Chem Film
AKA Aluminum Chromate, Chem-Film, Alodine, Iridite, Chromate Conversion Coating, Trivalent or Hexavalent
About Chem Film
Chemfilm is a “chemical conversion coating formed by the reaction of chemical conversion materials and the bare surfaces (not alclad) of aluminum alloys.” It is used primarily for aluminum and aluminum alloys where increased corrosion resistance, improved paint adhesion, and a low electrical resistance coating is required. It is also useful for improving the corrosion resistance and paint adhesion properties of a braded or discontinuous anodized surfaces of aluminum and aluminum alloy parts.
There are two classes of coating:
Class I-A
This is a thick coating for use as a final finish or pre-treatment to paint or powder coating. This is gold, brown or yellow depending on coating weight and alloy. Also provides maximum protection against corrosion, painted or unpainted.
Class III
This is a thin coating for low contact resistance. The coating weight lower as is the corrosion resistance. There may be some advantages using this thinner film in bonding applications. This film will appear to be a lighter color of gold.
As of Revision F of this specification, there are also two types of coatings:
1) Compositions containing hexavalent chromium. We use Electro Brite Aluminum Chromate 190 in our solution.
2) Compositions containing non-hexavalent chromium. We use a trivalent chemical conversion.
The Process
The process begins with a two step cleaning procedure on the part. First, the parts are dipped in degreasing solvent to remove dirt and oil from the surface. Then the parts are dipped into a caustic etch to remove any oxides (rust). The second step produces a thin black film on the aluminum surface that must be removed by a deoxidizer. The combination of etching and de-smutting results in a loss of up to .0009 inches on the aluminum surface. After desmutting the part, a thorough rinsing is needed to remove excess cleaning material and keep the part wet to prevent further oxides from forming.
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