Ever wondered how picture frames obtain that bright sheen? Or how your car manages that shiny look? Get a refresher on the basics of metal finishing.

From household fixtures to cars, virtually every piece of metal we see in everyday life has undergone some kind of surface finishing process. Surface finishing is very common because it fulfills three important functions. First, it makes metals look better; second, it deters rust or corrosion; and third, it boosts the metal’s strength and resistance to wear. Metals are finished for any or all of these reasons.

There are many kinds of surface finishing—such as painting, electroplating, electroless plating, anodizing, polishing, and powder coating. Painting is the most common type of surface finishing, encompassing lacquers, enamels, varnishes and stains. Many steel products, such as cars, are painted instead of plated.

However, bright automobile parts, such as bumpers, door handles, emblems, script, etc., are electroplated. Electroplating involves the coating of a metal object with a thin layer of metal through electricity. The most commonly deposited metals are gold, silver, chromium, copper, nickel, tin and zinc. The piece to be plated is typically a different metal, but it can be the same metal or even a non-metal, such as plastic. Other electroplated items include low-priced jewelry and kitchen and bathroom faucets.

In electroplating, the object to be plated is connected to a negative electrical lead and becomes negatively charged. It is immersed in a tank of solution containing the positively charged ions of the metal to be deposited. Because opposites attract, the positive metal ions get drawn to the negatively charged object. The ions thus coat the object, regaining their lost electrons and becoming metal again when they touch the piece. They continue to build up on the object’s surfaces for as long as the electrical current is on. Most electroplates are not thick, however. One one thousandth of an inch (0.001 inch) is considered pretty thick.

Electroless plating is another plating process. As its name suggests, however, it does not need electricity to work. The process is purely chemical and it proceeds by itself, once initiated. Certain chemicals, most notably electroless nickel, cause this reaction.

Electroless plating is employed for complex parts because deep recesses and holes often elude electroplating. Electroless plating, in contrast, plates anything that the solution wets. The chemical process is confined to certain applications only, however, because it generally costs more than electroplating and takes longer to deposit a given thickness of metal.

Aside from electrical and chemical processes, another important way to coat an object is vapor deposition. In this process, the metal to be deposited is heated and turned into a vapor inside a vacuum chamber. The part to be coated is placed in the chamber, where the metal vapor moves through the vacuum and condenses on the part’s surfaces.

Vapor deposition, also referred to as vacuum coating, is an inexpensive way to yield bright finishes. Shiny objects, such as picture frames and mirrors, are produced by this process.

Powder coating is another common surface finishing process. Just like electroplating, it relies on opposite charges being drawn to each other. In this process, the piece to be coated is electrically charged and then spray coated with a non-metallic powder that adheres to its surfaces. The object is then put inside an oven, which heats it up and causes the powder particles to melt and bleed together to produce a smooth finish. Powder coated objects include many lawn mower frames, sports equipment, playground equipment, lawn furniture and the interior of refrigerators, washing machines and dryers.

Conversion coating is another important process because it boosts a metal’s corrosion resistance. Unlike plating processes such as electroplating and electroless plating, conversion coating does not involve one metal coating another. Instead, a solution converts the surface from bare metal to a chemical combination of that metal and the solution. For example, in chromating, a solution containing chromic acid turns the surface of an object, say a zinc-plated part, to zinc chromate, which improves corrosion resistance. Examples of zinc-plated parts that have been chromated include nuts and bolts that you can buy from a hardware store.

Anodizing is a conversion coating process done mainly on aluminum. This process converts the surface from aluminum to aluminum oxide—a chemical combination of aluminum and oxygen. Not only is the aluminum oxide coating harder and more rust resistant than plain aluminum, it is also more versatile, able to assume most any color. It can even be made to look like bronze or brass.

Many items are made up of anodized aluminum—from pots and pans, to door and window frames. Anodized aluminum can also be found in exteriors, roofs, curtain walls, ceilings, floors and staircases in buildings throughout the world. It is also used in satellites because it can withstand the severe conditions of space.

Metal parts must also go through a cleaning process—which is generally performed as a preplate operation. Alkaline materials are often used to remove oils, dirt and rust from parts to be plated. Meanwhile, polishing or buffing ensures that surfaces are smooth. Electropolishing, for example, involves a tank of solution, which deplates high points in the immersed part when an electrical current is applied. The result is a smooth finish.

Vibratory finishing also accomplishes the same goal. Parts are placed inside a vibrator filled with small pebbles or other media and a cleaning solution. The vibrator then shakes to clean and smooth out the surfaces of the part.

Sources: Surface Finishing Basics
The American Electroplaters and Surface Finishers Society, Inc.
http://www.aesf.org/finishingbasics.html

Five Minute Guide to Plating
Products Finishing, Nov. 8, 2002
http://www.pfonline.com/articles/web110202.html

Anodizing…The Finish of Choice
Aluminum Anodizers Council
http://www.anodizing.org/what_is_anodizing.html