Chem Film Over A Black Anodized Coating

Question:

We are working on some parts with the following customer specification: Black hardcoat anodize (Type III, class 2) to match #37038 (FED-STD-595), followed by a Class 3 chromate conversion coating over which we are to apply a primer and black paint to #37038. Can the black color to #37038 really be controlled as a dyed anodic coating? Where can I find a supplier for that? J.L.

Answer:

Any anodizer in your area who does Type III (hardcoat) anodizing should be able to help you with this. The specification you mention sounds inaccurate though. The chromate conversion coating (MIL-C-5541) applied over the anodic coating is of no help to the adhesion of the organic coating. The conversion coat, when applied over bare aluminum chemically reacts with the aluminum to form a tight bond to which the paint will adhere. There is no chemical bond formed when the conversion coating is applied over the anodized finish. The anodic coating itself is an excellent paint base if it is sealed in hot (210–212°F) deionized water that has no additives having surfactants. In other words, just pure, buffered (for pH control) DI water. Surfactants are "wetting agents" and can be detrimental to the bonding of organic coatings to anodized aluminum.

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It also occurs to me that Class 3 chromate conversion coatings are generally used where good electrical conductivity is required on the aluminum part. Class 1a, a heavier film, is used as a paint base. It might be wise to review the finishing specification with your customer to make sure it is the best possible.

There are several black dyes available for aluminum. I don't know if any of them match the particular Federal Standard Color called out. Your local anodizer can help you with that.