Chromate conversion coating is used to passivate zinc-plated Steel, aluminium, copper, silver, magnesium and tin alloys. Conversion coatings converts surface properties of substrate materials.
It is primarily used as a corrosion inhibitor, primer and decorative finish.
Conversion coatings are amorphous with a gel like composition hydrated with water. This characteristic helps aluminum conversion coating in self healing while scratched.
Hexavalent coatings are banned because of their poisonous nature. Trvalent coatings are more popular these days.
Properties of Chromate Conversion Coatings
- Excellent Corrosion Resistance
- Conductive in nature
- Used as primary coat for paint.
- Can withstand 336 hour salt spray test
Conversion Coating Specifications ( MIL-C-5541E)
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.
Conversion Coating Specifications (ASTM-B449-93)
|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|