CHROMATE CONVERSION
In the chromate conversion process, the metal surface of a part is converted to a superficial layer containing a complex mixture of chromium compounds. The chromate film is soft when freshly formed, but once dried and aged, the film becomes more abrasive resistant. Chromate conversion coatings provide excellent corrosion resistance. Paint, lacquer, and organic finishes bond exceedingly well to chromate conversion coatings. In addition, the chromate coatings prevent loss of pain adhesion due to under film corrosion.
Chromate conversion films vary in color depending on the chemical being used and the base metal being coated. Colors range from clear, to light iridescent, to gold. Greater corrosion protection is provided with formulations that give a golden color, as these coatings tend to be thicker. A variegated appearance may be obtained when the base metal is non-uniform (i.e., welds in wrought materials and castings).
(All example pictures are parts done by T & T Anodizing.)
Major Industry Specifications Chromate Conversion:
Type I – Compositions containing hexavalent chromium
Type II – Compositions containing no hexavalent chromium
Class 1A - For maximum protection against corrosion, painted or unpainted
Class 3 - For protection against corrosion where low electrical resistance is required
MIL-STD-171
MIL-S-5002
T&T now supplies MIL-DTL-5541F finishing to meet the RoHS Directive.