Chromate coating
Similar to the phosphating process on iron and steel materials, chromate coating is applied to aluminum.The term chromating refers to the chemical and electrochemical treatment of metals and metal coatings in solutions in which chromic acid, chromate, or dichromate are the main components. The result of such an operation is the formation of a protective coating consisting of trivalent and hexavalent chromium compounds on the metal surface. Chromate coatings can be observed in yellow and green colors.
Yellow chromate is based on Cr 6+, green chromate is mainly based on Cr 3+. The coating weight can be between 0.4 and 1.5 g/m2 depending on the time and type.
It is important that the metal material is free from oil, oxides and other impurities and that it is properly abraded before the chromating process. The aluminum surface, which has been degreased and its surface has been cleaned, is ready for chromate coating and subsequently for painting.
Aluminum Chromate and Applications
The purpose of chromate coating on aluminum is to provide better paint adhesion and increased corrosion resistance. Types of chromate coatings can be applied by dipping or spraying.
With yellow chromate coating, good paint adhesion and corrosion resistance are achieved. The chromate bath also consists of Cr 6+. The coating formed on aluminum is mainly Cr 6+. When discharging the effluent, the Cr 6+must be converted to Cr 3+.
In the green chromating process, the bath contains Cr 6+. The coating formed on the aluminum is predominantly in the form of Cr 3+.
Characteristics of chromate coatings
1- This type of coating provides good resistance to corrosion for the metal.
2- Good conductivity is one of the characteristics of these coatings. Today, in the electronics industry, parts that must be in electrical contact are covered with chromate coating.
3- One of the weaknesses of these coatings, especially thick chromate coatings, is their low abrasion resistance. However, abrasion resistance can be increased by drying the part after the chromate treatment.
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