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 underfilm 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).
Major Industry Specifications Chromate Conversion:
MIL-DTL-5541F
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Type I – Compositions containing hexavalent chromium
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Type II – Compositions containing no hexavalent chromium
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Class 1A - For maximum protection against corrosion, painted or unpainted
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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
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