plater in Illinois is using this process on zinc die castings as a replacement for brass plate (Peden 1996).
Cadmium is extremely toxic and tightly regulated by EPA and OSHA. Because of its regulatory status and the high cost of cadmium plating, many platers are substituting cadmium with zinc where possible. Metal finishers have found some problems with finding substitute bath solutions or low-cyanide cadmium solutions for many applications. No single cadmium substitute has stood out as a drop-in solution. The primary problems with cadmium substitutes are customer acceptance, the characteristics of the finish, and the higher cost of the plating solution in some cases (Davis 1994).
Cadmium exhibits superior corrosion resistance (especially in marine environments), lubricity, and other specific engineering properties. Cadmium also is easily welded. Moreover, because of its toxicity, fungus or mold growth is not a problem. Often, cadmium-plated material is chromated to increase corrosion resistance. The largest segment of the cadmium plating market is the military, which is beginning to change its specifications to less toxic products (Haveman 1994).
Common Bath Solution
The most common method for electroplating cadmium is an alkaline cyanide bath. Cadmium is supplied to the bath in the form of metallic cadmium and cadmium compounds. An all-purpose, bright cadmium bath has a sodium cyanide to cadmium ratio of 5:1. Sodium hydroxide and sodium carbonate also are used in the bath solution. Operating temperatures range from 24 degrees Celsius to 32 degrees Celsius. A current density of 20 to 40 amperes per square foot is required to achieve a uniform plating thickness (ASM 1982).
Alternative Process Solutions for Cadmium Plating
Cadmium plating solutions that do not use cyanide are commercially available. These include cadmium acid and cadmium alkaline plating solutions. Given the toxicity of cadmium, however, the most environmentally preferrable substitutes do not use either cadmium or cyanide. Replacing cyanide-based cadmium coatings with one of the non-cadmium, non-cyanide alternatives eliminates workplace exposure to both cadmium and cyanide and reduces environmental releases of both chemicals. Tables 9 and 10 present an overview of the available alternatives. These alternatives include several non-cyanide based cadmium baths, various combinations of zinc-based chemistries, and two tin-based alternatives. Some of the alternatives have improved performance when compared to cadmium. These benefits include:
Some of the limitations of cadmium alternatives include:
Cadmium Neutral or Acid Sulfate/Cadmium Acid Fluoroborate
Three non-cyanide, cadmium-based alternatives are available: neutral sulfate, acid fluoroborate, or acid sulfate. However, these cadmium-based alternatives do not have the throwing power of cadmium cyanide processes. The only substitute that is capable of high cathode efficiency is acid fluoroborate, but only at high current densities. Since cadmium is also a highly regulated substance, non-cyanide alternatives that still use cadmium are not as preferable as those substitutes that contain neither cadmium or cyanide (Pearlstein 1991).
Numerous zinc alloy processes are commercially available including zinc-nickel, zinc-cobalt, zinc-tin, and zinc-iron. The use of zinc alloys has grown because of their potential to replace cadmium, particularly in countries such as Japan where the use of cadmium has been strictly curtailed or prohibited. Zinc alloys were introduced in the Japanese and German automotive industry for use in fuel lines and rails, fasteners, air conditioning components, cooling system pumps, coils, and couplings. Improved warranty provisions in 1989 from vehicle manufacturers such as Honda, Toyota, and Mazda further boosted the use of zinc-nickel and zinc- cobalt in the automotive industry. Other industries that use zinc alloys as a substitute for cadmium include electrical power transmitting equipment, lock components, marine, and aerospace industries. Metal finishers also have substituted zinc-nickel coatings for cadmium on fasteners for electrical transmission structures and on television coaxial cable connectors (EPA 1994).
Plating with zinc alloys requires that operating parameters are controlled and maintained at much tighter standards than with cadmium cyanide plating. Critical parameters include pH, chemistry, temperature, and agitation level. Zinc-nickel alloys can be plated from a chloride-based process that is similar to chloride zinc baths or from an alkaline non-cyanide zinc solution. Brightening agents and other additives make these alloy processes more expensive to purchase and operate than cadmium baths. The alloying metal usually is added as a chemical concentrate, which is purchased from the supplier. Zinc anodes generally are used with this solution because alloy anodes are not readily available (Altmayer 1993a).
For cadmium applications that require enhanced corrosion resistance to salty environments, zinc alloys are suitable substitutes. Pure zinc also can be used as a substitute for heavy cadmium deposits (more than 1 millimeter thick). However, zinc alloy deposits can fail to be suitable substitutes when cadmium is specified for the following characteristics: enhanced lubricity, solderability, low electrical contact resistance, ease of disassembly after corrosion has occurred, or inhibition of fungus or mold growth (Bates 1994).
Treatment of rinsewater from zinc alloy electroplating usually is simply adjusting the pH, eliminating the need for cyanide oxidation. The zinc-cobalt, zinc-tin, and zinc-iron processes do not add any metals to the process that are presently regulated under federal water programs (Altmayer 1993a). The following sections provide a brief description of several of the most common zinc alloys.
Alkaline zinc-nickel baths produce a deposit that tends to favor applications that do not require bendability. Those applications are better suited for the laminar structure of acid baths. Alkaline zinc-nickel coatings, however, provide one of the highest corrosion protection ratings available with a chromate conversion coating. High corrosion protection is a result of the chromate solution dissolving some of the zinc from the surface, leaving a nickel-rich layer. Zinc-nickel finishes provide good corrosion properties after parts-forming operations and heat treating. Other features of alkaline zinc-nickel are low metal formulation, limited range of chromate colors, difficulty in chromating because of nickel content, and temperature constraints that require a chiller for control (Zaki 1989).
Zinc-cobalt deposits contain approximately 1 percent cobalt and 99 percent zinc. The acid bath has a high cathode efficiency and high plating speed. The deposit also has reduced hydrogen embrittlement when compared to alkaline systems. Thicknesses of the deposits tend to vary substantially with the current density of the process bath (Murphy 1993).
Zinc-Iron Acid or Alkaline
The primary advantage of zinc-iron is its ability to develop a deep uniform black conversion coating. Additionally, the alloy is easily welded and machined and is used easily on strip steel. This coating has been used successfully as a base coat prior to painting. The primary disadvantage of zinc-iron coatings is their limited ability to provide corrosion resistance (Murphy 1993).
Zinc-nickel acid solutions provide bright coatings that exhibit high throwing power. Good corrosion properties are maintained after parts- forming operations and heat treating because acid zinc-nickel delivers a higher nickel content than the alkaline zinc bath, which tends to increase corrosion. Unfortunately, acid solutions also tend to produce deposits with poorer thickness distribution and greater alloy variation between high and low current density areas than its alkaline counterpart. Other disadvantages include the limited range of chromate colors, required use of additional inert anodes, and segregated treatment systems. For workpieces that are being chromated after a cadmium plate, this solution is difficult to work with because higher brightener levels and nickel content create a more brittle coating, making it more difficult to chromate (Zaki 1989).
A tin-zinc alloy has been developed in the United Kingdom as an alternative for cadmium plating. The proprietary solution, Stanzec, was developed by the International Tin Research Institute (ITRI) in Uxbridge, Middlesex, United Kingdom. It contains 75 percent tin and 25 percent zinc and can be used in either rack or barrel plating. Research is underway to develop a high-speed tin-zinc plating line for the continuous plating of steel strip (Plating and Surface Finishing 1994).
Zinc chloride process baths were tested to assess the feasibility of using this solution as an alternative for cadmium cyanide in rack plating operations. Performance results demonstrated that while the zinc chloride finish was similar to the cadmium finish, the cadmium-plated parts, however, exhibited superior corrosion resistance. The main advantage of using the zinc chloride over cadmium is a greatly reduced hazard risk at the facility. High capital costs ($2 million for the purchase of new equipment, cleanup costs for old equipment, and waste disposal costs) gave this investment a payback period of 115 years.
The process change, therefore, cannot be justified on economics alone (PNWPPRC 1996).
CorroBan was developed by Boeing scientists in the early 1980s. It is a proprietary zinc-nickel alloy that is electrodeposited from a cyanide-free solution. The process was licensed to Pure Coatings, Inc. The zinc provides galvanic protection similar to cadmium while the nickel imparts extra hardness. Deposits from this process pass 2,600-hour salt spray tests and ASTM F 519 hydrogen embrittlement tests and are compatible with aluminum. This deposit also has lubricity (torque-tension) characteristics similar to cadmium. Testing also has shown that CorroBan provides better sacrificial corrosion protection than cadmium because of the improved electrode potential of the coating in a sodium chloride solution (EPA Region 2 1995).
Alternative Deposition Processes for Cadmium Plating
50/50 Zinc-Cadmium Alloy Using In-situ Reclaim
The 50/50 zinc-cadmium alloy using an alternative deposition processes has shown promise as an alternative to cadmium plating. This alloy uses 50 percent less cadmium, but exhibits superior corrosion resistance. The coating is applied using a dry plating technique developed by IonEdge Corporation for use specifically with this alloy. In this dry plating process, simultaneous plating of zinc and cadmium species is conducted under neutral gas-flow discharge conditions. Details of the process are of a proprietary nature and, therefore, further information is not available (Sunthankar 1994).
Ion Vapor Deposition of Aluminum
Aluminum coatings deposited through ion vapor deposition (IVD) can replace cadmium coatings in some applications, eliminating both the use of cadmium and cyanide. This technology is suited especially to applications that require cadmium to protect steel substrates from corrosion and to inhibit the growth of organisms such as mold and fungus. Ion Vapor deposition aluminum coatings can be applied to a wide variety of metallic substrates including aluminum alloys and plastic/composite substrates. This process does not use or create any hazardous materials.
This technology has been used mainly on high-strength steels in the aerospace industry and in marine applications. Some facilities have converted to IVD coatings, eliminating the anodizing process on aircraft components that are subject to fatigue (EPA 1994). Ion Vapor Deposition aluminum has found applications in naval aircrafts, particularly those manufactured by McDonnell Douglas Corporation. This company has found IVD aluminum coatings are especially suited for parts where temperatures can exceed 450 degrees Celsius and/or when contact with titanium parts is expected. This process also is used when working with high-strength steels that preclude using cathodic processes such as electroplating. However, IVD coatings lack the frictional properties of cadmium and are expensive to implement (Lansky 1993).
The advantages of IVD aluminum coatings include the uniformity in finish thickness and excellent throwing power. Deposits can be plated on difficult-to-reach places, making IVD attractive for coating complex shapes. The process is limited, however, in its ability to deposit coatings into deep holes and recesses, particularly in configurations where hole depth exceeds the diameter (Pearlstein 1991). IVD processes are discussed in more detail in Alternative Methods of Metal Deposition.
Copper plating is widely used as an underplate in multi-plate systems and as stop-offs for carburizing as well as in electroforming and the production of printed circuit boards. Although relatively corrosion resistant, copper tarnishes and stains rapidly when exposed to the atmosphere. Copper alone is rarely used in applications where a durable and attractive surface is required. Copper plating is used generally as an underplate or pre-plate before a final finishing operation such as nickel or gold. Bright copper is used as a protective underplate in multiple layer systems or when a decorative finish is desired. The copper finish often is protected against tarnishing and staining by the application of a clear lacquer. Copper plating can change the appearance, dimensions, or electrical conductivity of a metal part. Jewelry manufacturing, aerospace, and electronics often use copper plating (ASM 1982).
Common Bath Solutions and Waste Treatment
The major constituents of copper cyanide baths are potassium cyanide, potassium hydroxide, and copper cyanide. Cyanide copper plating requires a two-stage waste treatment procedure. The first step is cyanide destruction using either chlorine gas or less hazardous, but more expensive, hypochlorite treatment. The second step is precipitation of metals (i.e., pH adjustment with a caustic). The sludge produced from this treatment contains trace amounts of cyanide, increasing disposal costs significantly (ASM 1982).
The benefits of replacing cyanide-based copper plating baths with a non-cyanide solution include reduced environmental exposure and employee health risks. Non-cyanide copper has the following benefits:
Issues Related to Non-Cyanide Substitutes
Non-cyanide copper plating requires more frequent bath analysis and adjustment than cyanide-based plating. Cyanide-based copper plating baths are relatively forgiving to bath composition because they remove impurities. Non-cyanide baths are less tolerant of poor surface cleaning so thorough cleaning and activation of the surface is critical to obtain a quality finish. Personnel should be capable of operating the non-cyanide process as easily as the cyanide-based process (EPA 1994).
Operating costs of the bath are substantially higher for the non-cyanide processes than the cyanide process, however, replacing the cyanide-based bath with a non-cyanide bath eliminates the need for treatment of cyanide-contaminated wastewaters. This reduces substantially the difference in cost between the two solutions. Given the higher operating costs, a facility might not be able to justify this conversion on economics alone unless the facility faces substantial treatment costs for cyanide emissions.
The use of non-cyanide copper plating baths is not widespread. The number of companies running non-cyanide trials is small, but growing (Altmayer 1993). An application where non-cyanide plating could be attractive from a cost perspective is selective carburizing. This process is used widely in the heavy equipment industry for hardening portions of coated parts such as gear teeth. Gears must be hard at the edges, but not throughout because hardness throughout could cause the part to become brittle. To achieve this selective hardening, the plater applies a copper mask to that portion of the part that is not targeted for hardening. The part then is treated with carbon monoxide and other gases (EPA 1994).
Alkaline non-cyanide processes are unable to deposit adherent copper on zinc die castings and zincated aluminum parts without a copper strike. The one exception is a supplier that claims to be able to plate these parts using a proprietary process. Several facilities are currently testing this method on a pilot scale (Altmayer 1993). Of these pilot tests, two facilities reported that costs are approximately two to three times more than other processes, even when waste treatment and disposal costs are considered. One of these facilities has discontinued the use of the process while the other facility has continued with the process believing that the benefits of increased safety and compliance are worth the cost (EPA 1994).
For plating copper, certain non-cyanide alkaline baths of proprietary composition have been developed. Four widely known alternatives to copper cyanide plating are copper acid sulfate, copper acid fluoroborate, copper alkaline, and copper pyrophosphate. Table 11 provides a brief overview of these four alternative bath solutions.
Specific Non-Cyanide Alternatives
Copper Alkaline Solutions
Non-cyanide alkaline baths yield fine grained, dense deposits similar to cyanide copper deposits. The one area where they might differ is in the purity of deposit. Additives in copper alkaline solutions incorporate a trace of organic material into the deposit. This solution is ideally suited for workpieces that require thick deposits such as those used as heat treating (carburizing) stop-off on steel parts. The dense deposit is an excellent diffusion barrier for carbon (Braun Intertec 1992).
The non-cyanide alkaline copper solution uses cupric copper ions while the cyanide process contains monovalent copper. The chemical composition of monovalent copper results in faster plating at the same current density levels. Platers can operate alkaline copper baths at higher current densities than cyanide solutions to yield faster plating overall. The throwing power of the non-cyanide process is superior to the cyanide process, especially in barrel plating. This process uses one-quarter to one-half of the copper contained in cyanide solution, resulting in lower sludge generation because of lower metal concentrations (Mabbett 1993).
The alkaline substitute has significant drawbacks. Copper alkaline non-cyanide baths operate at significantly lower pHs (8.0 to 8.8) than traditional cyanide copper lines. Despite the lower pHs, non-cyanide baths have trouble tolerating zinc contamination and have not been successful at plating copper over zinc surfaces. The alkaline copper process also is more sensitive to impurities and the chemistry can be difficult to control. In addition, changing over to alkaline copper requires a lined tank and, in some cases, the addition of a purification tank. Overall, the cost for substitution is fairly high when compared to the cost of copper cyanide solutions (Mabbett 1993).
Copper Acid Sulfate
The copper sulfate bath is the most frequently used of the acid copper electrolytes. An acidic copper plating bath using sulfate ions has proved versatile. However, the low pH can sometimes attack the substrate and increase iron concentration in the process bath. The process is used primarily in printed wire board manufacturing and electroforming operations and for the application of copper as an undercoating for chromium. By altering the composition of the bath, platers can use copper sulfate in through-hole plating of printed circuit boards where a deposit ratio of 1:1 is desired. With additives, the bath produces a bright deposit with good leveling characteristics or a semi-bright deposit that is easily buffed (Braun Intertec 1992).
In contrast to heavy copper cyanide plating baths, copper sulfate baths are highly conductive and have simple chemistries. Sulfate baths are economical to prepare, operate, and treat. Previous sulfate bath problems have been overcome with new formulations and additives. The copper cyanide strike might still be needed for steel, zinc, or tin-lead base metals (Braun Intertec 1992).
Fluoroboric acid is the basis for another copper plating bath that provides enhanced solubility and conductivity as well as high plating speeds. This bath is simple to prepare, stable, and easy to control. Operating efficiency approaches 100 percent. Deposits are smooth and attractive and can be easily buffed to a high luster. The addition of molasses to the bath, when operated at 120 degrees Fahrenheit, results in deposits that are stronger and harder (Weisenberger 1982). Additional agents must be used to avoid excessive porosity in thicknesses greater than 20 mils. The drawbacks of this bath solution are that it is more costly, has fewer additive systems available, and is more hazardous to use than other non-cyanide alternatives. Treatment of wastewater also is more costly (Murphy 1993).
Copper pyrophosphate is used primarily to produce thick deposits. These baths are used for decorative multi-plate applications, through-hole plating of printed circuit boards, and a stop-off in selective case hardening of steels. The types of plates obtained with this solution are similar to those obtained with a high efficiency cyanide bath. However, a strike is required if plating over steel, magnesium, aluminum, or zinc. Alkaline pyrophosphate baths exhibit good throwing power, plating rates, and coating ductility. In addition, the bath normally operates at an almost neutral pH. Deposits from this bath are fine-grained and semi-bright. The main disadvantage of copper pyrophosphate is that the chemistry is expensive and wastewater is harder to treat when compared to traditional copper cyanide wastewater (Braun Intertec 1993).
Acid Copper Versus Alkaline Copper Solutions
Plating of copper from acid baths is used extensively for electroforming, electrorefining, manufacturing of copper powder, and decorative electroplating. Acid copper plating baths contain copper in bivalent form and are more tolerant of ionic impurities than alkaline baths. However, they have less macro-throwing power and poorer distribution rates than alkaline solutions. Acid baths have excellent micro-throwing power, which can be effective in sealing porous die castings. As with the alkaline baths, the plater must apply a strike to a workpiece prior to plating on steel or zinc (Braun Intertec 1993).
Alternative Deposition Processes for Copper Plating
The Department of Defense is testing the feasibility of depositing copper using new deposition technologies such as plasma spraying, ion plating, and sputter deposition. For more information on these technologies, refer to Alternative Methods of Metal Deposition.
Waste Treatment of Alkaline Non-Cyanide Copper
Wastewater treatment of non-cyanide copper solutions is simpler than those for copper cyanide processes because of the elimination of cyanide removal. Another benefit is reduced sludge generation because the non-cyanide process contains one-half to one-fourth as much copper as a full-strength cyanide copper bath. Furthermore, non-cyanide alternatives eliminate the two-stage chlorination system that uses chemicals such as chlorine or sodium hypochlorite that can increase sludge generation. One potential disadvantage of the non-cyanide bath is that it frequently can become contaminated beyond control (as happened in pilot test), requiring increased treatment and disposal for the process line (Freeman 1995).
Separation Technologies for Copper Plating
This section provides specific examples of recycling and recovery technologies for copper plating including ion exchange, electrodialysis, electrolytic recovery, and reverse osmosis. A more in-depth discussion of individual recycling technologies is included in Pollution Prevention in Rinsing.
Copper platers can use ion exchange to recover a high percentage of the copper from contaminated plating baths and rinsewaters. For example, a Montreal plating shop sent rinsewater with copper concentrations of about 300 parts per million from a copper sulfate plating solution (acid copper) to an ion exchange resin unit. The unit reduced the concentration of copper to about 1 part per million. Every 20 to 30 minutes, the resin would be regenerated with dilute sulfuric acid, exchanging copper ions in the resin with hydrogen ions. The concentrated copper sulfate solution produced from the regenerating process was added to the plating tank as needed. Through this ion exchange process, the company recovered 95 percent of the copper from the running rinse (RI DEM 1995a).
Used on a stagnant rinse line, electrodialysis can recover 90 to 95 percent of the dragout from heated copper plating solutions. This concentrated dragout goes back into the plating tank while the dilute stream is returned to the rinse tank. Electrodialysis can run continuously without regeneration, requires only a DC power source for operation, and consumes relatively small amounts of electricity. A disadvantage of electrodialysis is that it recovers plating bath impurities along with the copper. The membranes in this process also are prone to fouling from either solids in the bath or from compounds forming on the sheets (RI DEM 1995b).
This process recovers only the metals that are dragged into the rinsewater. Enough metal must be present in the solution to form a usable strip. A homogenous copper deposit requires the rinse solution to have concentrations of 2 to 10 grams per liter. Cathodes with greater surface area can recover copper from much lower concentrations (in the 10- to 50-milligrams-per-liter range).
An electroplater in Providence, Rhode Island, reported recovery of 85 grams per minute over 9 days using a 5-square-foot electrode. The unit received flow from a dragout tank and returned the clean water to the same tank. This particular tank's copper concentrations dropped from 150 to 10 milligrams per liter. Other companies have experienced similar reductions of approximately 88 percent of the copper from both standing rinse tanks and running rinses (RI DEM 1995a).
EPA performance tests have shown reverse osmosis (RO) to be successful in recovering metals from both acid copper and copper cyanide plating baths. Reverse osmosis membranes used in cyanide applications might need pretreatment. A copper cyanide plater reported that its RO unit recovered 98 to 99 percent of the copper from its plating wastes and 92 to 98 percent of the cyanide. The type of membrane used is a major factor in determining the effectiveness of RO. Cellulose membranes cannot withstand cyanide and solutions with either high or low pH. However, many membranes are resistant to these conditions (RI DEM 1995a).
Copper strikes often are used to deposit a thin intermediate layer (strike) of copper over a variety of substrates including steel and zinc die castings before those metals are plated with other metals. This layer is required for successful plating because it promotes adhesion on difficult-to-plate metals and protects some substrates from degradation in subsequent plating solutions. Because copper strikes are applied frequently, finding replacements for cyanide solutions can greatly assist facilities in reducing the amount of cyanide that they use (Hughes 1991).
Copper Strike Alternatives
Dilute copper pyrophosphate has been viewed as a feasible replacement for the cyanide strike because the solution does not degrade substrates.The main disadvantage of this chemistry is that it usually takes three times longer to plate than traditional cyanide solutions (Hughes 1991). For more information on this alternative, refer to the previous section on copper cyanide plating in this chapter.
High-pH nickel plating solutions have been available for a long time as a substitute for cyanide copper strike on zincated surfaces and zinc die castings. To obtain optimum results with high-pH nickel, the plater must balance the ratio between nickel sulfate and sodium sulfate. The proper ratio depends on several factors including part geometry; parts with complex shapes require higher sodium sulfate concentrations than parts with simple geometry. For plating operations above a 5.4 pH, platers use ammonium hydroxide and sulfuric acid for pH control. Zinc contamination should be removed continuously through low current density dummying in a purification cell. Cleaning parts prior to plating is more critical in high-pH nickel plating than traditional copper cyanide strikes. Because the bath chemistry is not proprietary and requires no additives, facilities can mix their own solutions. This makes the cost of operating this bath lower than operating cyanide copper strike lines and significantly lower than the cost of operating alkaline non-cyanide copper baths (Freeman 1995).
Using sodium in the bath will affect the deposit characteristics of the strike. The higher the sodium content of this nickel-plating bath, the more brittle the deposit becomes. The bath, therefore, should be used only as a strike before conventional nickel or copper plating. Parts that undergo fatigue cycles or extreme temperature changes can experience early fatigue failures and less corrosion resistance (Freeman 1995).
Substituting high-pH nickel for a copper cyanide strike will eliminate a cyanide wastestream. However, the ammonium ion present in the high-pH nickel formulation can cause waste treatment problems unless the concentration can be minimized through dragout recovery techniques. Another disadvantage of this technique is that the bath contains a higher metal content than the cyanide copper process and twice the metal content of the alkaline non-cyanide process. Sludge volume from wastewater treatment would be affected accordingly (Freeman 1995).
The electrodeposition of precious metals for decorative and engineering purposes is an important part of the metal finishing industry. Given the high cost for a gallon of precious metal solution, platers have used many methods to conserve and recover precious metal solutions. Because of this, more information is available on recovery technologies for precious metals than common metals (e.g., copper, nickel, and zinc). Common forms of metal recovery in precious metal operations include ion exchange or electrowinning (Ford 1994).
Until recently, gold plating was used primarily for decorative purposes in jewelry and flatware. Currently, gold is widely used in the electronics industry because of its good electrical contact properties as well as corrosion and oxidation resistance. Typical applications for gold plating include printed circuit boards, contacts, connectors, transistor bases, and integrated circuit components. Gold plating also is widely used in the chemical industry for reactors and heat exchangers (ASM 1982).
Traditionally, gold has been plated from potassium gold cyanide solutions, although many different types of gold and gold alloys are available. However, gold plates can be broken down into eight general classes:
In general, platers use high gold contents at heavy thickness because this permits higher current densities and higher cathode efficiencies. Other methods that platers can use to increase plating speed include higher operating temperatures and increased agitation (ASM 1982).
Common Bath Solutions
The four general groups of gold plating solutions are alkaline gold cyanide, neutral gold cyanide, acid gold cyanide, and non-cyanide solutions (generally sulfite-based). Alkaline cyanide baths have been used for the past century. Because of the complexing action of cyanide, however, obtaining consistent co-deposit of gold alloys is difficult unless the process is operated at high current densities. As a result, platers have limited the use of alkaline cyanide baths to flash deposits. Around the 1950s, bright baths were developed using silver and selenium as alloying agents. Some success has been demonstrated with neutral baths. Free of cyanide at the start, these baths build up potassium cyanide by adding gold potassium cyanide to replenish the gold in the process bath (Braun Intertec 1992). Each of the groups can be paired with the different classes of plating operations discussed above.
Alternative Gold Plating Solutions
The high cost of gold has made conservation critical and has led to a search for substitutes. Table 12 provides an overview of the alternatives to gold cyanide plating.
*Little information on this solution is available
In gold plating, firms can substitute a sulfite bath for a cyanide bath. For example, a study performed at Sandia National Laboratories compared coatings on microelectronic circuits produced by the gold cyanide process and the gold sulfite process. The test results showed that gold sulfite plating solutions are compatible with a wide variety of substrates used in electronics including quartz, aluminum oxide, silicon, glass, cordierite, duriod, and gallium arsenide. The study also found compatibility with surface treatment compounds. The sulfite bath formed a gold plate with similar weld bond strength and a coat density similar to pure gold. The study concluded that the gold sulfite bath produced nearly equal, if not slightly better, coatings and was far less hazardous to use. Another study found that a non-cyanide sulfite gold plating solution is capable of stable operations at pH values as low as 4.0. At pH values lower than 6.5, sulfur dioxide is released at a controlled level during operation (Hughes 1991).
Palladium, a precious metal, has emerged as a feasible substitute for hard gold and, in some instances, soft gold finishes within the last decade. Palladium's attributes include lower cost, lower specific gravity, comparable attributes to gold, and solution composition. Palladium and palladium-nickel alloys have been used primarily for separable connectors and printed wiring board fingers. Recently, many additional applications have been found including contact finishing for edge card connectors, lead frames for IC packaging, solderable contact and end terminations for multi-layered ceramic capacitors, semiconductor optoelectronic devices for packing, etch resists for printed wire boards, battery parts, and decorative items for jewelry and consumer hardware. These applications all take advantage of palladium's lower cost and material properties, which, in many instances, are superior to gold. The use of palladium also eliminates the use of cyanide because palladium is plated from non-cyanide solutions. The two major solutions for palladium are ammonia-based and organic amine-based (Abys 1993).
Alternative Deposition Processes for Gold Plating
Several facilities are testing the use of alternative deposition processes for gold plating. Processes such as ion plating and sputter deposition are being tested. For more information on these processes, refer to Alternative Methods of Metal Deposition.
Recovery Technologies for Gold Plating
Because of the high cost of the metal salts for gold plating, recovery technologies are widely used. Even with the high cost of some of the technologies, it still is economically feasible for companies to use technologies such as ion exchange and reverse osmosis.
The largest use of silver plate is in the flatware and hollowware trade. The second largest use is in the electronics industry where large amounts of silver are plated onto conductors, wave guides, and similar items because of silver's unsurpassed electrical conductivity. In most of these applications, silver is plated over copper and copper alloys. The aerospace industry uses silver as a plate over steel in aircraft engine manufacturing (SME 1985).
Common Bath Solutions
Commercial silver electroplating has been practiced since the middle of the nineteenth century. The plating bath contains silver in the form of potassium silver cyanides and free potassium cyanide. Platers also can use sodium cyanide, but they generally prefer the potassium form. The amount of free cyanide in silver solutions is extraordinarily high. For example, a common copper-cyanide bath has 2 to 4 ounces of free cyanide per gallon while the amount of free cyanide in silver solutions commonly is 16 to 22 ounces. Large quantities of cyanide are required to increase the throwing power of the solution. Usually, a small amount of potassium carbonate and/or potassium hydroxide also is added to the bath. Silver baths usually are operated at room temperature although high-speed plating has been performed at temperatures as high as 120 degrees Fahrenheit (SME 1985).
When hard, bright silver deposits are desired, proprietary additives containing metals or organic brighteners generally are used. Some additive combinations increase the tarnish resistance of the silver deposit. As with all bright solutions, the metal and free cyanide content of the bath must be closely monitored (SME 1985).
Alternative Solutions for Silver Cyanide
Given the large amounts of cyanide used in silver plating, finding suitable alternatives could greatly reduce cyanide levels in wastewater. Several attempts have been made to introduce non-cyanide alternatives. Most of these solutions are based on ammonium, halide, and aminothio complexes containing silver and a variety of conductivity salts and brightening agents. In almost all cases, the non-cyanide solutions have had problems especially in producing thick, bright deposits. Many of the alternatives that have been tested are unsuitable because of photosensitivity. However, some proprietary formulations are worthy of mention. Table 13 provides an overview of the alternatives for silver cyanide plating.
*No additional information on this solution is available
RCA Silver Solution
RCA, Inc. obtained a patent for silver iodide in 1977. Silver iodide is a stable and easy-to-use solution. However, the solution was unsuitable for electronics and decorative coating because of sensitivity to light and the high cost of the solution. Another problem with this solution is that it is toxic and is likely to complicate waste treatment operations (Braun Intertec 1992).
In 1968, IBM, Inc. obtained a patent for a bath that uses silver ammonium complexes. This solution's optimum performance was found to be in the pH range of 11.0 to 12.5. At this pH level, the bath generates ammonium hydroxide, which poses a concern for employee health and safety (Braun Intertec 1992).
Silver Methanesulfonate-Potassium Iodide
Researchers have investigated a silver methanesulfonate-potassium iodide bath to study the effects of additives. This bath produced a deposit with a fine grain structure and appearance that was comparable to or better than a conventional cyanide bath. However, this solution has not been tested on a commercial scale (Braun Intertec 1992).
Technic Non-Cyanide Silver Solution
Some platers have successfully applied Technic Inc.'s proprietary non-cyanide silver solution for applications where a thin deposit is required. However, it has not been applied universally. A facility in New York tested Technics, Inc.'s non-cyanide silver solution, Technic-Silver CyLess, as a replacement for their bright silver cyanide line. The facility decided not to implement this system for the following reasons:
More work on Technic's non-cyanide solution is being performed by Lawrence Livermore National Laboratory through a cooperative research and development agreement.
Silver in the Electronics Industry
Researchers have developed a new silver plating bath with no free cyanide especially for high-speed plating in the electronics industry. This solution also can be formulated for standard systems. Silver coatings from the no free cyanide bath have good contact properties and are less susceptible to tarnishing than those from conventional alkaline cyanide silver baths. These solutions are easy to maintain and require less complicated waste treatment procedures. Silver can be precipitated as silver cyanide and reused. The neutral pH and no free cyanide properties cause the system to be less likely to leave residuals on parts, a property known as free rinsing (Braun Intertec 1992).
Alternative Deposition Processes for Silver Plating
Several facilities are testing alternative deposition processes for silver plating. Processes such as sputter deposition are being tested. For more information on this process, refer to Alternative Methods of Metal Deposition.
Recovery Technologies for Silver Plating
Because of the high cost of the metal salts used in silver plating, recovery technologies are widely practiced. Even with the high cost of the some of the technologies, it is still economically feasible for companies to use technologies such as ion exchange and reverse osmosis. Silver cyanides can be quite problematic because the complexed cyanide is somewhat resistant to oxidation using conventional alkaline chlorination.
Electrolytic Recovery Technology for Silver Cyanide Recycling
Wastewater generated from the rinsing of silver cyanide parts contain silver and cyanide-containing compounds. The wastestream requires pretreatment to reduce these toxic materials prior to discharge. Electrolytic recovery technology uses an electrical current to plate out the silver metal and oxidize the cyanides in spent rinsewater. The silver metal is recovered from the electrolytic recovery unit (ERU) as a metal foil that can be returned to the plating process bath as an anode source. The purity of the recovered silver should meet the specifications for anode purity as long as the water from the rinse tanks is used to rinse parts that are plated only in the silver cyanide tank. The ERU should be plumbed to a static rinse tank in a closed-loop fashion. The cyanides are partially oxidized to cyanates in the electrolytic process. This technology can be used to remove more than 90 percent of the silver metal in the rinsestream and oxidize 50 percent of the cyanides (NFSESC 1995).
The benefits of electrolytic recovery for silver cyanide recycling include cost savings and reduced hazardous waste generation. The cost savings will vary for each installation, however, cost savings can be expected from reduced use of treatment chemicals for cyanides and heavy metals in the wastewater treatment plant, reduced costs for silver anodes and chemicals, and reduced cost for disposal of hazardous waste sludge generated from the treatment process. For more information on electrolytic recovery, refer to the recycling/recovery section in Pollution Prevention in Rinsing.
Silver Recovery with Ion Exchange and Electrowinning
Ion exchange systems can be used to remove silver cyanide complexes from rinsewater. These metal complexes are strongly retained by anion resins and are difficult to remove with conventional strong base regeneration. Often, the exhausted resin is simply shipped off site for silver recovery by incineration, resulting in high operating costs for the ion exchange unit because of resin costs. A study done in Wisconsin found that by combining ion exchange and electrowinning technology facilities can expect that:
For more information on ion exchange and electrowinning, refer to the recovery/recycling section in Pollution Prevention in Rinsing.
A new technology is under development at Los Alamos National Laboratory to selectively recover silver ions from electroplating rinsewaters. The silver ions are recovered in a concentrated form with the appropriate counter ions ready for return to the original electroplating bath. The technology is based on the use of specially designed water-soluble polymers that selectively bind with silver ions in the rinse bath. The polymers have such a large molecular weight that they can be separated using ultrafiltration technology. The advantages of this technology are high metal selectivity with no sludge formation, rapid processing, low energy, low capital costs, and small size.
The electroplating industry uses approximately 88,000 tons of zinc in the United States per year. Approximately 40 percent is used in cyanide baths and another 40 percent is used in chloride zinc solutions. The remainder is used in alkaline non-cyanide baths (Davis 1994).
Zinc plating is versatile and used for many different applications. Because zinc is relatively inexpensive and readily applied in barrel, tank, or continuous plating, platers prefer it for coating iron and steel parts when protection from either atmospheric or indoor corrosion is the primary objective (Ford 1994).
Common Bath Solutions
As stated above, zinc is deposited electrolytically from three different solutions: a cyanide bath, an acid chloride bath, and an alkaline non-cyanide (or zincate) bath. Zinc is also used in the galvanizing process. Workpieces usually are chromated after plating. The conventional zinc coating is dull gray in color with a matte finish. Another common zinc coating is bright zinc with a bleached chromate conversion coating or a clear lacquer coating, which is sometimes used as a decorative finish (Mabbett 1993).
Alternatives to Cyanide Zinc Baths
Two bath solutions are currently used as alternatives to zinc cyanide plating: zinc alkaline and zinc acid chloride. Tables 14 and 15 present these alternatives and their characteristics. Proper matching of the bath solution to the substrate characteristics is important to successfully implement a non-cyanide zinc plating system. Regular steel and leaded steel substrates are both compatible with acid chloride and alkaline non-cyanide processes. Substrates other than steel tend to be more compatible with acid chloride zinc than alkaline zinc (TURI 1994).
Alkaline non-cyanide electrolytes consist of sodium and zinc hydroxide. In the absence of cyanide, platers sometimes use proprietary sequestering agents to yield grain refinement. When operated with concentrations and other parameters in control, zinc alkaline baths perform as well as cyanide-based baths and are the least expensive of all the zinc plating baths. This solution has excellent throwing power and rinsewater generally is easy to treat. Also, sludge generation is low because of the low metal content of the solution (Murphy 1993).
A common problem with alkaline baths is the control of the zinc metal level. During idle periods, the caustic is aggressive toward the zinc anodes and metal concentration rises. Platers often are forced to remove anode baskets at the end of a work shift or prior to the weekend. Some opt to store the solution in an anode-free storage tank. In the past, yellowing of the plate has been a problem, however, advances in technology have resolved this issue and many proprietary bath solutions can provide excellent brightness and good color. Another drawback often cited about zinc alkaline baths is low cathode efficiency. While this is a problem for barrel platers, those that rack plate actually can find that an increase in cathode current density can provide excellent metal distribution on parts with intricate designs. Blistering also can be a problem with this solution, especially in thicker deposits. Blistering can be attributed to poor cleaning or high brightener levels. Good housekeeping is imperative to avoid this problem (Natorski 1992).
Alkaline zinc baths also form carbonate similar to cyanide solutions. Symptoms of this include yellows in blue bright chromate, a drop in brightness, and poor coverage. If carbonate levels become too high, platers should consider one of the following options:
Zinc Acid Chloride
Chloride zinc baths have been available since the 1960s. The original baths used chelates or ammonium chloride. Today, however, most baths use either potassium or ammonium chloride. The advantages of the chloride systems include brilliant deposits, high cathode efficiency, good leveling properties, low energy consumption, and easily treated non-toxic electrolyte. The disadvantages are poor throwing power, higher initial equipment investment, and higher brightener costs compared to the alkaline processes. In the past, chloride solutions had a foaming problem, especially in air-agitated rack plating. However, new surfactants in the solution produce low-foaming electrolytes. Facilities using atmospheric evaporators should use low-foaming solutions. Another advancement in chloride solutions are their ability to plate efficiently at higher temperatures (Murphy 1993). Higher temperature baths increase the number of potential recycling/recovery applications that this process can use.
Alternative Deposition Processes
A number of electroapplied organic coatings, also known as Ecoat, and at least one commercially available autophoretic coating are feasible as non-metallic substitutes for zinc electroplated coatings, especially when they are used for corrosion resistance on steel substrates (Altmayer 1993a).
In August 1988, Plating Inc., a subsidiary of Superior Plating, installed a 5 gallons-per-minute reverse osmosis system on their automated zinc cyanide line to recover rinse and process bath solution. In a 7-month study funded by the Minnesota Waste Management Board, the system achieved its objectives. It maintained rinse quality standards, recovered 2,480 gallons of plating solution, avoided shipment of thousands of gallons of dead rinse for treatment, and was projected to eliminate the need for shipment of 700 cubic feet annually of resins containing cyanide. Payback for the system was expected to be less than 1 year (Rich 1989).
While cyanide is a major contributor to pollution generation at a metal plating facility, other constituents are of concern because of the toxicity of the metal contained in the solution. Most common of these processes include nickel, tin, and chrome. The following section covers nickel and chromium plating.
Electroplating processes and aluminum finishing use chromium plating heavily. The most common hexavalent chromium-bearing solutions include decorative and hard chromium, aluminum conversion coating, bright dipping of copper and copper alloys, chromic acid anodizing, aluminum deox/desmut, chromate conversion coatings on cadmium and zinc, and copper stripping with chromic acid. This section will cover hard and decorative electroplating. Conversion coatings such as anodizing and chromating are covered in this chapter. Chromium use with aluminum and stripping are covered in this chapter.
Because of hexavalent chromium's high toxicity and cost for treatment and disposal, the industry has focused on reducing or eliminating its use. Hexavalent chromium is a known carcinogen and a designated hazardous air pollutant. Approximately 80 percent of the available power supplied to hexavalent chromium processes generates hydrogen gas. Evolution of the gas produces a mist of fine water particles with entrained hexavalent chromium. This mist is regulated under the Clean Air Act and the Occupational Safety and Health Administration (OSHA). Protection of employee health and safety as well as the environment requires a high level of emissions control (PTAPS 1995). Chromium, especially hexavalent, also is very easy to operate in a closed-loop system using simple technologies (Gallerani 1996).
When the plater's goal is a pleasing appearance that has durability, the plating is considered decorative. Decorative chromium plate is almost always applied over a bright nickel-plated deposit, which is usually deposited on substrates such as steel, aluminum, plastic, copper alloys, and zinc die casting. Chromium has a pleasing appearance when plated over bright nickel. Decorative chromium plating typically ranges from 0.005 mils to 0.01 mils in thickness. Decorative chromium plating can be found on numerous consumer items including appliances, jewelry, plastic knobs, hardware, hand tools, and automotive trim (EPA 1994).
When chromium is applied for almost any other purpose, or when appearance is an incidental or lesser feature, the process is commonly referred to as hard chromium plating or functional chromium plating. Functional chromium plating normally is not applied over bright nickel plating although, in some cases, nickel or other deposits are applied first to enhance corrosion resistance. Functional chromium plating tends to be relatively thick, ranging from 0.1 mils to more than 10 mils. Common applications of functional chromium include hydraulic cylinders and rods, crankshafts, printing plate/rolls, pistons for internal combustion engines, molds for plastic and fiberglass parts manufacture, and cutting tools. Functional chromium commonly is specified for rebuilding worn parts as rolls, molding dies, cylinder liners, and crankshafts (Chessin 1982).
Common Bath Solutions
The traditional chrome plating process is the 100:1 bath, which means that the ratio of chromic acid (CrO3) to sulfate (SO4) should be 100:1 by weight, that is, 250g/l CrO3 to 2.5 g/l SO4. Excess sulfate in these solutions can affect plating quality and should be removed by the addition of barium carbonate. The addition of this chemical causes the formation of barium sulfate, which can be precipitated. This solution contains chrome in the hexavalent form, which is regulated far more stringently than trivalent chrome. For this reason, development of trivalent chromium plating solutions is proceeding rapidly (Ford 1994).
To function as a suitable substitute for chromium, an alternative coating must offer a combination of wear resistance, corrosion resistance, lubricity, high-temperature tolerance, low friction coefficient, heavy thickness deposition, and high impurity tolerance. No single coating can replace the properties and processing ease of traditional hexavalent chromium, however, several alternatives have shown promise in replacing chromium for specific applications.
In some applications, especially decorative plating, the use of trivalent chromium has been proven successful as an alternative for hexavalent chrome for certain thicknesses. Use of trivalent chrome eliminates misting problems and the added reduction step in wastewater treatment. Adherence, throw, and coverage also are improved. Higher rack densities also can be achieved because bath concentration is much lower, dragout is less, and the amount of sludge produced by wastewater treatment is reduced substantially. However, plating thickness is limited to 0.1 mil; coatings thicker than this usually have problems with cracking and palling. Therefore, this technique usually is not suitable for hard chromium coatings, which can require finish thicknesses of 20 mils or more. Although the color tones of trivalent chromium coatings are different from those of hexavalent chromium, additives to the trivalent chromium bath often can ameliorate the difference. One of the main barriers for increased use of this solution is customer acceptance. Primarily, customer concern is related to the color of the deposit (Shahin 1992).
Electroless Nickel Phosphorous
The use of electroless nickel finishes from conventional hypophosphite solutions has been investigated as an alternative. The use of electroless nickel as an alternative is limited by its somewhat poorer physical properties including lessened hardness and abrasion resistance. The corrosion and wear properties depend on the phosphorous content, which can vary from 1 to 12 percent. Electroless nickel deposits from borohydride chemistry rather than from hypophosphite chemistry have shown better wear, lower friction, and improved hardness (Lindsay 1995). Additionally, heat treatment is required to achieve full hardness. Brittleness of the deposits makes some final finishing applications, such as grinding, difficult on thick deposits. Also, thick deposits of electroless nickel cannot be plated as cost effectively as chrome. However, electroless nickel plates more evenly so that the need for substantial overplating often can be eliminated. An advantage of electroless nickel is that the deposit follows all the contours of the substrate without excessive buildup at the edges and corners, which is a common problem in chrome plating (Meyers 1994).
The process bath, however, is more sensitive to impurities than the chrome plating bath. As a result, it must be monitored closely to maintain the proper concentrations and balance of the metal ions and reducing agents. In addition, the bath life is finite and requires frequent disposal and replenishment, especially when thick deposits are being applied. Deposition rates and coating properties are affected by temperature, pH, and metal ion-reducing agent concentrations (Meyers 1994).
Electroless nickel has been well accepted for ground-based hydraulic component use, however, it has not been used in aerospace applications. For more information on electroless nickel, refer to the section on electroless plating in this chapter.
Two nickel tungsten-based alloy electroplating processes are available as potential alternatives to chrome plating: nickel-tungsten boron (Ni-W-B) and a nickel-tungsten silicon carbide composite (Ni-W-SiC). The two processes are similar in that they are both electrolytic and they deposit a coating of nickel and tungsten with minor percentages of either boron or silicon carbide to enhance the coating's properties (Meyers 1994).
Both substitutes use less energy than chrome plating both for rectification and heating, resulting in reduced energy costs. Additionally, the deposits are more uniform than chrome, increasing plating line throughput and reducing reject rates. Each coating exhibits many of the same desirable properties as chrome plating, but additional testing is needed before widespread use can be expected. The major disadvantages of these two processes are their lack of maturity, potential for increased chemical costs, and their reliance on nickel (a metal targeted by EPA for reduction) (Meyers 1994).
Following several years of development, an alloy of nickel, tungsten, and boron has been introduced recently under the trade name AMPLATE. The plating solution is mildly alkaline and far less toxic than chromium. The alloy is reflective and has an appearance similar to chromium, bright silver, or bright nickel. The coating has favorable chemical and abrasion resistance, high ductility, a low coefficient of friction, and a uniform finish (Meyers 1994). Unlike most metals that exhibit a crystalline structure at ambient temperatures, the AMPLATE alloy is structureless so that the plate replicates the appearance of the substrate. For instance, if the substrate has a bright appearance so will the finish, but if the substrate is etched or patterned, the plated workpiece will appear etched (Scruggs 1992).
Nickel-Tungsten Silicon Carbide
This technology has been patented by Takada Inc. to replace functional (hard) chromium coatings. Nickel-tungsten silicon carbide is similar to nickel-tungsten-boron, except that it uses silicon carbide particles interspersed in the matrix to relieve internal stress and improve coating hardness (Meyers 1994). Nickel and tungsten ions become absorbed on the suspended silicon carbide particles in the plating solution. The attached ions are then adsorbed on the cathode surface and discharged. The silicon carbide particle becomes entrapped in the growing metallic matrix (EPA 1994).
This process has several advantages over hard chromium plating including higher plating rates, higher cathode current efficiencies, better throwing power, and better wear resistance. The main disadvantage of this process is its susceptibility to metallic and biological contamination. Much is still unknown about this process including its susceptibility to hydrogen embrittlement, fatigue, and corrosion as well as its maximum finish thickness, lubricity, grinding characteristics, and facility requirements (EPA 1994).
Tin-cobalt alloys provide a finish that is similar in appearance to chromium. The tin-cobalt appearance ranges in color from a bright, chromium appearance to a warm, silvery gray color. Color is controlled by varying the percent of tin in the alloy. To achieve the appearance of a chromium plate, the optimal tin-cobalt ratio in solution is 50:50. This ratio results in a plate that is 80 percent tin and 20 percent cobalt. Reducing the cobalt content of the plate below 17 percent results in a matte gray appearance. Additional operating parameters include a pH of approximately 8.5 and an operating temperature of between 38 and 43 degrees Celsius. The tin-cobalt finish provides a hardness and wear resistance that is sufficient for most indoor, decorative applications. The process, either in rack or barrel operations, uses an alkaline sulfate system with optional wetter/amine-based liquid brighteners. Current applications of this plating alternative for chromium include automotive interior parts, computer components, bicycle spokes, flexible shower hoses, and screws (Davis 1994).
Tin-Nickel Acid or Near Neutral
Tin-nickel alloy plating can be used as a replacement for decorative chromium plating for both indoor and outdoor applications because of its faint rose pink cast. This alloy is resistant to corrosion and tarnish and has good contact and wear resistance. Tin-nickel's hardness lies between that of chromium and nickel. Other advantages of this coating include excellent frictional resistance and ability to retain an oil film on its surface. Tin-nickel alloy plating solutions have a high throwing power, which enables the solution to function where plating chromium in deep recesses is a problem (Plating and Surface Finishing 1994).
Alternative Deposition Methods
Aluminum Ion Vapor Deposition
Ion vapor deposition (IVD) produces a multi-purpose coating that has excellent corrosion protection and no embrittlement problems. This technology has been used as an alternative to chromium coating in several applications. Extensive testing has shown that IVD aluminum protects substrates better than electroplated or vacuum-deposited chromium in acetic salt fog and outdoor environments. IVD also provides greater resistance to cracking (Muehlberger 1983). For more information on IVD, refer to the section on IVD in Alternative Methods of Metal Deposition.
Metal Spray Coating
Several metal spray coatings processes have shown promise as potential alternatives to chrome plating. These technologies are not new, however, recent regulation of chrome has made these technologies economically desirable. Variations on the spray technologies include arc spray, flame spray, plasma spray, and high velocity oxy-fuel (HVOF) spray. From a materials standpoint, HVOF spray results in coatings with the best properties (Meyers 1994).
HVOF coatings are used currently in many industrial applications because they develop very hard, wear-resistant surfaces that are comparable to those of chrome plating. In HVOF coating application, an explosive gas mixture ignites the barrel of the spray gun, which melts a powdered coating material and propels it at supersonic speeds toward the substrate. The superior properties achieved using this technology are a result of the high speed of the material. The higher the velocity, the greater the force of impact on the substrate, resulting in fewer voids in the coating. Several of the potential alternatives contain chromium, yet the HVOF coating will generate a significantly smaller mass of chromium-containing waste and will emit less chromium. The powdered overspray can be captured and recycled easily by a dry filter system and, unlike conventional chrome electroplating, no chemicals are added to the total waste volume when precipitating the metals (Meyers 1994).
A disadvantage of the HVOF process is that the application is limited to line-of-sight areas of the part. Complex shapes, threads, and bores/holes cannot be coated. Unlike the chemical substitutes that use conventional finishing methods, metal spray coatings will require changes in finishing and grinding operations. Given the hardness of the coating, stripping and reworking these finishes might prove difficult (Meyers 1994). For more information on this process, refer to Alternative Methods of Metal Deposition.
Physical Vapor Deposition
Physical vapor deposition (PVD) is one of the many emerging replacements for chromium electroplating. PVD encompasses several deposition processes in which atoms are physically removed from a source and deposited on a substrate. Thermal energy and ion bombardment methods are used to convert the source material into a vapor. Specific processes for applying chromium include ion plating and sputtering. For more information on PVD, see Alternative Methods of Metal Deposition.
Titanium nitride using PVD is a prime replacement for chromium coatings. This material exhibits greater hardness than chromium and can be applied cost effectively in a thinner coating. Titanium nitride applied with PVD is not subject to hydrogen embrittlement. However, because of its hard nature, titanium nitride coating cannot replace chromium in highpoint or line-load applications. This material also does not provide the corrosion protection of the thicker chromium plates (Lindsay 1995).
Other Emerging Technologies
The government is evaluating several other technologies as alternatives to hexavalent chrome under the Environmental Technology Initiative including:
For more information on these alternative deposition processes, refer to Alternative Methods of Metal Deposition.
Process Modifications for Chromium Plating
Inhibiting the release of chromium into the air by forming a physical barrier atop the plating bath with plastic balls or mist suppressants or by altering the chemistry of the bath with the addition of wetting agents is one way to prevent pollution from a chromium plating line (PTAPS 1994).
Floating Plastic Ball
Placing solid polypropylene balls 3/4 to 1½ inches in diameter on top of the plating bath will retard mist formation and evaporation. The balls can prevent up to 70 percent of the mist from escaping the plating solution and can be used effectively in both decorative and hard chrome plating processes. Polypropylene is reasonably resistant to chromic acid solutions at temperatures of up to 140 degrees Fahrenheit. Higher temperatures can cause the balls to break. Platers should use solid balls because hollow ones tend to trap solution inside from seam leakage (PTAPS 1994).
Polypropylene balls cost approximately $45 to $200 per 1,000 balls depending on the ball size and the total quantity purchased. An average tank requires about 2,500 balls. There are no additional operating costs for using this method. However, some of the balls might need to be replaced on an annual basis depending on the operating conditions of the tank (PTAPS 1994).
When using balls in a plating solution, pre-cleaning of parts is essential. Small amounts of oil and grease from workpieces can float onto the bath surface and adhere to the balls. As parts are raised and lowered in the bath, oil-covered balls can drag across the workpiece surface and prevent effective plating and rinsing, resulting in a flawed coating.
The most common problem associated with this method is that the balls become trapped in recessed areas of parts or equipment (e.g., barrels) and prevent plating or cause burning or dulling of the plated workpiece. Whether the balls become entrapped usually is associated with their size. To help prevent entrapment of balls, platers can use plastic mesh bags. The bags can keep the balls together on the surface and reduce the likelihood of balls being carried into subsequent tanks (PTAPS 1994).
A mist (or fume) suppressant is a chemical that forms a barrier on the surface of the bath solution to prevent mist from escaping. During operation of the plating or anodizing process, the mist suppressant generates a foam blanket and traps the process gases either between the bath surface and the blanket or within the foam blanket. Mist suppressants can be more than 99 percent effective in reducing emissions from decorative chrome plating and anodizing (PTAPS 1994).
Suppressants are chemical additives that can affect the chemical balance of the plating or anodizing solution. For this reason, a generic suppressant is not available for widespread use. Depending on the various types of baths within a shop (e.g., hard, decorative, or proprietary), a different mist suppressant or concentration of suppressant might be required for each bath to achieve the desired result. Some mist suppressants must be replaced because of the degradation of the active ingredient. These are know as temporary suppressants. Other mist suppressants only have to be replaced when they are diminished because of bath dragout (PTAPS 1994).
Another factor to consider when deciding whether to use mist suppressants is the amount of foam generated during bath use. Too much mist suppressant will cause a large foam blanket that can result in excessive dragout into subsequent rinse tanks. This dragout will lead to an increase in the amount of materials necessary to replenish the suppressant in the tank. The foam blanket also can be drawn into the exhaust system, increasing the likelihood that a more concentrated chromium mist will be released from the stack. Finally, too much mist suppressant can spill onto the facility floor or into other tanks, generating large amounts of waste that require clean up and increase treatment and disposal costs (PTAPS 1994).
Because hydrogen is the primary gas formed during plating, dissipation of the gas is important. Build up of hydrogen with the foam or under the foam poses a serious explosion risk, especially when the parts are removed while "hot" (i.e., the electric current is still on). When hot parts are removed, the hydrogen gas can ignite spontaneously, resulting in equipment damage, serious personal injury, and an increased risk of fire (PTAPS 1994).
Mist suppressants come in liquid and free compressed-powder form and range in cost from $10 per pound to $60 per pound. The amount of mist suppressant chemical necessary to form a sufficient barrier varies depending on the type of chemical mist suppressant, tank size, and frequency of plating. According to manufacturers' instructions for different mist suppressants, the recommended amount of chemical to add ranges from 0.001 percent of the total volume of the plating bath to 0.1 percent (approximately 1 ounce per 500 gallons initially with infrequent additions thereafter). When used in the appropriate amount, dragout and replenishment costs are minimal. Because mist suppressants also are stand alone emission controls, utility requirements are nonexistent, except perhaps for makeup water in the case of free-form powders (PTAPS 1994).
When using mist suppressants, operators should start with half the manufacturer's recommended amount and increase levels slowly to determine the actual amount required to achieve the desired result. The necessary amount of suppressant often depends on the activity of the plating line. Less active plating lines might require the use of more suppressant while a simila amount on a busy day might generate an unmanageable foaming problem (PTAPS 1994).
Reduction of surface tension of the chrome plating or anodizing bath reduces the rate of mist generation by causing the gas bubbles to burst with less intensity. For chrome finishing, decreasing the surface tension to 40 dynes per centimeter will achieve excellent chromium emission reductions. However, wetting agents can affect the quality of the deposit; too much can cause burning, pitting, or poor adhesion; too little can result in little or no reductions in emissions (PTAPS 1994). For a complete description of wetting agents, refer to the first section in this chapter.
Static Rinse Tank
Many facilities use a static rinse tank (often known as dead rinse) after the process bath. Water from this tank is used as makeup water in the process bath. Using this method has assisted many facilities in closing the loop on chromium contamination.
Thin plastic sheets can be placed over the plating bath to reduce emissions by trapping and condensing vapors from the tank. The cover can be placed almost directly on the chromic acid solution, resulting in little free space between the cover and the solution. Tank covers can be constructed of plexiglass or other suitable plastic and cut to fit the size of the tank. The facility should determine how to remove the cover when transferring workpieces during plating or anodizing operations. Rigid covers are most easily made by using anchors and hinges that operate like a window or door. Platers can use flexible sheeting by mounting it on one side, rolling it over the top, and anchoring it to the other side like a window shade. A drawback of this method is that chromium can dry out or corrode plastics. Another consideration is how the cover will affect the movement of parts through the process line (PTAPS 1994).
Recycling and Recovery Technologies for Chromium Plating
For hexavalent chromium plating baths, firms can use porous pots to extend bath life. During plating, the concentrations of iron and other cationic impurities build up in a hexavalent chromium bath so that the finish is unsatisfactory. When the solution reaches this point, operators can use porous pot technology to purify the process solution. This technology uses a porous pot in which a semi-permeable membrane separates a cathode from an anode along with an applied power source. In this operation, the iron and other contaminant metal ions pass through the membrane and accumulate in the cathode chamber. Once the contaminants are contained in the chamber, they can be removed periodically for disposal. Chromate ions remain in the anode compartment as part of an anolyte that, after purification, can be returned to the plating tank for further use. Using this technique, companies not only reduce waste but also use less chromium. The liquid in the cathode compartment must be handled as waste (IAMS 1995).
Two basic design configurations exist for this technology. One type consists of a tank holding four to eight pots. Plating solution is pumped to the tank on a continuous basis and returned by gravity flow to the plating tank. The cells are powered by a rectifier (1,000 to 2,000 ampheres) that is dedicated to the purification unit. A second type of unit exists that consists of a single pot that is suspended directly in the plating bath. This unit is powered by the tank rectifier and draws up to 240 ampheres. The advantage of the smaller unit is that it does not require extra equipment (e.g., rectifier, fume exhaust system, and overhead hoist). However, there are disadvantages to the smaller units. They include limited capacity and operation that only occurs when the tank is energized.
Membrane electrolysis is similar to the ion transfer technology used in porous pots, however, this technology is primarily used in chrome applications. The unit employs a separate tank and power source for operation. Plating solution is circulated through the unit, which contains an anode compartment and 10 cathode modules. When the unit is energized, bath cations pass through the membrane and deposit on the cathodes. The membrane is not anion or cation selective. Selectivity is a result of the electrical force. This selectivity distinguishes this technology from electrodialysis equipment. For more information on this technology, refer to the recycling/recovery section in Pollution Prevention in Rinsing.
Ion exchange has been applied to chromium solutions for the removal of trivalent chromium, iron, and other metallic contaminants. Facilities using this technology usually treat the solution on a batch basis, requiring a shutdown of the chromium line. However, a continuous process has been used. Generally, the plating solution is cooled and diluted prior to treatment.
Eco-Tec Inc. in Canada has developed an ion exchange system for use in hexavalent chrome plating operations. Initial results are promising, however, other platers have had problems in the past with other ion exchange systems. Problems usually are a result of fouling membranes and sensitivity to chromium concentrations (Cushnie 1993). For more information on ion exchange, refer to Pollution Prevention in Rinsing.
Companies have plated nickel since 1842, but modern nickel plating began in 1916 with the introduction of the Watts formulation. Typically, nickel ingots or balls are dissolved into a metal salt solution that is used in the plating baths. However, nickel salts have some negative characteristics including allergenic properties and carcinogenicity (ASM 1982).
Nickel plating commonly is used to impart corrosion resistance or to act as an intermediate layer prior to plating silver, chrome, or gold. Nickel also is valued for its leveling and brightening properties. Because of these properties, nickel can eliminate the need for polishing work and can improve the quality of an inferior substrate. Several types of electrolytic nickel plating are available including sulfamate nickel and bright nickel. Many industries use nickel plating for decorative or functional purposes including jewelry, automotive parts, tools/dies, and lighting fixture manufacturing (RI DEM 1995c).
Typical wastes from nickel plating operations include nickel-contaminated water from running rinses and excess dragout solution. If a plater uses traditional chemical precipitation to treat the nickel-laden wastewater prior to discharge, the resulting sludge automatically is classified as a F006 hazardous waste. Iron and chromium contamination is common in acidic nickel baths. In most formulations, this contamination can be removed with peroxide combined with pH elevation and batch filtration (SME 1985).
Common Bath Solutions
Sulfamate nickel, also known as dull nickel or engineering nickel, is used for engineering (usually non-decorative) applications to produce low-stress deposits. This plate is ductile and can be used in many applications. This bath also is useful for electroforming and for parts that are susceptible to fatigue failure. The primary constituents of this bath are nickel sulfamate, nickel chloride, and boric acid (Ford 1994).
Another common nickel plate is bright nickel, sometimes referred to as a Watts bath. This solution imparts a bright and hard finish, which is used mainly for decorative purposes. The process bath in nickel plating contains both inorganics (i.e., nickel salts and acid) and organics (i.e., brighteners and wetting agents). Additional chemicals ultimately determine the characteristic of the plate. These include brightening and wetting agents that account for the brittle nature of the deposit (Ford 1994).
Alternative Metal Processes Baths for Nickel Plating
Alternatives to nickel as intermediate layers include bronze, palladium, and cobalt. Like nickel, however, cobalt is under examination as an allergen and carcinogen and might be regulated in the future.
Processes available for white or yellow bronze deposits include cyanide or alkaline-based systems. Like the nickel baths, operators can add buffers, brighteners, and levelers to the bath to create the needed characteristics. Bronze exhibits better throwing power than nickel, resulting in a more evenly distributed thickness. This alloy can be used when superior solderability, hardness, corrosion resistance, brightening properties, thickness distribution, wear resistance, or diamagnetic properties are needed. Bronze alloys also kill bacteria, just like copper, and have better bactericide properties than silver, making them attractive plating materials for bathroom fittings and door handles (Simon 1994).
White bronzes are hard and tarnish resistant. This metal also is corrosion resistant. Yellow bronzes are hard, but do not have the corrosion resistance properties of white bronze because of their high copper content. Yellow bronzes, however, have the brightening and leveling effects of nickel plating. Platers use a layer of white bronze on top of a yellow bronze to replace bright nickel of the same layer and thickness (Simon 1994).
Bronzes can provide a surface that is harder than nickel and, in decorative applications, protect workpieces from deterioration or tarnishing. However, for technical applications where the workpiece will be subjected to high temperatures, bronzes are not an appropriate substitute.
Metal finishers can use palladium as a substitute for nickel as an intermediate layer when nickel's property as an allergen is a consideration. Some platers also consider palladium a feasible replacement for gold because of palladium's lower cost. One of the benefits associated with substituting palladium is that it is not listed as a chemical that facilities must report under the TRI reporting requirement.
A relatively new system for using palladium chloride as a palladium salt combined with proprietary additives exists. This system produces ductile deposits that are crack-free and bendable as well as resistant to corrosion. To avoid contamination of the electrolyte, a gold strike or palladium strike is recommended before applying the main palladium layer (Simon 1994).
Issues Concerning Palladium and Bronze as Nickel Substitutes
Many questions remain unanswered about the feasibility of replacing nickel with bronze and palladium, but some advantages are well known. Bronzes and palladium are comparable to nickel with regard to hardness, color, corrosion protection, solderability, further plating ease, and wear resistance. Bronzes, especially the yellow-white combination, are superior to palladium in this area. However, palladium is similar to nickel in its diffusion properties. The use of bronze as a diffusion barrier is limited to decorative purposes. In regard to ductility, bright nickel is the most brittle of the three applications. Palladium is ductile, yellow bronze has average ductility, and white bronze is brittle. With regard to cost, palladium is the most expensive of the three processes (Simon 1994).
Recycling/Waste Reduction Technologies
Common pollution prevention options in nickel plating include electrolytic dummying, spill and leak prevention (especially from filtration systems), countercurrent rinsing, evaporation, and ion exchange. This section covers particular recycling/recovery technologies for nickel plating. For more detailed information on a specific recycling/recovery technology, refer to Pollution Prevention in Rinsing.
Electrodialysis Reversal Process (EDR) in Nickel Plating
Electrodialysis is an electrochemical separation process in which ions are transferred through a pair of ion-selective membranes from a less concentrated to a more concentrated solution as a result of the flow of direct electric current. Initially, these systems could only transfer ions in one direction. Typical problems associated with the system included membrane fouling and organic buildup. Newer electrodialysis systems, however, eliminate these problems by allowing the flow to periodically reverse itself in order to clean the membrane. Recently, electrodialysis has been considered a promising method for the recovery of nickel ions from rinsewater, recycling them back to the plating solution and simultaneously generating clean water for reuse in the plant. Important parameters to evaluate include limiting current density, current efficiency, and water transport through the membranes (RI DEM 1995c). Small-volume shops might find that the costs associated with electrodialysis are too high. But effort to build smaller systems that are feasible for all manufacturers is increasing (Girasole 1996).
Electrolytic Recovery in Nickel Plating
Nickel can be recovered from a variety of concentrated solutions including dragout tanks, ion exchange regenerants, and concentrated membrane fluids. The metal that is recovered can be sold as scrap metal or, as some facilities have done, returned as reclaimed material to the plating bath for use as a solid material in the anode. Electrolytic recovery works best in nickel plating applications when pH values are between 3 and 9. Generally, high energy is required (100 to 500 ampheres). Anode and cathode materials are important design parameters for this technology. Stainless steel or graphite are the best choices. Trace metal contamination in the electrowinning solution can sometimes affect the overall efficiency of this operation (Girasole 1996).
Electrolytic recovery of nickel cannot eliminate nickel because the nature of the process makes it less efficient with low metal concentrations. However, it will have a significant effect on sludge generation. In general, for every pound of metal reclaimed, sludge generation is reduced by 4 pounds (Girasole 1996). For more information on electrolytic recovery, refer to Pollution Prevention in Rinsing.
Ion Exchange in Nickel Plating
Ion exchange is a frequently used and effective method to recycle nickel rinsewaters and capture nickel metal either for reuse or recycling. Specifically made resins are manufactured to remove particular metal ions from the solution through the exchange of similarly charged ions. At some point, the resin becomes saturated with the metal ions and must be regenerated with an acid to remove the captured metals. The metals sometimes can be reused in the nickel plating tank if the ion exchange regenerant is matched correctly because the residual material consists of concentrated nickel salts. Some companies also choose to send the saturated resin off site for metal recovery (RI DEM 1995c).
The final rinse in a nickel plating line can be continuously processed through the resin columns to ensure a nickel-free final rinsewater. The upstream rinses and dragouts are returned to the plating tank to make up for evaporative losses. As a result, platers can reduce generation of F006 hazardous waste sludge. Initial capital costs for ion exchange can vary depending on the size of the shop, and setup costs can range from several thousand dollars for a small line to $10,000 to $20,000 for a larger facility. Operating expenses include chemical costs and labor (RI DEM 1995c).
Reverse Osmosis in Nickel Plating
While ion exchange is used more frequently, interest among platers in reverse osmosis (RO) is growing. Nickel is the most common plating salt reclaimed with RO because it is expensive and the pH and temperature requirements are handled easily by a RO system. Reverse osmosis can separate dissolved components such as nickel ions, significantly reducing or possibly eliminating F006 sludge. Depending on the type of RO membrane used, the recycled rinsewater might still contain some metal ions. While a perfect membrane theoretically could separate 100 percent of metal ions, the commercially available membranes are usually 95 to 99 percent effective. Whether this is clean enough depends on the ultimate use of the plated part (Cushnie 1994).
For many non-decorative products, water produced by RO is clean enough to return to the process. However, in decorative and electronic applications where nickel is a base for precious metals, a nickel-free final rinse is necessary to avoid contaminating the precious metal plating solution. Modifying the RO process to ensure a nickel-free final rinse is possible with the addition of a small ion exchange system. Operating costs for these systems include membrane replacement, electricity, and labor. Average labor costs can total $2,000 to $3,000 per year. Systems for smaller units are not yet commercially available, however, they are under development (RI DEM 1995c).
Electroless plating is a growing segment of metal finishing, especially in the electronics industry. In electroless plating, metals are deposited onto the surface of a part without the use of electricity as a source of electrons. Instead, the bath solution supplies the electrons for the deposition reaction. These baths are extremely complex using a variety of chelating and/or complexing agents that hold the metals in solution. Common chelating agents include ethylenediaminetetraacetic acid (EDTA), citrates, oxalates, cyanides, and 1,2 diaminocyclohexanetetraacetic acid (DCTA). Nickel, copper, cobalt, and gold are the most common metals plated in this process. Deposition rates are controlled by the amount of reducing agent present and the type of chelating agent used. Figure 7 presents a flowchart of a typical electroless plating process.
Electroless plating results in a fine-grained metal deposit similar to traditional electroplating finishes. Industries use this process to plate on non-conductors such as plastic, electroformed dies, and printed circuit boards or to obtain an extremely uniform plate (ASM 1982).
Waste segregation is especially important in electroless systems because of the presence of chelators. Chelated metal solutions are not responsive to conventional neutralization, precipitation, flocculation, and settling treatment techniques. Therefore, electroless platers require alternate treatment methods. Because of the affinity of metallic ions for chelating agents, combining waste streams will cause unchelated metallic ions to mix with unreacted agents, increasing the load of difficult-to-break chelated metals to the recovery equipment (Jordan 1985).
Although extremely similar to electroplating, electroless operations feature four rather distinctive characteristics:
A common method of treating electroless wastes is the addition of reducing agents such as sodium borohydride, sodium hypophosphite, or sodium hydrosulfite at elevated temperatures to reduce the soluble metals to their metallic or oxide forms. Sludges produced in these processes contain relatively impure metal powders that are susceptible to air oxidation and require further treatment because of the presence of interstitial water containing relatively large amounts of free chelating agents. Over a relatively short period of time, these chelates can cause redissolution of some of the metal oxide in the sludge. As a result, platers must consider this sludge a hazardous waste and manage the waste accordingly (Richmond 1991).
Housekeeping in Electroless Plating
Electroless solutions are especially susceptible to impurities affecting the process solution. Impurities in the solution can cause reduced ductility and corrosion resistance as well as pitting, adhesion, and roughness problems. Facilities should identify sources of contamination and take steps to avert them including worker training or equipment modifications. Common sources of contamination include cleaners, pickling solutions, airborne particulates, hard water, and defective equipment (ASM 1982).
Metal Recovery in Electroless Plating
Electrolytic techniques use high surface-area cathodes and/or non-conductive fluidized beds to recover metal that can be sold for scrap. The process uses a high surface-area cathode that attracts the metallic ions out of solution. Platers then must strip the cathodes and treat the resulting solution by chemical or conventional electrolytic means to remove the remaining metal content. In some cases, concentrate can be returned to the process solution. Using a process such as this will result in decreased sludge generation and increased production rates from the electroless bath (Jordan 1985). Pollution Prevention in Rinsing presents a more thorough review of electrolytic recovery.
Electrodialysis uses a membrane that allows for the separation, removal, or concentration of ionized chemicals. These functions are accomplished by selective transport of ions through ion exchange. Electrodialysis uses two different membranes: an anionic permeable (AP) membrane that allows passage of only anions and a cationic permeable (CP) membrane that allows cation ions to pass through. The result is two streams: a demineralized rinsewater stream suitable for reuse and a concentration of metallic salts usually returned directly to the plating solution. For example, electroless nickel and copper plate rinsewaters generated during the production of printed circuit boards can be directed to an electrodialysis unit. Processed concentrate can be returned to the plating tank and water reused in the rinse process (Kamperman 1991).
Ion Exchange in Electroless Plating
The use of conventional ion exchange systems as well as newly developed resin technology has been proposed for the treatment of electroless wastes. Conventional cation exchange resins are extremely inefficient. Systems involving these resins, therefore, require large columns and frequent regeneration. Most chelating resin systems available today are not effective on all chelators. In addition, the regenerant from most ion exchange or chelate resin systems requires further treatment in order to reclaim or otherwise remove the metal content (Cushnie 1994).
Electroless copper is used commonly to plate parts for engineering applications, particularly to provide conductivity for electronics and printed circuit boards or plastics that are going to receive further plates for decorative applications (ASM 1982).
Common Process Solutions
Electroless copper uses copper salt as the metal salt, often cupric chloride, EDTA as a chelating agent, and formaldehyde (a suspected carcinogen) as the reducing agent. The reductive reaction is favored at high pHs so caustic soda is used to keep the pH above 11.0. Reducing agents often react with the bath, resulting in slower deposition rates and poorer deposit quality. This also can mean that the bath will need to be rejuvenated after several metal turnovers.
This solution also is subject to spontaneous decomposition. Copper built up on the tanks from the process solution must be stripped with an etching solution (e.g., sulfuric acid/hydrogen peroxide etchant). This results in an additional wastestream of copper and etching solution (Ford 1994).
Alternatives to Electroless Copper
The printed circuit board industry is testing a proprietary technology, called the Blackhole Process, as an alternative to electroless copper plating. This process uses conventional plating equipment and aqueous black carbon that is dispersed at room temperature. The carbon film that is obtained provides the conductivity needed for the through-holes. The following qualities make Blackhole environmentally attractive:
The chemistry in the Blackhole process avoids the use of metals (i.e., copper, palladium, and tin) and formaldehyde (a suspected carcinogen) used in electroless copper plating. Compared to conventional electroless copper plating, the Blackhole technology uses fewer individual steps than electroless plating. The smaller number of process steps reduces the use of rinsewater, decreasing waste treatment requirements (EPA 1994).
Printed circuit boards are prepared prior to carbon coating in the same manner as electroless copper including etchback. Immediately prior to carbon coating, the boards are cleaned with proprietary cleaners and conditioning solutions which are alkaline and contain weak complexing agents. The carbon coating solution also is slightly alkaline and contains extremely fine carbon particles. The process has been available commercially since 1989. It is used in many printed circuit board facilities and has been approved by the United States Military (MIL-55110D)as a substitute for electroless copper in military applications (Altmayer 1994). Making the transition from an electroless copper plating system to the Blackhole technology requires only the disposal and cleaning of the existing electroless line and purchasing of the new solutions (EPA 1994).
Elimination of electroless copper removes a chelated process from wastewater, however, the substitute might have some disadvantages. The extremely fine, suspended carbon might cause problems in wastewater treatment operations by clogging filters, coating probes, and interfering with clarifier operations. Carbon cannot be removed by precipitation and must be controlled at the source. Carbon in wastewater will increase loading to the publicly owned treatment works (POTW) significantly. The carbon also can act as an organic collector, increasing total organic concentrations in the wastewater. In areas where POTWs have excessive coloration regulations, discharges containing carbon are unlikely to meet this requirement (Altmayer 1993).
Electroless nickel is used normally as an engineering coating to impart corrosion and wear resistance to a workpiece. Platers also commonly use the process on aluminum to provide a solderable surface and to improve lubricity and the release of molds and dies. Because of these properties, this technology is used widely in petroleum, chemicals, plastics, optics, printing, mining, aerospace, nuclear, automotive, electronics, computers, textiles, paper, and food machinery manufacturing (Fields 1982).
Common Bath Solutions
Metal finishers have used electroless nickel since the 1950s. The most common baths use nickel sulfate salts with sodium hypophosphite as the reducing agent. Platers frequently use hypophosphite in metal applications and a warm, alkaline hypophosphite solution in plastics applications. In either case, decomposition of the sodium hypophosphite during the reduction reaction results in the formation of a compound that increases deposition rates. Generally, this occurs between five and seven metal turnovers (Fields 1982).
Some advantages of this process include:
Some disadvantages of the system include:
Another disadvantage is that the baths have a tendency to decompose spontaneously causing the entire tank to become nickel plated. When this occurs, the tank must be drained of the plating solution and filled with a nitric acid solution to dissolve the metal and repacify the tank. The nitric acid solution can be retained and used several times, but at some point it must be disposed (Davis 1992).
Bath Life Extension
Because of the frequency of bath change-out, the primary pollution prevention goal in electroless nickel baths is bath life extension. Bath life extension technology performs two functions: removal of the chemical byproducts formed during the processing of parts and the continuous addition of bath chemicals to maintain the overall chemical balance of the bath. Typical byproducts of the process are orthophosphite, sulfate, and sodium ions. Process bath chemicals and operating parameters such as nickel concentration, hypophosphite, reducing agents, complexing agents, pH, temperature, and bath stabilizers influence the effectiveness of different bath life extension methods (DoD 1996).
Recovery technologies such as ion exchange and reverse osmosis have been used to remove contaminates. Other methods include the precipitation of orthophosphite contaminants with calcium or magnesium ions, however, this method is only useful if the sulfate ion also is removed. Some treatments have extended the bath life from seven to ten times the original life (extensions such as these can reduce waste generation by 90 percent) while others have claimed increases of 50 times the original. Facilities should be aware that the concentration of inhibitors, catalysts, and exaltants will change as the lifetime of the bath is extended, requiring monitoring and additions of the chemicals (Bishop 1993).
Regeneration of Electroless Nickel Baths
Electroless nickel solutions are degraded by the buildup of orthophosphite, a breakdown product of sodium hypophosphite that platers use in the solution as a reducing agent. Studies are underway to see if electrodialysis is capable of removing the orthophosphite selectively, increasing the life of the solution vastly. Initial results for this are not promising, however, research centers such as the Toxics Use Reduction Institute continue to work with companies on making this technology feasible (Palepu).
Prolonging Bath Life with Lime
A pilot-scale study was conducted by TecKote, in Brampton, Canada, to determine if it is possible to precipitate out phosphite contamination of electroless nickel baths using lime. The test procedure was as follows:
The plating run lasted 44 hours and yielded some promising results. The test found that the addition of lime slurry doubled the life expectancy of the plating solution, however, the process also produced a sludge that was determined to be hazardous. The study did not determine whether this process would yield cost savings for plating facilities (Richmond 1991).
Immersion plating is a process similar to electroless plating. In this process, the metal finish is placed on the workpiece by displacing base metal from the workpiece with another metal ion in the plating solution. The metal ions in the plating solution have a lower oxidation potential than the displaced metal. This process, like electroless plating, uses chemical reactions to apply a metal finish to the substrate. Immersion plating differs from electroless plating in that the reducing agent is the base metal of the workpiece and not a chemical additive, as is the case in electroless plating (Davis 1994).
The thickness of deposits obtained in immersion plating is limited because deposition stops when the entire surface of the base metal is coated. Higher temperatures and agitation can increase the reaction rate of the immersion process. These baths usually are inexpensive to operate and deposit well. Other benefits of immersion include its ability to deposit on difficult surfaces such as bores or holes. When working with this solution, be aware of the safety hazards associated with bases and acids (Hirsch 1993). Table 16 identifies deposit-base pairs that can use this plating technique without a cyanide solution.
Chemical and electrochemical conversion treatments are designed to deposit a coating on metal surfaces that perform corrosion protection and/or decorative functions and, in some cases, to prepare for painting. Processes include anodizing, chromating, passivation, phosphating, metallic coating, and electropolishing. The converted surface is not superimposed on the underlying metal, but rather is a strongly adherent chemical entity formed at the interface by an interaction between the chemical coating solution and the ions formed from the metal surface immersed in the solution.
As mentioned in the previous section, anodizing is a specialized electrolytic surface finish for aluminum that imparts hardness, resists corrosion, increases paint adhesion, provides electrical insulation, imparts decorative characteristics, and aids in the detection of surface flaws on the aluminum. This process employs electrochemical means to develop a surface oxide film on the workpiece, enhancing corrosion resistance.
Anodizing differs from electroplating in two ways. First, the workpiece is the anode rather than the cathode as in electroplating. Second, rather than adding another layer of metal to the substrate, anodizing converts the surface of the metal to form an oxide that is integral to the substrate (SME 1985).
Industry uses three principal types of anodizing: chromic acid anodizing (called Type I anodizing), sulfuric acid anodizing (called Type II anodizing), and hard coat anodizing, a combination of sulfuric acids with an organic acid such as oxalic acids (called Type III anodizing). Because of the structure, the anodized surface can be dyed easily. These dyes are organic or organometallic and often contain chrome in the trivalent state. Whether the pieces are dyed or not, they need to be sealed. Sealing can be performed with hot water, nickel acetate, or sodium dichromate, depending on the required properties (SME 1985).
Type I (Chromic Acid) Anodizing
Chromic acid anodizing takes place in a solution of chromic acid. The hexavalent chrome solution creates a thin hard coating (Ford 1994).
Type II (Sulfuric Acid) Anodizing
Sulfuric acid anodizing takes place in a 15-percent solution of sulfuric acid. During the anodizing process, aluminum dissolves off the surface of the part and changes the surface characteristics to an oxide coating. This process creates a surface structure that is both porous and harder than the base aluminum. Sealing this coating provides greater corrosion protection. When the aluminum concentration in the bath solution builds up to a certain level (15 to 20 gallons per liter), the process becomes less efficient and requires treatment (Ford 1994).
Type III (Hard Coat) Anodizing
Hard coat anodizing is a form of sulfuric acid anodizing in which the acid content is slightly higher (20 percent) and an organic additive is added to the bath. This additive helps to create a tighter pore structure that increases the hardness of the oxide coating. Hard coat anodizing has a high resistance to abrasion, erosion, and corrosion. This type of coating also can be applied in much thicker layers than Type I or Type II anodizing (Ford 1994).
Platers use various methods to treat wastes generated from anodizing bath solutions. Technologies that have been employed successfully include evaporation systems operating under reduced pressure, sedimentation, reverse osmosis, filtration, and anion and cation exchangers.
Substituting Type I Chromic Acid Anodizing with Type II Sulfuric Acid Anodizing
Because of federal and state mandates being imposed on operations using hexavalent chrome, researchers have investigated the feasibility of substituting Type I anodizing with Type II sulfuric acid anodizing. The results of a NASA study indicate that in applications where anodizing is used to impart corrosion protection on aluminum, Type II sulfuric acid anodizing is superior to Type I chromic acid anodizing (Danford 1992).
Conversion from chromic acid to sulfuric acid anodizing is not a simple chemical substitution according to suppliers. The conversion requires a complete changeover of anodizing equipment and partial modifications to downstream waste treatment facilities. Replacement of the anodizing tank often is required because of the differences in acidity between sulfuric acid and chromic acid. Sulfuric acid anodizing processes also have different voltage and amperage requirements, necessitating replacement of the rectifier. The operating temperature of the electrolytic bath also is different for the two processes. The chromic process is usually maintained by steam heat at an operating temperature of 90 to 100 degrees Fahrenheit whereas the sulfuric acid process must be chilled using cooling water to an operating temperature of 45 to 70 degrees Fahrenheit.
Operation and maintenance costs are typically much lower for sulfuric acid anodizing than for chromic acid because of lower energy requirements. Wastewater treatment costs are lower as well because sulfuric acid only requires removal of copper whereas chromic acid requires more complex chrome reduction techniques. The change in materials also means that the cost of sludge disposal is greatly reduced.
Ion Vapor Deposition as a Substitute for Anodized Coatings
The brittle nature of anodized coatings can cause fatigue failure on aluminum alloy structures. However, the soft ductile ion vapor deposition (IVD) aluminum coating will not affect mechanical properties of the base metal detrimentally. In addition, the IVD coating offers excellent sacrificial and stress-corrosion protection, increasing the service life of products using this coating. This coating can allow for stronger constructions that save weight, particularly important in the design of new aircraft (Muehlberger 1983). For more information on IVD coatings, refer to Alternative Methods of Metal Deposition.
Chromic Acid Regeneration
Chromic acid anodizing solutions can be regenerated by the use of a cation exchanger which removes the accumulating aluminum together with other impurities such as copper. The life expectancy of the resin is much shorter than on normal waste treatment applications, but the method is still practical and economical (Steward 1985).
Chromic Acid Case Study
NASA conducted a case study comparing the corrosion protection between Type I (chromic acid) anodizing and Type II (sulfuric acid) anodizing. After using several analytical techniques, the study found the corrosion protection obtained by Type II anodizing superior to Type I anodizing. (Danford 1992)
Sulfuric Acid Anodize Regeneration with Ion Exchange
Traditionally, platers use ion exchange to remove metallic contaminants from wastewater streams. However, ion exchange resins remove the hydrogen and sulfate components of the sulfuric acid/aluminum anodizing solution. As the solution passes through the columns, the acid is removed. Then the wastestream, which consists of a small amount of acid plus all the aluminum from the anodizing solution, flows to the wastewater treatment system. To recover the acid, platers use water to flush the acid components from the resin, which forms a sulfuric acid solution that is low in dissolved aluminum and can be used again in the anodizing process (Ford 1994).
Sulfuric Acid Anodize Regeneration with Electrodialysis
Electrodialysis removes metal ions (cations) from solutions using a selective membrane, an electrical current, and electrodes. This technology uses a chemical mixture (catholyte) as a capture and transport media for metal ions. This catholyte will form a metal sludge and will require periodic change-outs. The recovered sludge is hazardous, however, companies might want to work with an outside firm to recover the metal in the sludge. Using electrodialysis, facilities can remove all the metal impurities from the anodizing bath, maintaining it indefinitely. By keeping the concentration of contaminants in the process bath low, the rinsewater potentially can be recycled back to the bath, closing the loop on the process. The cost to operate this system will depend on the size of the acid anodizing bath, the level of metal concentration, the metal removal capacity of the electrodialysis unit, and the ability to reclaim metals in the sludge. For more information on this technology, refer to the recycling/recovery section in Pollution Prevention in Rinsing.
Sulfuric Acid Anodize Regeneration Using Acid Retardation
Theoretically, sulfuric acid anodize solution and the phosphoric acid bright dip bath can both be regenerated using acid retardation, which is a sorption process using ion exchange resins. The cost for such a recovery operation is likely to be economically feasible for only very large operations. For more information on acid sorption technologies, refer to Pollution Prevention in Rinsing (Steward 1985).
It is also possible to collect sludges from rinsewater neutralization and from treatment of batch dumps of anodizing and caustic soda etch, press the sludges as dry as possible, and then dissolve the sludge in sulfuric acid to make a concentrated alum solution, which can be sold as a byproduct for coagulation in wastewater treatment operations. Facilities should ensure that they have a market for the alum cake prior to undertaking this option (Steward 1985).
Creating an antique finish by coating a workpiece with a black substance and mechanically relieving it so that the only black remains in recesses has been a common practice in jewelry electroplating shops for many years. Typically, the black is applied in one of several ways: as a paint, as an oxide coating (usually applied by immersion), or as an electroplated deposit (METFAB 1995).
Common Bath Solutions
The paints are solvent-based, the oxide solutions often contain hazardous materials (e.g., arsenic, lead, permangate, antimony, and dichromate), and the most popular electroplating process can contain more than 1 pound per gallon of solution of free sodium cyanide (METFAB 1995).
Alternative Bath Solutions for Blackening
The Versy Black process by Zinex Corporation of Oxnard, California, is a feasible alternative for traditional antiquing operations. This process does not contain any cyanide or chelators and contains only small quantities of zinc, copper, and cobalt. Substitution of this process for existing blackening operations eliminates cyanide, arsenic, antimony, permanganate, dichromate, and tellurium from a facility's wastestream. The proprietary nature of the Zinex product imposes limits on discussing the chemistry behind it (METFAB 1995).
The Rhode Island Department of Environmental Management in conjunction with METFAB Sales and Service tested the Versy Black solution over a 14-month period at several locations. The study showed that Versy Black is not only a feasible alternative, but also a superior product. Versy Black outperformed traditional blackening in the following ways:
Waste treatment of this process is simple. Because the process uses no chelators, treatment can be accomplished simply be precipitation with caustics (pH adjustment). The absence of chelators in the process also means that treating the metals that enter the wastestream from other processes is less troublesome. Chelators make the precipitation less effective because of their ability to keep metals from reacting with caustics to form insoluble hydroxides (METFAB 1995).
Platers often use chromate coatings to minimize rust formation and to guarantee paint adhesion after anodizing aluminum parts. These coatings also are used over zinc and cadmium to simulate the appearance of bright nickel and chromium. Other applications include use as a coating over zinc or cadmium-plated parts to prevent the formation of white rust. Depending on the color, chromating takes place in a solution of chromic acid and additives. Although these baths contain hexavalent chrome, they are not electrolytic baths and, therefore, do not generate the same level of mist/fumes of chrome electroplating or anodizing. For this reason, the chromating process is not regulated under the Chrome MACT standard (Katz 1992).
The operator immerses anodized parts in a solution that contains a hexavalent chrome salt, either chromic acid or chromate, and an acid, often nitric acid. This solution dissolves the outer layers of the substrate and causes a pH increase at the surface-liquid interface. This change results in the precipitation of a thin complex chromium gel on the surface. The gel is composed of hexavalent and trivalent chromium and the substrate itself. These chromate films provide further corrosion resistance and are formed in a wide range of colors: clear, yellow, gold, and drab olive (Ford 1994). Table 17 presents an overview of the common chromating uses for different substrates.
Unfortunately, like chromium plating, chromating involves highly carcinogenic and toxic materials. If inhaled, chromate mists can eventually cause lung cancer. Health and safety considerations and the increasing cost of disposal of chromium-containing wastes have prompted users to evaluate alternative treatments. A number of alternatives exist, however, few provide the corrosion protection of chromate conversion coatings. Sulfuric acid anodizing can be substituted for some chromium conversion coatings although the coatings are more brittle and significantly thicker than those produced with chromate (Freeman 1995).
Alternatives for Chromating
Cobalt/molybdenum (Alodine 2000) is a developmental conversion coating process that was originally developed and patented by Boeing Aircraft Company. The process is being developed further by Parker+Amchem in preparation for commercial availability. The process uses an undisclosed proprietary formulation identified generally as cobalt- and molybdenum-based. The cobalt and molybdenum ions are much less hazardous than chromium and behave similarly. The coating does not have the ability to inhibit pitting corrosion as effectively as chromium, therefore, a second step is required to meet military specifications. The second step is an organic emulsion seal (Alodine 2000) that enhances corrosion resistance and paint adhesion characteristics. The process is estimated to cost two to three times more than conventional chromating. The process also requires an additional tank for the sealing process (Meyers 1994).
Gardolene VP 4683
A new chrome-free, post-rinse called Gardolene VP4683 has been developed for use on phosphated steel zinc, and aluminum surfaces prior to painting. The rinse contains only inorganic metallic compounds as the active ingredient with no hexavalent or trivalent chrome. The rinse is applied at temperatures up to 100 degrees Fahrenheit and at a slightly acidic pH. The manufacturer describes corrosion protection and paint adhesion as equal to that of hexavalent chrome (Finishers Management 1991).
Oxide Layer Growth in High-Temperature Deionized Water
The oxide layer growth system was developed and refined within the past decade. The process coating is applied in a series of steps, including an oxide layer growth step in boiling deionized water, to build a corrosion-resistant paint base on aluminum. Both immersion and steam spray methods are being developed. The process does not use any hazardous materials and is completely inorganic and non-toxic. Depleted bath and rinsewaters require limited treatment before discharge to a sanitary sewer. The process can withstand greater temperature exposure than the chromate conversion coating and is thin, yet abrasion resistant. The chemical solutions used to apply the coatings are very dilute, facilitating long solution life and simple monitoring and control (Meyers 1994).
The major drawback with the oxide layer growth process is cost. The process requires many additional steps involving numerous tanks of chemicals at elevated temperatures. Consequently, energy and capital costs increase substantially. While energy costs are offset by waste disposal reductions, this technology is estimated to cost up to ten times more than conventional chromating methods. A variation on the process involves spray application within a cabinet coater. This device is a chamber or series of conveyerized chambers. This method reduces some of the associated heating and chemical requirements and requires much less floor space (Meyers 1994).
Non-Chromate Passivation of Zinc
The Centre for Advanced Electroplating in Denmark has developed two different treatment methods, both based on passivation using molybdate and phosphate (referred to as MolyPhos) as alternatives to chromating. Chromated zinc often is used in the automotive, aerospace, and electronics industries. Platers can use MolyPhos for passivation of electroplated zinc instead of chromate. Depending upon the zinc substrate and the environment in which the workpiece will be placed, this method will function similarly to yellow chromate. MolyPhos performs well in outdoor exposure tests, adhesion tests, and CMT tests, but does not fair well in salt spray tests. The results of numerous corrosion tests are summarized in Table 18 (Tang 1993).
Another promising alternative is the SANCHEM-CC chromium-free aluminum pretreatment system. The following is a summary of the process:
In cases where maximum corrosion resistance for certain aluminum alloys is required, the process requires a fourth stage. The developers claim that this process closely matches the performance of a chromate conversion process (EPA 1994).
Zirconium oxide, an organic conversion coating, is an alternative to chromating for some applications. This coating is one of the only commercially available non-chromating treatments for aluminum. This process usually involves immersion of the substrate in an aqueous solution containing a polymeric material and a zirconium salt. The zirconium deposits on the surface in the form of zirconium oxide. These coatings have been used on aluminum cans for some time, but they have not been tested in the kinds of environments in which chromate conversion coatings are typically used. Wider application of this coating as a total replacement for chromating must be based on further testing (EPA 1994).
Other Chromate Conversion Coating Alternatives
Several additional processes might prove feasible in eliminating chromium from conversion coatings. These include SBAA and other emerging technologies.
SBAA, developed by Boeing, might prove valuable as a replacement for chromating on certain aircraft parts. The process imparts excellent paint adhesion and corrosion protection at a cost that is comparable to chromating. However, because SBAA is an anodic process, it might not be technically feasible to use it on all parts, especially parts with steel inserts or those having sharp edges, crevices, or areas that entrap fluids (Meyers 1994).
Several experimental and developmental technologies might lead to breakthroughs as replacements for chromium in conversion coatings. These technologies include hydrated alumina coating, hydrated metal salt coating, oxyanion analogs, potassium permanganate, rare earth metal salts (cerium), zirconium oxide/yttrium oxide in aqueous polymeric solution, and lithium-inhibited hydrotalcite coatings.
Regeneration of Chromating Solutions
Both ion exchange and electrochemical methods have been demonstrated as effective methods to regenerate spent chromates; however, in almost all cases, the metal finisher relies on a proprietary chemical supplier for the appropriate balance in the chromating bath. Either of these regenerating technologies make the metal finisher responsible for the overall chemical maintenance of all constituents in the bath. Proprietary suppliers might provide this service to further assist finishers in maintaining a proper balance when one of these techniques is used (Steward 1985).
Passivation is a process by which protective films are formed on metals through immersion in an acid solution. In stainless steel passivation, embedded ion particles are dissolved and a thin oxide coat is formed by immersion in nitric acid, which sometimes contains sodium dichromate. During forming, machining, tumbling, lapping, and other processing operations, iron particles can be embedded or smeared onto the surface of stainless steel. If these remain, the iron corrodes and gives the appearance of rust spots on the stainless steel. In order to prevent this, platers immerse the parts in a solution of nitric acid that sometimes contains oxidizing salts (generally sodium dichromate) depending on the alloy. Generally, 300-series stainless steel and chromium steel with 17 percent or more chromium are passivated in a solution of nitric acid. Series-400 stainless steel with less than 17 percent chromium is passivated in nitric acid and sodium dichromate (Ford 1994).
Chelant-based Solutions for Passivation
Nitric acid is a fuming, suffocating, and corrosive liquid. The acid's fumes are toxic and the liquid causes severe tissue burns. Because of these attributes, Cal-chem of South El Monte, California, has developed a safer substitute solution for use with passivation on steel. The solution contains chelants as opposed to nitric acid. Chelants provide an attractive alternative because they are non-toxic and biodegradable, however, platers need to be careful of this solution in waste treatment (Microcontamination 1993). For more information, refer to the first section of this chapter.
Phosphating is used to treat various metals (mainly steel and iron) to impart corrosion resistance and to promote the adhesion of finishes such as paint and lacquers. Phosphating treatments provide a coating of insoluble metal phosphate crystals that adhere strongly to the base metal. Generally, phosphating solutions are prepared from liquid concentrations containing one or more divalent metals, free phosphoric acid, and an accelerator (Ford 1994).
Iron and zinc phosphate coatings often are used as paint bases and manganese phosphate coatings are applied chiefly to ferrous parts for break-in and galling (e.g., engine parts). Other metallurgical uses for phosphate coatings are aiding in the forming of steel, wear resistance, and corrosion protection (with the addition of oils or waxes). The choice of iron or zinc phosphate coating depends on product specifications. In general, the more extensive multi-stage zinc phosphate processes provide better paint adhesion, corrosion protection, and rust protection than iron phosphate processes. Zinc phosphate baths, however, tend to be more expensive, require more maintenance, and often result in more sludge disposal (SME 1985).
The phosphating process consists of a series of application and rinse stages typically involving the application of either an iron, manganese, or zinc phosphate solution to a substrate. A simple iron phosphating system is comprised of two stages: an iron phosphate bath that both cleans the part and applies the conversion coating followed by a rinse bath to remove dissolved salts from the treated surface. An advanced zinc phosphating line might feature seven stages of spray/dip and rinse baths. In addition, a final seal rinse comprised of a low-concentrate acidic chromate or an organic non-chromate often is applied to further enhance corrosion resistance. Following the conversion application, the parts are dried to prevent flash rusting (Ford 1994).
Pollution Prevention in the Phosphating Process
Since the 1970s, a trend in the metal finishing industry toward reducing heating costs, improving working conditions, prolonging equipment life, reducing sludge, and reducing processing steps has resulted in low-temperature iron and zinc phosphate coatings and, to a limited degree, solvent phosphating solutions.
Regeneration of Phosphating Baths
Precipitates are formed continuously in phosphating operations presenting maintenance headaches. Often, this results in dumping of the solution. Usually, the precipitates accumulate in the tank, primarily on the heating coils. When the solution is removed from the tank, this accumulation of sludge can be manually removed. The solution should be decanted back into the tank to minimize waste because this uses space and time; this is rarely done. A more efficient system involves the use of a continuous recirculation system through a clarifier with gentle agitation in the sludge blanket zone. This allows for indefinite use of the solution and allows easy removal of dewatered sludge from the bottom of the clarifier (Steward 1985).
Aluminum finishing uses mainly anodizing and chromating treatments. Prior to anodizing or chromating, a workpiece usually proceeds through a cleaning and etching process. Both anodizing and chromating use similar cleaning schemes. If required, platers perform solvent or less toxic immersion cleaning. The cleaner used for aluminum workpieces should not contain sodium hydroxide because this chemical attacks aluminum. An example of an acceptable cleaning solution is borax (sodium tetraborate). Technical assistance providers should be aware that, even if a cleaner is made of a non-toxic substance, if the cleaning solution becomes contaminated with oils and lubricants, the spent solution can be classified as a hazardous waste (SME 1985).
The next step prior to finishing is etching. The surface is etched by an alkaline or acid-based etchant. Platers perform this etch step to thoroughly clean and prepare the surface for further treatment. The caustic etch used in many aluminum finishing lines and the chemical milling solution used for aircraft components can both be regenerated by crystallization and removal of sodium aluminate. However, the process must be carefully controlled and maintained. Many operations might not find this option economically feasibe because the technology is capital intensive (DOD 1993).
During the etching process, smut often is formed on the aluminum workpiece. The smut must be removed prior to further treatment. Usually this treatment is accomplished with a nitric or nitric/hydrofluoric acid dip, otherwise known as desmutting (Cushnie 1994). The desmutting process involves the use of large amounts of acids. Several products currently on the market use approximately 10 percent of the usual amount of nitric acid. Commonly, ferric nitrate is substituted as a desmutting agent in the presence of small amounts of nitric acid. The rate and desmutting action of ferric nitrate are comparable to the traditional 50-percent nitric acid bath (Ford 1994).
Occasionally, workpieces that have a metallic coating must be stripped to the base metal. Most immersion stripping is accomplished with cyanide, which does not attack steel substrates but dissolves many of the metals used as coatings. There are seemingly as many different stripping solutions as there are base metal/applied coating combinations. In general, they tend to be either acid-, alkali-, or cyanide-based. They can be chelated heavily or not at all. The most important property of these solutions (besides the ability to strip the coating) is that they must not attack the base metal. The use of cyanide-based metal strippers results in the generation of cyanide-contaminated solutions and a host of associated occupational health and hazardous waste compliance issues. These solutions require special treatment and disposal procedures. Interestingly, stripping can be a part of the finishing process, particularly for complex parts that only require plating on certain surfaces. The entire part is plated and then the areas where plating should remain are masked off and the entire part is immersed in the stripping solution to remove the undesired finish (Ford 1994).
As the baths used in stripping are similar to those used in plating, similar techniques of pollution prevention and waste minimization are applicable. Attention to cleanliness and process control are important in reducing stripping wastes. Stripping usually is accomplished either by chemical immersion or by electrolytic processes. Although mineral acids, suitably inhibited, are useful for stripping some coatings, they tend to attack the metal substrates and, therefore, are limited in their application.
Several non-cyanide alkaline immersion stripping baths are available to remove copper or nickel from various substrates. These baths typically use either the ammonium ion or an amine to provide complexing. Persulfate or chlorite anions can be used as well as proprietary formulations. The use of non-cyanide strippers eliminates cyanide from the spent stripper solution. In general, these non-cyanide strippers are less toxic than their cyanide-based counterparts and are more susceptible to biological and chemical degradation, resulting in simpler and less expensive treatment and disposal costs. In addition, the use of a non-cyanide stripper can simplify the removal of metals from spent solutions. These metals are difficult to remove from cyanide-based solutions because they form a strong complex with the cyanide ligand (EPA 1994).
Because non-cyanide stripping solutions are typically proprietary formulations, the detailed chemistry of coating removal is not available for most solutions. Stripping solutions are available for a wide variety of coating metal/base metal combinations and processing characteristics can vary widely (EPA 1994).
Non-cyanide strippers have been available for many years. The major drawbacks of this technology include lack of speed, etching of some substrates, and the need for electronic current. As the disposal costs for cyanide strippers increase, many companies have converted to non-cyanide stripping and have adjusted production cycles accordingly for the slower stripping speed (EPA 1994).
The wide variety of non-cyanide strippers makes it difficult to generalize about operating parameters. Some strippers are designed to operate at ambient bath temperatures whereas others are recommended for operating temperatures as high as 180 degrees Fahrenheit. Stripping processes range from acidic to basic. Bath life is longer because higher metal concentrations can be tolerated. In general, the same equipment can be used for cyanide-based and non-cyanide stripping, however, acidic solution tank liners might be needed to prevent corrosion (EPA 1994).
The impacts on costs when using non-cyanide strippers are:
Facilities should be aware that treatment costs might not change or might increase if cyanide is still used in other processes in the facility.
Hazards and Limitations
Non-cyanide metal strippers have some disadvantages. The stripping rates for some coatings might be lower than for comparable cyanide strippers. Some strippers can produce undesirable effects on substrate metals even if the stripper has been recommended by the manufacturer. Also, for some non-cyanide strippers, the recommended operating temperatures are high enough to cause safety concerns and reduced temperatures can lead to slower stripping times and reduced effectiveness (EPA 1994).
A major use for non-cyanide strippers is the removal of nickel coatings. Advances in non-cyanide alternatives have been spawned by the difficulty in treating nickel cyanide waste-streams. Opportunities for further improvement still remain though as the non-cyanide process is significantly slower than cyanide (8 hours versus 1 hour) (EPA 1994).
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