logo Pollution Prevention in Metal Finishing: Plating
A Northwest Industry Roundtable Report

Appendix B: Metal Plating Process Description

This section provides a brief overview of the electroplating process. While other metal plating processes are under development or limited in use, electroplating remains the predominate method for metal plating in the Northwest and throughout the United States. The information in this section is based on the Metal Finishing Industry Market Survey by the Metal Finishing Suppliers Association and the National Association of Metal Finishers; and the report, Profile of the Metal Finishing Industry, written by the Waste Reduction Institute for Training and Applications Research.

Metal finishing involves a series of processes that provide the surfaces of manufactured parts with a number of desirable physical, chemical, and appearance qualities. Nearly all manufactured or fabricated products made of metal or having metal components feature some type of metal finishing. There are 46 different processes regulated under metal finishing standards featuring different technologies, operational steps, inputs, and outputs.

A significant amount of metal finishing is found within companies that manufacture products rather than those that specialize in metal finishing. These are referred to as "captive" operations. However, a great deal of metal finishing is contracted to independent establishments, called "job shops." The existence of the job shop finishing industry can be understood by looking at the relationship of metal finishing to the rest of the manufacturing process. Metal finishing is generally the last operation before sale or assembly. It can require capital intensive operations but may have a minor financial impact on the overall value-added of the product. Metal finishing is also chemical intensive, generates waste streams that are expensive to treat, and is heavily impacted by environmental regulations. As a result of these characteristics, many firms decide to outsource their metal finishing to job shops.

Like many other industries, quality, low price, and delivery time are three important competitive issues for metal finishing companies in the Northwest. In fact, it may be one of the most price competitive industries in existence. This may be because there are many firms providing metal finishing, and the service itself is relatively undifferentiated. As a result, manufacturers can aggressively pursue the best price. The differentiation that does exist in metal finishing in some circumstances can be overcome by relatively small investments in different plating processes and chemistries. In addition, international competition is on the rise. As more companies fabricate and assemble outside the U.S. to take advantage of cheaper labor, more metal finishing will be done overseas.

The result of all of these factors combined makes for a highly price-competitive industry. A finisher who incorporates a new, capital intensive pollution prevention technology may be forced to raise his prices to cover the costs, and price himself out of a contract or out of business entirely.

Metal finishing can be divided into four main categories: metal deposition technologies (the application of a metal coating onto a metal part, referred to as "plating"); organic finishing technologies (the application of paint and related materials onto a metal part); conversion technologies (finishing methods in which the "plated" materials interact with and physically change the make-up of the metal part); and removal technologies (subtractive processes that involve the removal of metal from the metal part either through physical action or chemical reaction). The roundtable discussions and this report focus on metal deposition technologies (i.e. plating).

Electroplating is used to favorably change the surface properties of a metal part by adding some type of metal coating to the part. Electroplating occurs when the part is placed in a chemical bath containing the desired metal ions and electrical current is passed through the bath. Examples of benefits of electroplating include making a part more durable or appealing in appearance.

While the exact process used at any one shop is very site specific, the simple, generalized process flow diagram is shown step by step below and is representative of the process used in most shops:

    1. Part Cleaning
    2. Post-Clean Rinse
    3. Acid Dip
    4. Post-Acid Dip Rinse
    5. Part Plating
    6. Post-plating Rinse
    7. Part Drying
Each individual step actually includes some type of tank or series of tanks that the parts are dipped or submerged in for a set period of time. These tanks are commonly referred to in the industry as "baths."

The two most common types of plating are barrel and rack plating. In barrel plating, the parts are loaded into a perforated container that is rotated in the chemical bath. Barrel plating is typically used for small, high-volume production parts, such as nuts and bolts. In rack plating, parts are clamped onto racks that are then dipped into the chemical baths. Rack plating is used for parts that are larger, fragile or complex in geometry.

Whether the barrel or rack plating process is used, parts go through two major steps surface preparation and surface treatment as shown in Figure 1. These two steps can be broken down as follows:

After basic plating operations are completed, a variety of post-treatment processes can be used on the part to further enhance the appearance or improve a property of the part. One example of a post-treatment process is heat treatment, which is used to optimize the hardness of a part.

Besides the core process described above, two ancillary processes are found in almost every metal plating shop metal stripping and wastewater treatment. Metal stripping is used to rework parts that were improperly plated or did not meet specifications. Wastewater treatment is used to prepare rinsewaters and other process solutions for legal discharge, usually under the limitations of a discharge permit issued by a state or local government.


Continue to Appendix C: Primary Chemicals Used in Metal Plating Processes and Waste Quantities.

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This report was developed with grant funding from the U.S. Environmental Protection Agency, and was a joint project of the Business Assistance Programs in Alaska, Idaho, Oregon and Washington.

© 1999, Pacific Northwest Pollution Prevention Resource Center
phone: 206-352-2050, e-mail: office@pprc.org, web: www.pprc.org