Anodizing “Aluminized” Steel

Q: Is it possible to anodize “aluminized” steel? We have a choice of aluminized steel with either a pure aluminum coating or a 10% silicon-aluminum alloy. Which of these aluminum coatings would be easier to anodize? Is anodizing always an immersion process? J.K.

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A: Theoretically, aluminum-coated steel could be anodized, but I don’t think the process is commercially viable. I have no knowledge of this ever being done. Perhaps it has been tried in a laboratory environment. However, I do not know the degree of success attained.

Anodizing aluminized steel would be problematic from the start. If there are any pin holes or voids in the aluminum, coating the part will “burn” where the steel is exposed to the process. There must be sufficient aluminum thickness to allow the anodizing process to go forward and it should be uniform over the entire steel part. If a nominal 1.0 mil (0.001 inch) of hard coating were desired, for example, approximately half of the anodic coating thickness would be penetration of the aluminum coating while the balance of the coating would be buildup. (Aluminum oxide takes up more space than the aluminum it replaces). There must be aluminum remaining as a substrate under the anodic coating in order to avoid burning of the part. If the aluminum coating is 10% silicon, anodizing would be problematic for many reasons that I won’t go into. If this process must be done then the pure aluminum route is the way to go.

Anodizing is usually an immersion process. However, aluminum parts can be “brush” anodized in the field or anywhere. The same chemistry and physics apply. There are people who specialize in this technique.