Chromate Conversion
Anodizing does not maintain the electrical conductivity of the metal, which is essential for the electronics industry in particular, necessitating the use of chromate conversion instead. Chromate conversion is often performed on
aluminum alloys because they are naturally less corrosion-resistant due to the alloying elements that they are formed from such as copper and iron. However, it can also be performed on materials such as zinc, cadmium, titanium and magnesium. As a result of its broad material capabilities, chromium conversion is often utilized in industries such as: construction, as hardware for buildings, doors and windows; industrial, in metal finishing as well as various other processing applications; automotive, for various parts and engine components; and medical, for prosthetic joints and other types of medical equipment. Chromate conversion processes are also used in industries such as pharmaceutical, optical, commercial and aerospace, and is capable of producing a wide range of colors for decoration. The variety of shade that chromate conversion produces is another similarity to
aluminum anodization, which is also known for its
color anodizing.
The chromate conversion process uses chromic acid, an unstable and corrosive acid, in the form of soluble salts, which are ionic compounds. The material requiring surface conversion treatment is dipped in a bath of the chromic acid, referred to as a chromate treatment bath, which passivates the surface of the material. The passivation results in a thin layer of gel-like film that hardens over time to become a hard protective coating. There are two main kinds of chromatic acid used in chromate conversion processes: hexavalent chromate and non-hexavalent chromate. Hexavalent chromate is a chemical compound that contains chromium in its +6 oxidation state. The use of this material is restricted by the Environmental Protection Agency because they have classified it as a human carcinogen. As a result, many companies have begun researching different forms of chromatic acid or other types of acid, which would be more environmentally acceptable. Non-hexavalent chromate is less readily-absorbed by the human body than hexavalent chromate, and meets the standards established by the Restriction of Hazardous Substances and the European Union's End-of-Life Vehicle recycling directives. An example of a non-hexavalent chromate is trivalent chromate, which is more eco-friendly than hexavalent chromate, but still being monitored by the National Toxicology Program.
