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Chromate conversion coating is a process used to protect aluminum and some other metals from corrosion, which is often called rust. Corrosion occurs from a reaction of metals and oxygen with moisture or water present. Rust is an oxide of the metal, where the oxygen atoms chemically react with the metal atoms. Left uncorrected, oxidation or rust can ruin metal parts very quickly in high-humidity climates.
Even into 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 dipped in a water-based solution of chromium salts and a small amount of acid will form a very thin layer of chromium oxide on the surface. The chromate conversion coating process can be done at room temperatures in just a few minutes.
Not all metals can accept a chromate conversion coating, as the process does not work well on iron or steel. A zinc coating, however, can first be applied, with the chromate layer added later. A chromate coating does not slow down the iron or steel rusting, but it protects the zinc that is protecting the steel. Chromate coatings vary in color from bright yellows to dark browns, part of which is caused by the thickness of the coating, and to a smaller degree what metal is being protected. Darker coatings typically are 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 mixtures with copper and other metals were used to make lightweight but strong parts for aircraft. Corrosion quickly became a problem, and normal paints did not prevent it. Chromium metal coatings were found to reduce corrosion to acceptable levels.
A chromate conversion coating is often called a chemical film, and the coating layer is quite thin, unlike paint or zinc plating. One advantage of chromate coatings is their ability to self-heal small imperfections, rubs or scratches. The chromium atoms can move slowly in the coating layer, and will eventually re-coat small scratches or damage. Large cuts or rubbed areas cannot self-heal and require re-treatment.
There are several processing steps needed to coat aluminum or other metals. Metals must be extremely clean, and are usually washed and rinsed multiple times with soap, solvents and water. The surface must be prepared for the chromate conversion coating by etching the surface with an acid batch. Etching is a process where the metal surface is roughened by reacting it with an acid solution that chemically attacks the metal surface. Rinsing must be done again after etching and before the chromate conversion coating can be applied.
Chromate 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. Pre-treating aluminum with a chromium coating creates a surface that works well with paints, and has allowed commercial airlines to paint aircraft with their company identification. Chromate coatings do not greatly reduce electrical conductivity, which is the ability to carry an electric current. This can be an advantage in aircraft or other vehicles that carry an electrical current through parts of the frame. Paints and other coatings form layers that block electrical flow, requiring expensive wiring designs to overcome the problem.
A major drawback of using chromium coatings was the toxicity of chromium. A type called hexavalent chromium is very toxic to humans and animals, and remains in the environment for a very long time. The use of less toxic trivalent chromium began in the late 20th century, which has had some success but is less effective as a metal protector than the hexavalent. Research also included other coatings with titanium and zirconium, which have seen some use for industrial applications.