Chromate conversion coating is a type of conversion coating. Conversion coatings generally converts surface properties of substrate materials.
Chomate conversion coating is used to passivate zinc-plated Steel, aluminium, copper, silver, magnesium and tin alloys. They are also known as chem film, alodine or iridite.
They are available as hexvalent and trivalent chromates. Hexavalent coatings are banned because of their poisonous nature. Trivalent coatings are more popular these days.
Properties of Chromate Conversion Coatings
- Excellent Corrosion Resistance
- Conductive in nature
- Used as primary coat for paint or powder coat.
- Can withstand 336 hour salt spray test.
- Chromate conversion coating are amorphous with a gel like composition hydrated with water. This characteristic helps aluminum them in self healing while scratched.
Type I (Hexavalent Chromium)
- Class-1A (Yellow) : Maximum corrosion protection and highest electrical resistance.
- Class-3 (Clear or Yellow) : Lowest corrosion protection and low electrical resistance.
Type II- (Trivalent chromium)
Class-1A (Clear) : Maximum corrosion protection and highest electrical resistance
Class-3 (Clear) : Lowest corrosion protection and low electrical resistance.
|Class 1||Yellow to Brown||Used as Final Finish Operation|
|Class 2||Colorless to Yellow||Moderate Corrosion Resistance, generally used as paint base|
|Class 3||Colorless||Low corrosion and electrical resistance, Used as decorative finish|
|Class 4||Light green to green||Moderate corrosion resistance|
To sum up, Chromate conversion coatings is a type of conversion coatings. They are used to passivate zinc, aluminum, copper, silver, magnesium and tin alloys.
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