METAL FINISHES Everything but Paint By: William C
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METAL FINISHES Everything But Paint By: William C. “Bill” Anderson, P.E. Reformatted for this website and published with the permission of Old Cars Weekly. Except for a few parts on an automobile, chrome plating which is used for wear most have some type of finish. Those resistance on some engine parts. It is that don’t include some castings of iron called hard chrome because it is several or aluminum, e.g., steering boxes and thousandths of an inch thick and can be alternator cases and forged steel pieces, measured for hardness as opposed to e.g., steering arms and roll bars This decorative chrome which is only article will cover typical finishes except millionths of an inch thick and would paint, including: electroplating, easily break if subjected to a hardness passivation, anodizing, and polishing. test. This article will concentrate on Most of these finishes are used on mild decorative chrome plating only. steel which comprise the bulk of automotive parts. The subject of chrome plating is filled with many buzzwords that are Electroplating essentially meaningless — “show chrome,” “triple chrome plating,” Electroplating is used on many different “double-nickel chrome,” tri-chrome” and parts and is used to apply chrome, more. Read the platers’ ads and probably nickel, copper, cadmium, zinc, and terne more will surface. The following will (a mixture of four parts lead to one part demystify this process and provide some tin). All electroplating requires a clean, guidance to ensure a successful result. smooth surface. The piece to be plated is negatively-charged and the metal ions to Decorative chrome plating requires be deposited on the subject piece, which polishing of the piece to be plated, are contained in an electrolyte (called a cleaning — chemically and electrically, bath), are positively charged. An activating the surface, plating copper electrical DC current is passed through (may be omitted), plating two or three the bath causing the metal ions to be coats of nickel and then plating a final deposited on the part being plated. layer of chrome. Between each step, the piece must be thoroughly rinsed so as to Chrome Plating not contaminate the succeeding plating bath. Cross contamination of the electrolytes will lead to peeling of the Various automotive parts of steel and plating and a white-colored corrosion on zinc die cast are chrome-plated. Of all the plated piece when exposed to the the metal finishes chrome-plating is elements. arguably the most misunderstood of the metal finishes. Most people associate New metal pieces have minimal chrome plating with decorative chrome preparation as compared to restoration of plating such as used on car bumpers a part for a collector car. A used piece until they were replaced by plastic being re-plated requires stripping of the covers. However, there is also hard 1 existing chrome, the existing nickel, and, process and this is the principal reason it if present, the existing copper. Then, any is so expensive. There are also other rust must be removed, dents straightened factors. Chrome plating uses many and pits and scratches polished out. hazardous chemicals and complying Next, a layer of copper is deposited and with environmental regulations has this layer buffed until a smooth surface increased production costs. Finally, in is obtained. Sometimes, this step must be recent years there has been a run-up in repeated. These restoration steps are commodity prices for nickel and copper essential to successful re-chroming of a greatly increasing the cost of the platers’ part. If these restoration steps are not supplies. carefully done, the original part dimensions will be changed and Other Electroplated Metals adjoining parts may not fit or details, such as those in zinc die cast parts, will The other commonly applied metals in be obliterated. Some parts, such as automotive use include cadmium, zinc chrome air cleaners, rocker arm covers, and terne. Terne, a mixture of four parts etc, are chrome plated without any lead and one part tin, was used for many copper, particular when new. years by GM to plate hydraulic tubing and gas tanks. After the part is prepared, it is ready for final steps of chrome plating. First, a Cadmium is distinguished by its whitish layer of semi-bright nickel is applied hue. It was the preferred silver-appearing followed by a coating of bright nickel. finish applied to small parts and The reason for two coatings of nickel is fasteners in the automotive industry until to provide a durable finish. The bright the early 1970s. Then, environmental nickel is anodic to the semi-bright nickel regulations were adopted that and sacrificially protects it by spreading significantly increased the cost of any corrosion forces laterally instead of cadmium plating to control the allowing them to penetrate through to hazardous chemicals involved. Pieces the steel leading to rust. Any rust such as relay covers, regulator bases, showing on chrome is the underlying washer bottle holders and the like plus steel rusting because the overlying fasteners on cars before the mid-1970s coatings have failed. The nickel layers are probably cadmium plated because of are then topped with chrome. Typically, cadmium’s superior (compared to zinc) once the chrome plate step is complete durability. the work is done. However, sometimes it is necessary to buff the part to remove Zinc plating was originally reserved for any excess in high-current density areas larger pieces, such as brake backing which is function of how the part is plates. It is distinguished by a bluish connected to the electric current. Special hue. Once the cost cadmium increased, connectors must often be fabricated to zinc plating was also used for small properly disperse the electrical current silver-colored parts and fasteners. Most uniformly through the part. silver colored parts available today, either for new cars or reproduction parts, As the foregoing description reveals, are zinc-plated. Often these zinc plated chrome-plating is a labor intensive parts are further treated by passivation. 2 later. A key characteristic of all Each of these metals is applied in a layer chromate conversion coatings, regardless of only a few thousandths of an inch of color, is an iridescent green, blue and thick or less. The coating is essentially red coloration. This coloration is a transparent. Therefore, the degree of happenstance of the shape of the part shine to the plated piece is determined and its natural reflectivity. by the surface preparation of the substrate material. For example, if a Once a clear chromate conversion restored relay cover or fastener is media process became available after World blasted and not polished, the resulting War II it was often applied to cadmium- finish will have a matt appearance. Like plated parts to enhance durability of the chrome-plating, the part to be plated finish. Yellow chromate on cadmium or must be thoroughly cleaned and zinc-plating is commonly seen on such activated before it is placed in the components as power brake boosters and plating bath. some air pollution control parts. Passivation Black Oxide Passivation is a family of processes used Black oxide is a caustic solution that to control corrosion by the spontaneous colors the steel black. It is one of the formation of a hard, non-reactive surface earliest passivation processes dating to th film, typically an oxide or nitride film the 18 century. Its earliest use was for that is a few molecules thick. Sometimes bluing gun barrels, a use continuing to it is used in conjunction with this day. Typically used for fasteners, electroplating. Several different black oxide is not long lasting. It can be passivation processes are employed to identified by a dull finish and protect automotive components and differentiated from other caustic fasteners. treatments by its dark flat black color. Sometimes, black oxide treated fasteners Chromate Conversion are oiled to protect the finish during storage before installation. Chromate conversion was developed during World War II to increase the Iron Phosphate corrosion resistance of zinc and cadmium-plated parts. The conversion Iron phosphate is another caustic-based coating uses a solution of various solution used to apply a protective chromium compounds. Originally, only coating to steel. A few parts that are olive was available but soon there were converted with iron phosphate are used more ranging from clear, to yellow, to without further treatment. Iron gold, to olive drab; the darker the phosphate also provides an excellent coating the more corrosion resistance. substrate preparation for paint and is Sometimes parts that have a chromate often used for that purpose, i.e., iron conversion are dyed black for phosphate followed by a coat of semi- appearance purposes; this treatment is gloss black paint. far more corrosion resistant than black oxides and iron phosphate discussed 3 Parkerizing is a proprietary process for aluminum parts that are typically passivating steel dating to 1912. It is anodized. This coating is very durable now owned by Henkel Surface and lasts for years under all types of Technologies and produces a flat black environments. appearance like iron phosphate and is slightly more durable. Other Finishes Zinc Phosphate Forged Pieces Zinc phosphate is an iron phosphate bath Forged steel pieces are typically found in enriched with zinc creating an iron-zinc suspension parts – tie rods, steering alloy. It provides far greater protection arms, anti-roll or sway bars, etc. Forging than iron phosphate because of the heats the steel and then presses it into the sacrificial properties of zinc. It is desired shape. A scaly-dark blue/teal recognizable by its dark gray, crystalline surface appears during cooling. No paint structure. The size and configuration of or other preservatives were typically this crystalline structure is determined applied.