Mil Spec: MIL-DTL-5541F

Classification. The chemical chromate conversion coatings are of the following types and classes.Types. The chemical conversion coatings are of the following types.

Type I – Compositions containing hexavalent chromium
Type II – Compositions containing no hexavalent chromium

Classes. The materials, which form protective coatings by chemical reaction with aluminum and aluminum alloys, are of the following classes

Class 1A – For maximum protection against corrosion, painted or unpainted.
Class 3 – For protection against corrosion where low electrical resistance is required.

ASTM Designation: B 449

Chromates on AluminumClassification of Chromate Coatings

Class* Appearance Coating Mass per Unit Area, g/m2 Corrosion Protection
1 Yellow to brown 0.4 to 2 Maximum corrosion resistance generally used as final finish
2 Colorless to yellow 0.1 to 0.4

Moderate corrosion resistance, used as a paint base and for bonding to rubber.

3 Color less<0.1 Decorative, slight corrosion resistance, low electrical contact resistance.4Light green to green0.2 to 5Moderate corrosion resistance, used as a paint base and for bonding to rubber

* No relationship exists between class numbers and degree of corrosion resistance. Class numbers have been assigned on the basis of traditional usage and for compatibility with national standards.