A used piece of trim being re-plated such as this grille divider from a 1957 Chevrolet requires stripping of the existing chrome, the existing nickel and, if present, the existing copper. Then, any rust must be removed, dents straightened and pits and scratches polished out.
Most parts on an automobile have some type of finish. Those that don’t include castings of iron or aluminum, such as steering boxes and alternator cases and forged steel pieces, such as steering arms and roll bars. This article will cover typical finishes except paint, including: electroplating, passivation, anodizing and polishing. Most of these finishes are used on mild steel, which comprise the bulk of automotive parts.
Electroplating
Electroplating is used on many different parts and is used to apply chrome, nickel, copper, cadmium, zinc and terne (a mixture of four parts lead to one part tin). All electroplating requires a clean, smooth surface. The piece to be plated is negatively charged and the metal ions to be deposited on the subject piece, which are contained in an electrolyte (called a bath), are positively charged. An electrical DC current is passed through the bath, causing the metal ions to be deposited on the part being plated.
Chrome plating
Various automotive parts of steel and zinc die-cast are chrome plated. Of all the metal finishes, chrome plating is arguably the most misunderstood. Most people associate it with decorative chrome plating, such as that used on car bumpers (until they were replaced by plastic covers). However, there is also hard chrome plating that is used for wear resistance on some engine parts. It is called hard chrome, because it is several thousandths of an inch thick and can be measured for hardness. At the opposite end of the spectrum is decorative chrome, which is only millionths of an inch thick and would easily break if subjected to a hardness test. This article will concentrate on decorative chrome plating only.
The subject of chrome plating is filled with many buzzwords that are essentially meaningless — “show chrome,” “triple chrome plating,” “double-nickel chrome,” “tri-chrome” and more. Read the platers’ ads and probably more will surface. The following will hopefully demystify this process and provide guidance to ensure a successful result.
Decorative chrome plating requires polishing of the piece to be plated, cleaning (chemically and electrically), activating the surface, plating copper (may be omitted), plating two or three coats of nickel and then plating a final layer of chrome. Between each step, the piece must be thoroughly rinsed so it doesn’t contaminate the succeeding plating bath. Cross contamination of the electrolytes will lead to peeling of the plating and a white-colored corrosion on the plated piece when exposed to the elements.
New metal pieces have minimal preparation as compared to restoration of a part for a collector car. A used piece being re-plated requires stripping of the existing chrome, the existing nickel and, if present, the existing copper. Then, any rust must be removed, dents straightened and pits and scratches polished out. Next, a layer of copper is deposited and this layer buffed until a smooth surface is obtained. Sometimes, this step must be repeated. These restoration steps are essential to successful re-chroming of a part. If these restoration steps are not carefully done, the original part dimensions will be changed and adjoining parts may not fit or details, such as those in zinc die-cast parts, will be obliterated. Some parts, such as chrome air cleaners, rocker arm covers, etc., are chrome plated without any copper, particular when new.
After the part is prepared, it is ready for final steps of chrome plating. First, a layer of semi-bright nickel is applied, followed by a coating of bright nickel. Two coatings of nickel are used to provide more durability to the finish. The bright nickel is anodic to the semi-bright nickel. It sacrificially protects it by spreading any corrosion forces laterally instead of allowing them to penetrate through to the steel, leading to rust. Any rust showing on chrome is the underlying steel rusting because the overlying coatings have failed. The nickel layers are then topped with chrome. Typically, once the chrome plate step is complete the work is done. However, sometimes it is necessary to buff the part to remove any excess in high-current density areas, which is the function of how the part is connected to the electric current. Special connectors must often be fabricated to properly disperse the electrical current uniformly through the part.
Chrome plating is a labor-intensive process, which is the principal reason it is so expensive. There are also other factors. Chrome plating uses many hazardous chemicals and complying with environmental regulations has increased production costs. Finally, in recent years, there has been a run-up in commodity prices for nickel and copper, greatly increasing the cost of the platers’ supplies.
Other electroplated metals
Other commonly applied metals in automotive use include cadmium, zinc and terne.
Terne, a mixture of four parts lead and one part tin, was used for many years by General Motors to plate hydraulic tubing and gas tanks.
Cadmium is distinguished by its whitish hue. It was the preferred silver-appearing finish applied to small parts and fasteners in the automotive industry until the early 1970s. Then, environmental regulations were adopted that significantly increased the cost of cadmium plating to control the hazardous chemicals involved. Pieces such as relay covers, regulator bases, washer bottle holders and the like — plus fasteners on cars before the mid-1970s — are probably cadmium plated because of cadmium’s superior (compared to zinc) durability.
Zinc plating was originally reserved for larger pieces, such as brake backing plates. It is distinguished by a bluish hue. Once the cost of cadmium increased, zinc plating was also used for small silver-colored parts and fasteners. Most silver-colored parts available today, either for new cars or reproduction parts, are zinc-plated. Often, these zinc-plated parts are further treated by passivation.
Each of these metals is applied in a layer of only a few thousandths of an inch thick or less. The coating is essentially transparent. Therefore, the degree of shine to the plated piece is determined by the surface preparation of the substrate material.
For example, if a restored relay cover or fastener is media blasted and not polished, the resulting finish will have a matte appearance. As with chrome plating, the part to be plated must be thoroughly cleaned and activated before it is placed in the plating bath.
William C. “Bill” Anderson, P.E., has been involved with the automotive hobby for more than 30 years with experience ranging from hot rods, to sports cars, to sports car racing, and to restoration of vehicles. He is an author, magazine editor, car show judge and professional engineer. Through Anderson Automotive Enterprises — www.andersonautomotiveenterprises.com — he restores and appraises cars.