Except for a few parts on an automobile, most have some type of finish. Those parts that don’t include some castings of iron or aluminum, e.g., steering boxes and alternator cases, and forged steel pieces, e.g., steering arms and roll bars.We’ll discuss passivation, anodizing and other finishes applied to automotive parts.
Determining the original finish of brightwork is difficult during restoration of collector cars because time and the elements typically have destroyed most of the original finish. However, with careful examination of the parts (often traces of the original finish can be observed) and having knowledge of typical manufacturing practices when the car being restored was manufactured, the correct original finishes can be determined and then applied to all components and fasteners.
Passivation
Passivation is a family of processes used to control corrosion by the spontaneous formation of a hard, non-reactive surface film, typically an oxide or nitride film that is a few molecules thick. Sometimes it is used in conjunction with electroplating. Several different passivation processes are employed to protect automotive components and fasteners.
Chromate conversion
Chromate conversion was developed during World War II to increase the corrosion resistance of zinc and cadmium-plated parts. The conversion coating uses a solution of various chromium compounds. Originally, only olive was available, but soon there were more ranging from clear, to yellow, to gold, to olive drab; the darker the coating the more corrosion resistance. Sometimes parts that have a chromate conversion are dyed black for appearance purposes; this treatment is far more corrosion resistant than black oxides and iron phosphate discussed later. A key characteristic of all chromate conversion coatings, regardless of color, is an iridescent green, blue and red coloration. This coloration is a happenstance of the shape of the part and its natural reflectivity.
Once a clear chromate conversion process became available after World War II, it was often applied to cadmium-plated parts to enhance durability of the finish. Yellow chromate on cadmium or zinc-plating is commonly seen on such components as power brake boosters and some air pollution control parts.
Black oxide
Black oxide is a caustic solution that colors the steel black. It is one of the earliest passivation processes dating to the 18th century. Its earliest use was for bluing gun barrels, a use continuing to this day. Typically used for fasteners, black oxide is not long lasting. It can be identified by a dull finish and differentiated from other caustic treatments by its dark flat-black color. Sometimes, black oxide treated fasteners are oiled to protect the finish during storage before installation.
Iron phosphate
Iron phosphate is another caustic-based solution used to apply a protective coating to steel. A few parts that are converted with iron phosphate are used without further treatment. Iron phosphate also provides an excellent substrate preparation for paint and is often used for that purpose, i.e., iron phosphate followed by a coat of semi-gloss black paint.
Parkerizing is a proprietary process for passivating steel dating to 1912. It is now owned by Henkel Surface Technologies and produces a flat-black appearance such as iron phosphate and is slightly more durable.
Anodizing combines some elements of electroplating and passivation to provide a more durable finish on aluminum parts than the naturally formed oxide, alumina.
Zinc phosphate
Zinc phosphate is an iron phosphate bath enriched with zinc creating an iron-zinc alloy. It provides far greater protection than iron phosphate because of the sacrificial properties of zinc. It is recognizable by its dark gray, crystalline structure. The size and configuration of this crystalline structure is determined by the variables of time, temperature and concentration of zinc in the bath at the time of deposition. There is no standard for this coating and, accordingly, appearance can vary. Zinc phosphate is typically used on hood latches. For a while in the 1960s and ’70s, General Motors used zinc phosphate on some hood hinges.
Anodizing
Anodizing is forced oxidation. This process combines some elements of electroplating and passivation to provide a more durable finish on aluminum parts than the naturally formed oxide, alumina. The part to be anodized is submerged in a solution of sulfuric acid and charged with a positive current. Because the part is the anode, the process was called anodize. The oxygen molecules in the sulfuric acid break down and oxidize the surface of the aluminum. Then, it is sealed in an extremely hot water rinse.
Window reveal moldings used in the 1960s and later and sill plates are aluminum parts that are typically anodized. This coating is durable and lasts for years under all types of environments.
Forged pieces
Forged steel pieces are typically found in suspension parts — tie rods, steering arms, anti-roll or sway bars, etc. Forging heats the steel and then presses it into the desired shape. A scaly dark blue/teal surface appears during cooling. No paint or other preservatives are typically applied. The forging appearance degrades over time, leaving a rusty surface unless coated with accumulations of grease as is often the case. Restorers use various techniques to attempt replication of this appearance where a high degree of authenticity is desired. Some of these techniques include specialty paints and bluing a media-blasted surface with gun bluing that is then sealed with flat clear paint.
Polishing
A durable metal finish that is used on decorative stainless steel trim pieces is polishing. Left alone, stainless steel resists corrosion and polishing provides a shine comparable to decorative chrome plating without the expense of electroplating.
Summary
Typically, the finish applied by the component manufacturer is specified by the car manufacturer. On the assembly line, these finishes are the ones remaining when the car leaves the line unless the component is part of an assembly that is painted afterwards, such as the basic engine. Ancillaries, such as generators, alternators, power steering pumps, air conditioning compressors, etc. and associated brackets added to the basic engine during assembly will have the various originally specified finishes.
Determining the original finish is difficult during restoration of collector cars because time and the elements typically have destroyed most of the original finish. However, with careful examination of the parts (often traces of the original finish can be observed) and having knowledge of typical manufacturing practices when the car being restored was manufactured, the correct original finishes can be determined and then applied to all components and fasteners.