Pioneer Metal Finishing
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Chromate Conversion Coating

When Bonding & Corrosion Is Critical

  • MIL-C-5541 Class 1A & 3
  • AMS 2473 & AMS 2474

Pioneer Metal Finishing's chromate conversion coating leaves no measurable build-up on the part and provides excellent corrosion resistance. In many paint or adhesive applications, chromate is used as a base for superior bonding. The coating is conductive and offered in yellow iridescent to clear iridescent. Electrical resistance increases with the darkness of the yellow color.

Applying chromate solution to clean aluminum produces a thin, gel-like film, which is an integral part of the metal itself, but converted to a non-metallic form.

Chromate treatments (sometimes called chemical film) produce effective paint bonds through molecular adhesions with the film being bound to the metal, in turn offering the same type of adhesion to the organic coating. The film lengthens the life of the paint, forming an effective barrier against corrosive attack through pores or scratches in the paint.

The great range of aluminum alloys creates a corresponding variation in appearance of chromate coatings. Using a given set of treatment conditions, five treated alloys could produce colors from pale yellow to deep brown. A high silicon aluminum casting and a low silicon wrought product could have widely different hues. Colors of coatings on "as-cast" surfaces are usually much lighter than those on wrought alloys.

The thickness of the film is .00001" to .00003". This coating is amorphous (which permits cold forming without rupture of the film) and non-porous (which aids against corrosion).

In spite of the fact that chromate doesn't have the hardness of an anodized finish, it stands up fairly well against abrasion of a non-cutting nature. An important quality for chromate coating is its ability to heal scratches in the film. The exposed metal gradually becomes covered by soluble chromate for areas surrounding the scratch.

Chromate films have low electrical resistance, varying slightly between Class 1A and Class 3, with Class 3 being the most electrically conductive. The film retards electrical action between dissimilar metals. For maximum protection, both parts should be chromated.

Chromate baths are operated at room temperature. Parts must not be subject to temperatures in excess of 140° F or a breakdown in corrosion resistance results.

Proper cleaning of the aluminum is essential in the chromating process. Oil, grease and other soils must be completely removed. Proprietary-inhibited alkaline cleaning compounds can be used for soak cleaning. Most aluminum alloying ingredients that produce smut on aluminum can be removed from etched surfaces with a proprietary acid deoxidizer.

Where dimensional tolerances permit, caustic etching imparts a pleasing appearance to the aluminum surface.