Metal Finishing Industry

Table of Contents  Overview  Regulatory  Planning P2 Programs   Common P2 Practices  Pre-Finishing Operations  P2 in Plating 
P2 in Rinsing  Alternative Methods of Metal Deposition  Facility Design

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.

General Pollution Prevention Techniques for Plating Solutions

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.

General Housekeeping

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.

Monitoring Bath Composition/Chemistry

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).

Additives for Plating Baths

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.

Wetting Agents

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).

Equipment Modifications to Prevent Pollution

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.

Purified Water

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.

Ventilation/Exhaust Systems

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).

Chemical Substitution

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.

On-Site Recycling and Recovery

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.

Figure 6. Multiple Reuse of Rinsewater (EPA 1992)

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:

• Metal precipitation: Non-chelated caustic solutions can be used to precipitate metals. However, cleaning solutions that contain detergents, surfactants, and high concentrations of wetting agents tend to destroy the flocculating/settling ability of the precipitated metal (Ford 1994).

• Chrome treatment: Solutions can act as pH adjusters in a precipitation tank (precipitation is discussed in the next section). For example, acid solutions can be used to adjust the pH in chromium reduction treatment. However, because these solutions typically have a high metal content, they should not be used for final pH adjustments. As with reusing spent rinsewater, facilities should check to make sure that the spent process bath solutions are compatible before they are used (EPA 1992).

• Chelated metals treatment: Spent acids, particularly those high in iron content such as ferric chloride etchants and steel and iron pickling solutions, are particularly desirable for pH adjustment (Ford 1994).

• Cyanide treatment: Non-chelated caustic solutions can be used to raise the pH in the first stage of cyanide treatment. In the second stage, the pH can be lowered with spent acid, but care must be taken not to use spent acids that contain nickel or iron as these metals form complexes with cyanide that are extremely refractory to alkaline chlorination (Ford 1994).

Waste Segregation

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).

Table 8. Precipitators for Common Plating Solutions (ASM 1982)

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).

Cyanide-Based Plating Processes

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).

Overview of Non-Cyanide Substitutes

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:

• Often, several non-cyanide solutions replace the single cyanide line.
• Process controls and cleaning practices must be maintained within tighter limits.
• Without the complexing ability of cyanide, periodic removal of iron and other potential contaminants might be required to ensure deposit quality. Filtration generally is necessary when using non-cyanide processes.
• The color shades obtained in chromating non-cyanide deposits do not always match those obtained with the same color chromates over cyanide deposits. Customers should be notified when segregating products with color shade differences is important.
• Some non-cyanide processes do not satisfactorily adhere to all surfaces and tend to become brittle at high temperatures.
• Alkaline non-cyanide processes generally provide more ductile deposits for subse- quent forming operations than do acid non-cyanide processes.
• In both acid and alkaline non-cyanide processes, higher levels of organic or non-organic brightening agents are required to achieve a more cosmetically appealing finish. However, residue left on the workpiece can cause problems in future finishing processes such as chromating.
• Acid substitutes require an appropriate liner such as plastic (TURI 1994).

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

Common Uses

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).

Bath Content

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.

Brass Pyrophosphate

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).

Brass Pyrophosphate-Tartare

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

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

Electrocoating

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 Plating

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).

Common Uses

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:

• Zinc substitutes exhibit improved corrosion resistance
• Zinc-nickel alloys have better wear resistance
• Zinc-cobalt deposits show good resistance to atmospheres containing sodium dioxide

Some of the limitations of cadmium alternatives include:

• Increased electrical contact resistance for zinc coatings
• Reduced lubricity
• Decreased throwing power
• Decreased corrosion resistance in marine environments

Table 9. Alternatives to Cadmium Cyanide—Product Quality Issues (TURI 1994)

Table 10. Alternatives to Cadmium Cyanide—Process Issues (TURI 1994)

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).

Zinc Alloys

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.

Zinc-Nickel Alkaline

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

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

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).

Tin-Zinc Alloy

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

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

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

Common Uses

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:

• Greatly reduces safety risks to workers
• Can reduce the costs and complexity of treating spent plating solutions
• Poses no risk of hydrogen cyanide (HCN) evolution from dragout to an acidic bath
• Can increase plating speed
• Eliminates a listed hazardous wastestream
• Eliminates or reduces Toxics Release Inventory (TRI) reporting requirements
• Can reduce sludge generation because of reduced metal concentrations
• Might not require treatment for carbonates in the plating solution (EPA 1994)

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.

Reported Applications

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).

Limitations

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.

Table 11. Overview of Alternatives for Copper Cyanide Plating (TURI 1994)

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).

Copper Fluoroborate

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

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.

Ion Exchange

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).

Electrodialysis

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).

Electrolytic Recovery

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).

Reverse Osmosis

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 Strike

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

Copper Pyrophosphate

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

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).

Precious Metals

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).

Gold Plating

Common Uses

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:

• Class A: Decorative 24K gold flash (2 to 4 millionths thick) plated in rack and barrel operations
• Class B: Decorative gold alloy flash (2 to 4 millionths thick) plated in rack and barrel operations
• Class C: Decorative gold alloy flash (20 to 40 millionths thick) plated in rack operations
• Class D: Industrial/electronic high-purity soft gold plated in rack, barrel, and selective plating operations
• Class E: Industrial/electronic hard, bright, and heavy 99.5 percent gold (20 to 200 millionths thick) plated in rack, barrel, and selective plating operations
• Class F: Industrial/electronic gold alloy heavy (20 to 400 millionths thick) plated in rack and selective plating operations
• Class G: Refinishing, repair, and general; deposits are pure bright alloys (5 to 40 millionths thick) plated in rack and selective plating operations
• Class H: Miscellaneous including electro forming of gold and gold alloys, statuary, and architectural applications (Weisberg 1993)

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.

• Alkaline gold cyanide: Class A, B, C, D, occasionally F, G, and H
• Neutral gold cyanide: This is usually used to achieve high-purity gold plated for Class D and G.
• Acid gold cyanide: This is used for bright, hard gold and gold alloy plating. It is used occasionally for Class B, C, E, F, and G.
• Non-cyanide gold: Occasionally, it is used for Class A, B, C, D, F, G, and H (Weisberg 1993).

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.

Table 12. Overview of Alternatives for Gold Cyanide Plating (Braun Intertec 1992)

*Little information on this solution is available

Gold Sulfite

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

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.

Silver

Common Uses

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.

Table 13. Overview of Alternatives for Silver Cyanide Plating (Braun Intertec 1992)

*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).

Silver Ammonium

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:

• Alternative is cost prohibitive: The facility estimated that the cost of operation for the non-cyanide system is three to four times more expensive than a conventional silver cyanide system.
• Difficult-to-polish surface: The non- cyanide silver plating process produces a surface that is difficult to polish and not as bright as conventional silver cyanide depos- its. Customer specifications require that the facility's silver-plated products are bright.

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:

• Greater than 99 percent of the silver cyanide can be removed from electroplat- ing rinsewater using a strong base anion exchange resin.
• Ion exchange resins can be effectively regenerated using sodium thiocyanate at a dosage of 40 to 50 pounds per cubic foot of resin.
• Silver can be completely recovered from spent thiocyanate regenerant using electrowinning. Thiocyanate destruction during electrowinning is fairly minimal so that regenerant can be reconstituted for reuse. Electrowinning also can be used to remove 67 percent of the copper contamination.
• Budgetary estimates for a 2 gallons-per- minute system indicate a 3- to 4- year payback is feasible (Lindstedt 1992).

For more information on ion exchange and electrowinning, refer to the recovery/recycling section in Pollution Prevention in Rinsing.

Polymer Filtration

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.

Zinc Plating

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).

Common Uses

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).

Table 14. Alternatives to Zinc Cyanide—Product Quality Issues (TURI 1994)

Table 15. Alternatives to Zinc Cyanide—Process Issues (TURI 1994)