Color conversion coating protects metals from corrosion. It involves immersing metal in an aqueous solution of chromium salts and acid to form a thin layer of chromium oxide. The process works on aluminum but not iron or steel. Chromium coatings can self-repair minor scratches and are useful for painting. However, hexavalent chromium is toxic, and trivalent chromium is less effective as a metal protectant.
Color conversion coating is a process used to protect aluminum and some other metals from corrosion, often called rust. Corrosion occurs from a reaction of metals and oxygen with moisture or water present. Rust is a metal oxide, where oxygen atoms react chemically with metal atoms. If not corrected, oxidation or rust can ruin metal parts very quickly in very humid climates.
Even in the 21st century, the chemical process for how chromium atoms build a layer on metal surfaces was not well understood. It was known that metal parts immersed in an aqueous solution of chromium salts and a small amount of acid formed a very thin layer of chromium oxide on the surface. The color conversion coating process can be performed at room temperature in minutes.
Not all metals can accept a color conversion coating, as the process doesn’t work well on iron or steel. However, a zinc coating can be applied first, with the chromate layer added later. A chrome coating doesn’t slow down the rusting of iron or steel, but it does protect the zinc that protects the steel. Chrome coatings range in color from bright yellow to dark brown, some of which is caused by the thickness of the coating and to a lesser extent the metal being protected. Darker coatings are usually slightly thicker and offer better corrosion protection.
Aluminum became a popular metal for the new aircraft industry in the early 20th century. Pure aluminum is very soft, so blends with copper and other metals have been used to make lightweight yet strong parts for aircraft. Corrosion quickly became a problem and ordinary paints didn’t stop it. Chrome plating has been found to reduce corrosion to acceptable levels.
A color conversion coating is often called a chemical film, and the coating layer is quite thin, unlike paint or zinc plating. One benefit of chrome trims is their ability to self-repair minor blemishes, rubs, or scratches. The chromium atoms can move slowly in the coating layer and will eventually cover small scratches or damage. Large cuts or chafed areas cannot heal themselves and require re-treatment.
There are several processing steps required to coat aluminum or other metals. Metals need to be extremely clean and are usually washed and rinsed multiple times with soap, solvents and water. The surface should be prepared for the color conversion coating by etching the surface with acid batch. Etching is a process in which the metal surface is roughened by reacting it with an acidic solution which chemically attacks the metal surface. Rinsing must be done again after etching and before the color conversion veneer can be applied.
Color conversion coating is also useful if a part needs to be painted; for example, aluminum does not accept paint well and painted surfaces degrade very quickly. Pretreating aluminum with a coating of chrome creates a surface that works well with paints and has allowed commercial airlines to paint aircraft with their company identification. Chrome platings do not significantly reduce electrical conductivity, which is the ability to carry an electric current. This can be an advantage in aircraft or other vehicles that carry electrical current through parts of the chassis. Paint and other coatings form layers that block electrical flow, requiring costly wiring projects to overcome the problem.
One major disadvantage of using chromium coatings was the toxicity of the chromium. One type called hexavalent chromium is very toxic to humans and animals and remains in the environment for a long time. The use of the less toxic trivalent chromium began in the late 20th century, which has had some success but is less effective as a metal protectant than hexavalent. The research also included other titanium and zirconium coatings, which have seen some use for industrial applications.
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