Chromate Conversion Coatiing

What is Chromate Conversion Coating ?

Chromate conversion coating is a type of conversion coating that converts surface properties of substrate materials. It is used to passivate zinc-plated Steel, aluminium, copper, silver, magnesium and tin alloys. It is also known as chem film, alodine or iridite.

Chromate conversion coating are available as hexvalent and trivalent chromates. Hexavalent coatings are banned because of their poisonous nature. Trivalent coatings are more popular these days.

Chromate Conversion Coatings Properties

  • 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.

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.

Specifications (ASTM-B449-93)

Coating ClassAppearanceCorrosion Protection
Class 1Yellow to BrownUsed as Final Finish Operation
Class 2Colorless to YellowModerate Corrosion Resistance, generally used as paint base
Class 3ColorlessLow corrosion and electrical resistance, Used as decorative finish
Class 4Light green to greenModerate corrosion resistance

Conclusion

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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