Metal Finishing Industry
Overview of the Metal Finishing Industry
These days everyone doing pollution prevention assistance seems interested in helping the metal finishing industry; ever wonder why? Metal finishing, when taken as a whole, is one of the largest users of many toxics chemicals in the country. Electroplating alone is the second largest end user of nickel and nickel compounds, and the third largest end user of cadmium and cadmium compounds. Electroplating also accounts for a substantial amount of chromium use in the United States. In other words, this industry is responsible for managing large amounts of hazardous materials (Davis 1994).
Many industries use metal finishing in their manufacturing processes including automotive, electronics, aerospace, hardware, jewelry, heavy equipment, appliances, tires, and telecommunications. Figure 1 shows the percent of markets served by metal finishers in 1992.
Figure 1. Markets Served by Metal FinishersPercent of 1992 Market (EPA 1995a)
Why is metal finishing so prevalent? Without metal finishing, products made from metals would last only a fraction of their present lifespan because of corrosion and wear. Finishing is also used to enhance electrical properties, to form and shape components, and to enhance the bonding of adhesives or organic coatings. Sometimes the finishes are used to meet consumer demand for a decorative appearance.
Overall, metal finishing alters the surface of metal products to enhance:
Metal finishers use a variety of materials and processes to clean, etch, and plate metallic and non-metallic surfaces to create a workpiece that has the desired surface characteristics. Electrolytic plating, electroless plating, and chemical and electrochemical conversion processes are typically used in the industry. Typical supporting processes can include degreasing, cleaning, pickling, etching, and/or polishing.
Some of the materials used in metal finishing are solvents and surfactants for cleaning, acids and bases for etching, and solutions of metal salts for plating the finish onto the substrate. Figure 2 presents an overview of the fabricated metal products manufacturing process and shows the types of emissions and wastes that are generated during production.
Figure 2. Overview of the Metal Fabricating Process (EPA 1995a)
The electroplating, plating, polishing, anodizing, and coloring industry is classified under the Standard Industrial Classification (SIC) code 3471 and includes establishments primarily engaged in all types of metal finishing. Companies that both manufacture and finish products are classified according to products they make. Nonetheless, they are still considered part of the metal finishing industry.
Firms that rely on one customer or that conduct metal finishing as part of a larger operation are referred to as captive shops. These companies tend to have larger operations than job shops. Independent facilities, often referred to as job shops, rely on a variety of customers and coat a variety of workpieces and substrates. In general, job shops tend to be small and independently owned. Enough similarities exist between the job and captive shops that they are essentially considered part of one industry. The job and captive shops use the same types of processes and fall within the same regulatory framework (EPA 1995a).
However, the barriers they face in deciding upon and implementing new technologies reflect the differences in their environmental performance and in the corporate capabilities of the two segments. Captive operations, which are more specialized, can focus their operations because they often work on a limited number of products and/or use a limited number of processes. Job shops, on the other hand, tend to be less focused in their operations because they can have many customers often with different requirements. In general, captive shops tend to have greater access to financial and organizational resources and, as a result, tend to be more proactive in their approach to environmental management. However, this is not always the case. The vastly different cultures in these shops greatly affects their perceived ability to implement pollution prevention (EPA 1994).
Job shops and captive shops do not ordinarily compete against each other because captive finishers seldom seek contract work. However, captive facilities might use job shops as subcontractors to perform tasks that their operations are unable to or that they choose not to do. As a nationwide trend, many manufacturers are choosing to eliminate or reduce metal finishing operations from their facilities because it is not of strategic importance for their long-term success. In some of these cases, the larger firms have shifted their plating activities to job shops (EPA 1995a).
Metal finishing comprises a broad range of processes that are practiced by most industries which manufacture metal parts. Typically, manufacturers perform the finishing after a metal part has been formed. Finishing can be any operation that alters the surface of a workpiece to achieve a certain property. Common metal finishes include paint, lacquer, ceramic coatings, and other surface treatments. This manual mainly addresses the plating and surface treatment processes.
The metal finishing industry generally categorizes plating operations as electroplating and electroless plating. Surface treatments consist of chemical and electrochemical conversion, case hardening, metallic coating, and chemical coating. The following sections briefly describe the major plating and surface treatment processes in order to provide a context for the more in-depth information in the chapters that follow.
Electroplating is achieved by passing an electric current through a solution containing dissolved metal ions and the metal object to be plated. The metal object serves as the cathode in an electrochemical cell, attracting ions from the solution. Ferrous and non-ferrous metal objects are plated with a variety of metals including aluminum, brass, bronze, cadmium, copper, chromium, gold, iron, lead, nickel, platinum, silver, tin, and zinc. The process is regulated by controlling a variety of parameters including voltage and amperage, temperature, residence times, and purity of bath solutions. Plating baths are almost always aqueous solutions, therefore, only those metals that can be reduced in aqueous solutions of their salts can be electrodeposited. The only major exception to this principle is aluminum, which can be plated from organic electrolytes (EPA 1995a).
Plating operations are typically batch operations in which metal objects are dipped into a series of baths containing various reagents for achieving the required surface characteristics. Operators can either carry the workpieces on racks or in barrels. Operators mount workpieces on racks that carry the part from bath to bath. Barrels rotate in the plating solution and hold smaller parts (Ford 1994).
The sequence of unit operations in an electroplating process is similar in both rack and barrel plating operations. A typical plating sequence involves various phases of cleaning, rinsing, stripping, and plating. Electroless plating uses similar steps but involves the deposition of metal on metallic or non-metallic surfaces without the use of external electrical energy (EPA 1995a).
Electroless plating is the chemical deposition of a metal coating onto an object using chemical reactions rather than electricity. The basic ingredients in an electroless plating solution are a source metal (usually a salt), a reducer, a complexing agent to hold the metal in solution, and various buffers and other chemicals designed to maintain bath stability and increase bath life. Copper and nickel electroless plating commonly are used for printed circuit boards (Freeman 1995).
Immersion plating is a similar process in that it uses a chemical reaction to apply the coating. However, the difference is that the reaction is caused by the metal substrate rather than by mixing two chemicals into the plating bath. This process produces a thin metal deposit by chemical displacement, commonly zinc or silver. Immersion plating baths are usually formulations of metal salts, alkalis, and complexing agents (e.g., lactic, glycolic, or malic acids salts). Electroless plating and immersion plating commonly generate more waste than other plating techniques, but individual facilities vary significantly in efficiency (Freeman 1995).
Chemical and electrical conversion treatments deposit a protective and/or decorative coating on a metal surface. Chemical and electrochemical conversion processes include phosphating, chromating, anodizing, passivation, and metal coloring. Phosphating prepares the surface for further treatment. In some instances, this process precedes painting. Chromating uses hexavalent chromium in a certain pH range to deposit a protective film on metal surfaces. Anodizing is an immersion process in which the workpiece is placed in a solution (usually containing metal salts or acids) where a reaction occurs to form an insoluble metal oxide. The reaction continues and forms a thin, non-porous layer that provides good corrosion resistance. Sometimes this process is used as a pretreatment for painting. Passivating also involves the immersion of the workpiece into an acid solution, usually nitric acid or nitric acid with sodium dichromate. The passivating process is used to prevent corrosion and extend the life of the product. Metal coloring involves chemically treating the workpiece to impart a decorative finish (EPA 1995a).
Other commonly used finishing technologies that do not fall into the plating or chemical and electrochemical conversion processes include cladding, case hardening, dip/galvanizing, electropolishing, and vapor deposition. The following sections provide brief overviews of these different processes.
Cladding is a mechanical process in which the metal coating is metallurgically bonded to the workpiece surface by combining heat and pressure. An example of cladding is a quarter. The copper inside is heated and pressed between two sheets of molten nickel alloy, bonding the materials. Cladding is used to deposit a thicker coating than electroplating, and requires less preparation and emits less waste. However, equipment costs are higher than electroplating (Freeman 1995).
Case hardening is a metallurgical process that modifies the surface of a metal. The process produces a hard surface (case) over a metal core that remains relatively soft. The case is wear-resistant and durable, while the core is left strong and pliable. In case hardening, a metal is heated and molded and then the temperature is quickly dropped to quench the workpiece. An example of a material made with case hardening is the Samurai sword. The hardened surface can be easily shaped, however, the sword remains pliable. This method has low waste generation and requires a low degree of preparation. Operating difficulty and equipment cost are approximately the same as for anodizing, although case hardening imparts improved toughness and wear (Freeman 1995).
Case hardening methodologies include carburizing, nitriding, micro-casing, and hardening using localized heating and quenching operations. Carburizing, the most widely used case hardening operation, involves diffusion of carbon into a steel surface at temperatures of 845 to 955 degrees Celsius, producing a hard case coating. Nitriding processes diffuse nascent nitrogen into a steel surface to produce case hardening. Nitriding uses either a nitrogenous gas, usually ammonia, or a liquid salt bath (typically consisting of 60 to 70 percent sodium salts, mainly sodium cyanide, and 30 to 40 percent potassium salts, mainly potassium cyanide). Carbon nitriding and cyaniding involves the diffusion of both carbon and nitrogen simultaneously into a steel surface.
Dip/galvanized coatings are applied primarily to iron and steel to protect the base metal from corroding. During the dipping process, the plater immerses the part in a molten bath commonly composed of zinc compounds. The metal part must be free of grease, oil, lubricants,and other surface contaminants prior to the coating process. Operating difficulty and equipment costs are low, which makes dipping an attractive coating process for most industrial applications. However, dipping does not always provide a high quality finish (Freeman 1995).
In electropolishing, the metal surface is anodically smoothed in a concentrated acid or alkaline solution. For this process, the parts are made anodic (reverse current), causing a film formation around the part that conforms to the macro-contours of the part. Because the film does not conform to the micro-roughness, the film is thinner over the micro-projections and thicker over the micro-depressions. Resistance to the current flow is lower at the micro-projections, causing a more rapid dissolution. Many different solutions are available for electropolishing depending on the substrate (Ford 1994).
Metallic coatings change the surface properties of the workpiece from those of the substrate to that of the metal being applied. This process allows the workpiece to become a composite material with properties that generally cannot be achieved by either material alone. The coating's function is usually as a durable, corrosion-resistant protective layer, while the core material provides a load-bearing function. Common coating materials include aluminum, coated lead, tin, zinc, and combinations of these metals.
Metallic coatings often are referred to as diffusion coatings because the base metal is brought into contact with the coating metal at elevated temperatures, allowing the two materials to interlace. These systems include various metallic spraying applications, cladding (application using mechanical techniques), hot dipping, vapor deposition, and vacuum coating. The main application for spray diffusion coatings is workpieces that are difficult to coat by other means because of their size, shape, or susceptibility to damage at high temperatures. Cladding uses a layer of metal that can be bonded to the workpiece using high-pressure welding or casting techniques. In some applications, cladding can be used as an alternative to plating. Hot dipping is another diffusion process that involves partial or complete immersion of the workpiece in a molten metal bath. The facility applies the coating metal in a powdered form at high temperatures (800 to 1,100 degrees Celsius) in a mixture with inert particles such as alumina or sand, and a halide activator. Vapor deposition and vacuum coating produce high-quality pure metallic layers, and can sometimes be used in place of plating processes (EPA 1995b).
In general, objects to be finished undergo three stages of processing, each of which involves moving the workpiece through a series of baths containing chemicals designed to complete certain steps in the process. The following list illustrates each of the three basic finishing stages and the steps typically associated with them:
Figure 3. Process for Surface Preparation for Electroplating (EPA 1995a)
Figure 4. Overview of the Metal Finishing Process (EPA 1994)
In each of these stages, opportunities for pollution prevention exist. For an overview of pollution prevention opportunities, refer to Figure 5 and Table 1. The two figures provide an overview of the different pollution prevention techniques/technologies that metal finishers can use and their place on the waste management hierarchy. Table 1 presents more detailed information on specific waste reduction techniques and an overview of the applications and limitations of each. The information provided in this table is consistent with the United States Environmental Protection Agency's (EPA) environmental protection hierarchy and their definition of pollution prevention.
Figure 5. Waste Minimization/Pollution Prevention Methods and Technologies (EPA 1995b)
EPA defines pollution prevention as any practice which reduces the amount of any hazardous substance, pollutant, or contaminant entering the wastestream or otherwise released to the environment (including fugitive emissions) prior to recycling, treatment, or disposal; and reduces the hazards to public health and the environment. Pollution prevention practices can include changes in the design, inputs, production, and delivery of a product including:
What we call pollution prevention often can be called something else in another profession. For instance:
Many waste minimization options, including process recovery and reuse as well as improved operating procedures, represent significant opportunities for waste reduction with relatively low investment costs. Similarly, such options as product replacement can represent the ultimate pollution prevention solution, however, the implementation of these options is largely driven by consumer preference and not favored by the industry (EPA 1995b).
Often, technical assistance providers can have greater success in getting companies to implement pollution prevention if they understand the nature of the industry. The following sections provide background on metal finishing, demographics, characterization, and motivations to assist in gaining that understanding.
There are an estimated 3,500 independently owned metal finishing shops, mostly small operations with limited capital and personnel (EPA 1994). A typical job shop is a small single establishment that employs 15 to 20 people, receives their workpieces from an outside source, and generates $800,000 to $1 million in annual gross revenues. Between 1982 and 1987, the number of smaller shops declined, while the number of larger metal finishers increased. This development appears to signal a trend toward smaller shops closing down and medium and larger shops incrementally increasing in number (EPA 1995a). Overall, however, there has been a sharp decline in the number of job shops in the United States. Moreover, there are an estimated 10,000 captive finishing operations in the United States that are not listed under SIC 3471.
Although geographically diverse, the metal finishing industry is concentrated in what are usually considered the heavily industrialized regions of the United States: the Northeast, Midwest, and California. This geographic concentration has occurred in part because small plating facilities locate near their customer base to be cost effective (EPA 1995a).
In describing the industry, EPA sometimes groups metal finishers into four categories or tiers with regard to their environmental performance. These groups each face different drivers and barriers in their environmental performance. The list below characterizes these categories and their most significant challenges. These challenges can affect a company's decision-making process. Understanding the various types of firms can help technical assistance providers determine the most effective way for different platers to implement pollution prevention.
u Tier 1: EPA characterizes Tier 1 companies as environmentally proactive firms that are actively pursuing and investing in strategic environmental management projects. These firms are in compliance with environmental regulations and are actively pursuing and investing capital in continuous improvement projects that go beyond compliance.
Some metal finishers (Tier 3 and some Tier 4 firms) might have a perverse incentive to operate, even in the face of disappearing profits, because of the potentially high environmental cleanup costs associated with shutting down and liquidating. These facilities, although operational, are not making any additional capital investments to improve environmental performance. Because they lack internal capital and cannot secure external financing to fund cleanups, these firms continue to perform poorly and represent a significant barrier to entry for more efficient firms that might have higher short-term costs (EPA 1994).
Assistance providers can use a number of pollution prevention motivators in discussions with company personnel. Using the information provided in the previous section, combined with the proper motivators, can help assistance providers develop successful strategies to sell pollution prevention to the facility management. Drivers for metal finishers seem to depend on the tier in which they are classified. The following list contains the drivers for different tiers:
The barriers that generally apply to some or all of the tiers are:
Davis, Gary A. et al. 1994. The Product Side of Pollution Prevention: Evaluating Potential Safe Substitutes. Cincinnati, Ohio: Risk Reduction Laboratory, Office of Research and Development.
EPA. 1995a. Profile of the Fabricated Metal Products Industry. Washington, DC: Office of Enforcement and Compliance Assurance.
EPA. 1995b. Metal Plating Waste Minimization. Arlington, VA: Waste Management Office, Office of Solid Waste.
EPA. 1994. Sustainable Industry: Promoting Strategic Environmental Protection in the Industrial Sector: Phase I Report Metal Finishing Industry. Washington, DC: Office of Policy, Planning and Evaluation.
Ford, Christopher J., and Sean Delaney. 1994. Metal Finishing Industry Module. Lowell, MA: Toxics Use Reduction Institute.
Freeman, Harry M. 1995. Industrial Pollution Prevention Handbook. New York, NY: McGraw-Hill, Inc.
Haveman, Mark. 1995. Profile of the Metal Finishing Industry. Minneapolis, MN: Waste Reduction Institute for Training and Applications Research.