Metal Finishing Industry
Virtually all products that are finished require some pre-finishing procedure such as cleaning, stripping, or pickling. Without a properly cleaned surface, even the most expensive coating will fail to adhere or protect the workpiece. Depending on the process being performed, the degree of cleanliness will vary. Finishing operations such as phosphating, chromating, or electropolishing do not require as high a degree of cleanliness as electroplating or electroless plating. For example, it would be unreasonable to set up the same system for phosphating and electroplating operations because the cleaning prior to phosphating is not as critical to achieving a finish of high quality. In some cases, cleaners that contain acids or alkalis can actually reduce the quality of the finishing process. In other words, the cleaning process should match the finishing operation so that cleaning is performed in the most economical way to meet requirements (Innes 1993).
Technical assistance providers should help companies determine feasible options by examining the overall cleaning operations of a facility. Often workpieces in plating lines are cleaned several times using water, acids, caustics, and detergents. An analysis of an entire finishing process can identify redundant or unnecessary cleaning steps. The cleanliness requirements also should be evaluated to see if they are too stringent. Second, assess the technical feasibility of the alternatives. To determine the best alternative, ask the following questions:
Then, compare the economics of technically feasible options. When comparing the economics, consider capital outlay, process operating costs, permit requirements, waste disposal costs, labor costs, and energy costs. Many facilities have substantially reduced waste generation by implementing alternative cleaning systems. Finally, consider any potential new regulatory requirements that might be required if the new process is installed (NFESC 1995).
In order to properly assess the cleaning operation, four parameters must be analyzed: the substrate, the degree of cleanliness required, the nature of the soil, and incoming water quality.
The composition, physical properties, and chemistry of the base metal influence the selection of the cleaning procedure. Hardness, porosity, thermal coefficient of expansion, conductivity, melting point, specific heat, and the effect of hydrogen embrittlement must be considered. For instance, hardened steels and other metals, such as titanium, can become embrittled by hydrogen. For this reason, these metals must be handled appropriately to minimize embrittlement (Innes 1993).
The condition of the base metal is equally important. For example, a piece of metal with heat or welding scale requires more processing than nonoxidized cold rolled steel. The cleaning medium must be designed so that it is compatible with the metal being processed. A cleaning process that attacks the metal surface is undesirable. Therefore, select a process that does not attack the metal or does so in a desirable way (e.g., a satiny or frosty appearance might be desirable on aluminum substrates). Other problems can be encountered when working with metals that have low melting points (200 degrees Fahrenheit). Such alloys cannot be cleaned in boiling aqueous solutions. Other metals can be distorted or bent in heated solutions (Innes 1993).
Degree of Cleanliness Required
The degree of cleaning required varies depending on the particular surface treatment it will receive. For instance, parts plated with a cyanide-based solution usually do not need a high level of cleaning because cyanide-based plating solutions clean the part during the plating operation. For a nickel plate to adhere to a metal surface, however, the surface must be extremely clean. Because the plating bath does not contain cyanide, it does not clean the part. Therefore, thorough and rigorous cleaning operations are needed prior to plating (EPA 1990).
Nature of the Contamination
In order to properly design the cleaning system and sequence of baths or other operations, it is important to know the composition of the contaminants on the material surface. Generally, soils can be broken down into two categories: organic and inorganic.
Other soils are paints, cleaning residues, fingerprints, inorganic coatings, and rust preventatives.
The method and medium used for soil removal depends on the composition and condition of the soil as well as its physical and chemical properties. Often a cleaning procedure can be recommended based on the chemical properties of the soil providing that a chemical change has not occurred after application because of age or drying out. For instance, alkaline cleaners often are used to remove heavy soils and some solid oils while caustics are good stripping agents. Acid cleaners and abrasives are used mainly to remove oxidation and rust. When parts have been contaminated with several materials, sequencing of cleaning operations can be important. For instance, a layer of oily contamination might be removed by an alkaline cleaner before abrasives are used to remove a rust layer (EPA 1990).
The condition of the incoming water often is overlooked in metal cleaning. Hard water can decrease the effectiveness of a cleaning system. Water with a hardness exceeding 25 grains is definitely a problem and must be treated in order to operate most cleaning systems adequately. Many of the chemical additives that are used in cleaning can be neutralized by minerals found in tap water. Filtration and deionizing can be used to correct water quality problems (Innes 1993).
Four types of metal cleaning can be used by the metal finishing industry: solvents (both halogenated and nonhalogenated), alkaline cleaners, electrocleaning, and acid cleaners. Alkaline and acid cleaners are generally referred to as aqueous cleaners. Mixtures of solvents and alkalines frequently used. Mixtures where water-immiscible solvent is emulsified in water are termed emulsion cleaners (EPA 1990). Electrocleaning uses electrical current to clean the workpiece.
Frequently, no one cleaning operation can be specified as the best. Several cleaning methods often appear appropriate and only through experimentation can one be selected. Some cleaning techniques involve the application of organic solvents to degrease the surface of the workpiece. Other techniques such as emulsion cleaning use common organic solvents dispersed in an aqueous medium.
The cleaning process generally can be divided into three distinct phases: immersion, power spray, and electrocleaning. The purpose of immersion and spray cleaning is to remove the bulk of or all soil contamination prior to electrocleaning, phosphating, pickling, or chromating. In some cases, the spray cleaner also can be an activation process. In other cases, spray cleaners can be used individually or in conjunction with one another. In fact, in a number of cleaning operations, success ultimately depends upon the soak and spray combination (Innes 1993). An immersion or spray cleaner can fall into any of the following categories:
The most common form of cleaning in metal finishing operations is chlorinated solvent vapor degreasing and ambient-temperature solvent immersion cleaning. The Clean Air Act Amendments of 1990 required new standards for vapor degreasing. These new regulations are pushing metal finishers to investigate alternative cleaning methods or improve current operating practices. The University of Tennessee Center for Industrial Services has developed a manual to assist manufacturers in complying with the CAAA standards for vapor degreasing.
Traditionally, vapor degreasing using chlorinated solvents such as trichloroethylene (TCE) or perchloroethylene (PERC) has been used to remove oils, grease, and wax-based soils. Unlike other cleaning processes involving water, vapor degreasing does not require downstream drying because the solvent vaporizes from the workpiece over time. However, vaporization results in significant VOC emissions and solvent losses.
Conventional open-top vapor cleaners (OTVCs) use an open tank where solvent vapor is maintained. A perforated basket containing soiled parts is retained in this vapor layer for a few minutes and rotated to expose all part surfaces to the vapor. As vapor condenses on the parts, the soil is dissolved and carried away by condensate. When the parts reach the temperature of the vapor, no more condensate is generated and the parts are removed (Ford 1994).
Methods for Improving Solvent Vapor Degreaser Efficiency
Certain equipment-related and operational factors can reduce emissions from traditional OTVCs including:
With the phaseout of chlorofluorocarbon (CFC)-based cleaners, facilities have been investigating chemical, mechanical, and specialty alternatives. Chemical alternatives include replacement solvents for use in existing cold cleaning and vapor degreasing systems. In contrast, mechanical alternatives commonly require replacement of an entire process. Generally, higher initial costs are offset by a safer workplace and reduced operating costs. Specialty alternatives include processes such as plasma systems and supercritical cleaning. Table 7 lists several methods to reduce the use of chlorinated solvents.
Many alternatives to methylchlorofluoro-carbons (MCF) and CFC-113 are available for use in cold cleaning and vapor degreasing applications such as wipe cleaning, dip cleaning, immersion soaking, pressure washing, and vapor degreasing. Some solvents are recommended only for specific applications while others are used for many applications. In general, the following properties are desirable when considering solvent alternatives: low surface tension to penetrate small spaces, high density to remove small particles, high volatility to provide rapid drying, good solvency to readily improve organic soils, low cost, low toxicity, non-flammable, little residue, and easy cleanup and disposal (NFESC 1995).
Drop-in solvent replacement of traditional solvents such as MCF and CFC-113 usually is not possible. However, because vapor degreasing is effective at cleaning delicate parts, facilities might want to consider maintaining the process with a substitute solvent. Some possible CFC-free alternatives include:
Aqueous cleaning uses a solution of water, detergents, and acidic or alkaline chemicals to clean parts. These cleaners also are made up of builders, surfactants, inhibitors, and chelators. Most cleaners include a variety of ingredients, many of which are not needed for a firm's cleaning process and can actually cause problems with cleaning systems. Before a facility purchases any equipment for aqueous cleaning, it should first identify an acceptable aqueous cleaner. Some vendors will work with a facility to develop a cleaner tailored to their application. Below are some common additive types, what they do, and the advantages and disadvantages of each (FL DEP 1995).
The newest aqueous cleaners clean by subverting the soil. Non-emulsifying surfactants have a higher affinity for the substrate than the soil. The surfactant lifts the soil from the part without chemically reacting with it. Non-emulsifying cleaners work well in spray applications. If a settling tank and oil skimmer are added to the system, soils can be removed and the cleaning solution can be reused, sometimes indefinitely without contaminating the part. In an immersion tank, however, non-emulsified oils rise to the surface. Parts can be contaminated with oils as they are withdrawn from the tank. This may be desirable because the oil can act as a rust inhibitor, but is not acceptable in most situations.
Weak emulsifiers offer the best of both worlds. This type of cleaner will keep the oils in suspension as long as the solution is agitated, but the emulsion breaks when the agitation stops, either in a holding tank or when the system is shut down. The soils can be removed and the solution can be reused.
The three most common equipment configurations of conventional aqueous processes for metal finishing are: immersion with ultrasonic agitation, immersion with mechanical agitation, and spray washing. Aqueous immersion cleaning with mechanical agitation or ultrasonics and spray methods are being used most widely as substitutes for solvent cleaning. Aqueous cleaners are generally better than solvent cleaners at removing soils or particulate matter. However, when oils or greases are part of the contamination, other steps might be needed to provide adequate cleaning. The rinsing and drying steps are of particular concern, especially with parts that have complex geometries or that are susceptible to corrosion because water can remain on the part and cause flash rust.
Advantages of these systems include increased safety and flexibility, decreased material and waste disposal costs, and multiple cleaning mechanisms (chemical reaction, displacement, emulsification, dispersion, and others). Aqueous cleaning solutions can be tailored for specific parts and contaminants. When compared with solvents, aqueous systems generally have higher capital costs and require elaborate rinsing and drying procedures as well as tighter process controls for optimum cleaning (NFESC 1995).
Alkaline tumbling and hand-aqueous washing are most often used although automated processes are available. In alkaline tumbling, the soiled parts are placed in an open, tilted vessel and an aqueous cleaning solution is introduced. As the vessel rotates, the parts tumble over each other. The cleaning solution overflows and clean tap water is added to rinse the parts.
In the hand-aqueous process, the workload (perforated basket of parts) is dipped into a series of tanks containing (successively) surfactant solutions and rinsewater. A continuous clean water flow must be maintained through the final rinse tanks, but the surfactant and dragout tank contents can be used for an entire day without changing. Both aqueous processes require drying at the end before further surface finishing treatment (Freeman 1995).
Automated aqueous washers use a helical screw to transport soiled parts through the five compartments. The parts are sprayed successively with solutions from the holding tanks. The helical screw agitates the parts as it carries them forward. The automated washer is used mostly for parts small enough to be conveyed by the helical screw. For larger parts, such as engines and transmissions, power washers can be used. The part(s) are placed on a turntable for the automatically timed cleaning cycle. High-pressure, high-temperature water, usually containing a detergent, blasts the parts clean. Rotation on the table and the number and angle of the sprays enable the water to reach all surfaces (Freeman 1995).
Regeneration of Aqueous Cleaners
Some of the cleaners used in aqueous cleaning can be regenerated for further use. Alkaline cleaners, for example, often are regenerated with microfiltration. In microfiltration, the membrane is a physical barrier with a pore size of approximately 1 to 2 microns. The microfiltration membranes reject grease, oils, and dirt while allowing the cleaning solution to pass through. For a detailed discussion of microfiltration, see Pollution Prevention in Rinsing.
Wastewater Treatment from Aqueous Cleaning Systems
In many instances because of local, state, or federal regulations, wastewater from aqueous or semi-aqueous processes must be treated before discharge to a municipal wastewater treatment plant. Contaminants of concern include organic matter (grease and oil), metals (dissolved or in suspension), and alkaline cleaners, which raise the pH to an unacceptable level. Pretreatment technologies include gravity separators, ultrafiltration, chemical treatment, precipitation, and carbon adsorption. For more information on these technologies, refer to Pollution Prevention in the Plating Process.
Acid Aqueous Solutions
Acidic solutions effectively and rapidly remove rust, scale, and oxides from metal surfaces. The solutions actually etch the surface of the metal and can improve coating adhesion. Inhibitors are used to control the etching rate. However, acid solutions are classified as hazardous waste and can cause hydrogen embrittlement as hydrogen gas formed during surface etching penetrates the metal and reduces its strength (KSBEAP 1996).
Aqueous Alkaline Cleaning
In alkaline cleaning, the cleaning action relies mainly on the displacement of soils rather than the breakdown of the soil, as is the case with organic solvents. Most alkaline cleaning solutions are comprised of three major types of components: builders such as alkali hydroxides and carbonates, which make up the largest portion of the cleaner; organic and inorganic additives, which promote better cleaning or act to affect the metal surface in some way; and surfactants. Other additives can include antioxidants and stabilizers as well as a small amount of solvents.
Mild alkaline detergent solutions such as sodium hydroxide, sodium carbonate, sodium phosphate, sodium metasilicate, and borax are used to clean many substrates because no hydrogen embrittlement results from alkaline cleaning (IHWRIC 1992). Alkaline cleansers also remove rust, scale, and oxides from metal surfaces. In general, the stronger the solution, the faster it cleans. However, relatively mild solutions often are used to easily accomplish thorough rinsing (KSBEAP 1996). Aqueous processes apply to a wide range of products and are environmentally safer than chlorinated solvent processes. Some disadvantages of aqueous cleaning are its high water consumption rate and its hazardous wastewater discharges (Freeman 1995).
Alkaline cleaning often is assisted by mechanical action, ultrasonics, or by electrical potential (e.g., electrolytic cleaning). Alkaline cleaning also can be used for the removal of organic soils. Alkaline cleaners and strippers are used to remove soil from metal parts, old paint, and plating. These types of solutions are beginning to be used in acid cleaning as well.
Regeneration of Alkaline Solutions
Most cleaning formulations resist treatment because they are designed to keep dirt and oil in suspension. If the concentration of cleaning chemicals is high enough in an effluent, it can prevent efficient removal of the precipitated metals. Slugs of alkaline cleaner passing through treatment systems are well known to upset the systems.
While alkaline solutions are not currently regulated by wastewater programs, they can have a significant impact in wastewater systems. In certain cases, large finishing operations on small sewer systems or small receiving streams might have a problem meeting organic content requirements because of wetting agents and detergents. Cleaners are also important contributors to a facility's total dissolved solids load in their effluent.
This method uses high-frequency sound waves to improve the cleaning efficiency of aqueous and semi-aqueous cleaners. By generating zones of high and low pressure in the liquid, the sound waves create microscopic vacuum bubbles that implode when the sound wave moves and the zone changes from negative to positive pressure. This process, called cavitation, exerts enormous localized pressures (approximately 10,000 pounds per square inch) and temperatures (approximately 20,000 degrees Fahrenheit on a microscopic scale) that loosen contaminants and actually scrub the workpiece, especially in hard-to-reach areas. Ultrasonic cleaning has allowed aqueous/non-chlorinated degreasing to be practiced in applications where solvents had been the only effective degreasing tool. Ultrasonic cleaning can be used on ceramics, aluminum, plastic, and glass as well as electronic parts, wire, cables, rods, and detailed items that might be difficult to clean by other processes (Freeman 1995). Ultrasonic cleaning can be used to increase the efficiency of virtually any immersion cleaning process.
Semi-aqueous cleaning, a combination of a non-aqueous cleaner with an aqueous rinse, is used frequently in metal cleaning, especially where aqueous methods alone are ineffective on heavy grease, tar, and soils. However, precision and electronics applications are limited. Primary concerns generally focus on the properties of the non-aqueous cleaners: volatility, flammability (especially in heated applications), exposure risks, residues requiring rinsing, and high costs of disposal. A nitrogen blanket can reduce the risk of combustion. Hydrocarbon and surfactant mixtures, alcohol blends, terpenes, and petroleum distillates are solutions available for semi-aqueous cleaning. Advantages include compatibility with most metals, plastics, and rust inhibitors. Other benefits include potential decreases in solvent purchases, decreases in evaporative losses, and reduction in metals entering the waste stream because of the non-alkaline nature of the cleaner.
Electrocleaners are basically heavy-duty alkaline types and are always used with an electric current either reverse, direct, or periodic reverse. These systems are designed for soil removal and metal activation. These solutions are heavily alkaline and often heated. Typically, an initial cleaning step precedes this operation, although electrocleaning alone will suffice. In electrocleaning, the work is immersed in the solution and current is applied. When water is electrolyzed by electric current, the following reaction occurs:
H2O + H2 +ŻO2
The objective of electrocleaning is to remove all soils and activate the metal surface. Activation is usually obtained by using reverse-current electrocleaning. The gas scrubbing of the oxygen assists in the removal of soils while the reverse current aids in soil removal and prevents the deposition of any metallic film or non-adherent metallic particles. A dilute mineral acid dip usually follows the final cleaner to neutralize the alkaline film on the metal surface.
Reverse or Anodic Cleaning
In reverse cleaning, the workpiece is made the anode. In this case, oxygen gas is evolved at the surface of the piece and assists in oil and dirt removal. Because of the reversed current, any metallics in the bath cannot deposit on the piece, making this method of cleaning preferable to others (Ford 1994). In this process, the current density, temperature, and concentration, particularly on non-ferrous materials, must be controlled to avoid etching and tarnishing. This type of cleaning is not recommended for use on aluminum, chromium, tin, lead, or other metals that are soluble in alkaline electrocleaners (Innes 1993).
Direct or Cathodic Cleaning
If the workpiece is the cathode, the cleaning is considered to be direct. In this case, hydrogen gas is liberated at the surface of the workpiece. The volume of hydrogen liberated is twice that of oxygen liberated at the anode for a given current density. Therefore, more gas scrubbing is achieved at the cathode than at the anode. For this reason, cathodic cleaning is sometimes used as a precleaner followed by anodic cleaning (Innes 1993). However, any positively charged particles (especially metallics) in the solution will tend to adhere to the workpiece forming a smut. This type of cleaning is generally used when reverse cleaning is harmful to the work (Ford 1994).
Any workpiece that is subject to hydrogen embrittlement should not be cleaned with this method unless adequate steps are taken after processing to remove the hydrogen. Generally, heat treatment for 1 hour at 400 degrees Fahrenheit immediately after processing will remove the embrittling effect of hydrogen.
Cathodic cleaning is used for the following applications:
Periodic reverse cleaning alternately makes the workpiece anodic and cathodic. This cleaning technique is used in oxide removal where acidic processes are harmful to the base material (Ford 1994). This method of cleaning generally is used to remove smut, oxide, and scale from ferrous metals. One of the advantages of this method is the elimination of acid on certain types of work (hinges) where entrapment of acid aggravates bleed-out during and after alkaline plating (brass, copper, zinc, cadmium, and tin). Oxides also can be removed without the danger of etching or the development of smut usually encountered from acid pickling (Innes 1993).
Solutions used in these types of cleaning operations often are sent off site for disposal because the chemical nature of the surfactants, wetting agents, inhibitors, and wetting compounds is such that they directly interfere with waste treatment operations.
Acid cleaning, or pickling, often is used to remove contaminants from the workpiece using an acid. Acid pickling is used to remove oxides (rust), scale, or tarnish as well as to neutralize any base remaining on the parts. Acid pickling uses aqueous solutions of sulfuric, hydrochloric, phosphoric, and/or nitric acids. For instance, most carbon steel is pickled in sulfuric or hydrochloric acids although hydrochloric acid can embrittle certain types of steel and is used only in specific applications. In the pickling process, the workpiece generally passes from the pickling bath through a series of rinses and then onto plating. Acid pickling is similar to acid cleaning, but is more commonly used to remove the scale from semi-finished mill products whereas acid cleaning is usually used for near-final preparation of metal surfaces prior to finishing.
Copper and Alloys
Straight electrolytic recovery as described earlier is highly effective on many copper pickling and milling solutions including sulfuric acid, cupric chloride, and ammonium chloride solutions. Solutions based on hydrogen peroxide generally are regenerated best by crystallization and removal of copper sulfate with the crystals sold as byproducts or redissolved for further treatment by electrolytic metal recovery (Steward 1985).
Highly concentrated bright dipping nitric/sulfuric acids are a difficult challenge for regeneration because of the small quantities (5 to 25 gallons) used and the high dragout losses. Regeneration is possible by distillation of nitric acid and removal of copper salts, however, the economics are usually not favorable.
Sulfuric and Hydrochloric Acid
Both sulfuric and hydrochloric acids are used commonly for cleaning steel. Sulfuric acid can be regenerated by crystallizing ferrous sulfate. Hydrochloric acid can be recovered by distilling off the acid and leaving behind iron oxide. These techniques have been used for many years in large facilities. The economics of these processes, however, usually are not favorable for smaller facilities (Steward 1985).
Waste pickle liquors from these operations often can be of use to sanitary waste treatment systems for phosphate control and sludge conditioning. Some industrial firms can use spent process waste from pickling operation. Iron in the waste is used as a coagulant in wastewater treatment systems (Steward 1985).
A vacuum furnace uses heat and a vacuum to vaporize oils from parts. Vacuum furnace de-oiling can be applied where vapor degreasing typically is used to clean metal parts. It also can remove oil from nonmetallic parts. Although capital costs for vacuum de-oiling are high, the operating costs are low. Unlike other clean technologies, vacuum de-oiling does not leave the cleaned parts water-soaked so they do not need to be dried. Because the time and temperature of the de-oiling process depends on the material to be cleaned and the oil to be removed, adjustments might be needed for each new material, oil, or combination. Also, the parts must be able to withstand the required temperature and vacuum pressure (Freeman 1995).
In this method, short pulses of high-peak-power laser radiation are used to rapidly heat and vaporize thin layers of material surfaces that form a dense cloud of hot vapors that will condense and recontaminate the surface if not removed immediately. Ablation must be carried out in an inert gas environment to avoid further contamination. Laser ablation can do localized cleaning in a small area without affecting the entire part. Laser ablation meets waste minimization goals with no solvents or even aqueous solutions needed. The only waste is the small amount of material removed from the surface of the item being cleaned (Freeman 1995). The use of laser ablation to clean metal surfaces is being explored by Sandia National Laboratories.
This method uses clean, dry, inert gas or air that is fed to a pressurized gas gun to physically remove the contaminant from the substrate surface. Advantages of this process are low capital cost and the fact that nonflammable gases are generally used. However, this technology might not be effective in removing all soils, and it might damage the substrate (Freeman 1995).
Supercritical Fluid Cleaning
This process involves the application of supercritical fluids at temperatures and pressures above their critical point to remove contaminants from parts. Carbon dioxide (CO2) is the most commonly used fluid in this process because it is widely available and considered to be environmentally sound. Supercritical fluid cleaning is compatible with stainless steel, copper, silver, porous metals, and silica. It leaves no solvent residue after cleaning and has low operating costs. However, capital costs are very high (Freeman 1995).
This method uses an electrically charged gas containing ionized atoms, electrons, highly reactive free radicals, and electrically neutral species to remove soils. Plasmas can be used in a wide range of temperatures and pressures. The advantages of this process include low operating costs and lessened disposal costs. However, initial capital costs can be high (Freeman 1995).
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Florida Department of Environmental Protection (FL DEP). 1995. Fact Sheet: Aqueous Cleaner Additives for Industrial Cleaning. Jacksonville, FL: Florida Department of Environmental Protection.
Ford, Christopher J., and Sean Delaney. 1994. Metal Finishing Industry Module. Lowell, MA: Toxics Use Reduction Institute.
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Kansas Small Business Environmental Assistance Program (KSBEAP). 1996. Environmentally Conscious Painting. Wichita, KS: Kansas Small Business Environmental Assistance Program.
Naval Facilities Engineering Service Center (NFESC) 1995. ODS-Free Metal Cleaning Overview. Department of Defense Pollution Prevention Technical Library. (Downloaded from Envirosense web site: http://www.es.inel.gov).
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