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Cracking of Hard Anodizing

I was wondering if visual cracking of hard anodizing is common. I have a pure aluminum rod that has been hard anodized and the coating displays several cracks, which I think is related to the original grain structure of the material. The anodizing is sealed in water, and the anodizer says that the cracking is typical and can't be avoided. I was wondering if it can be minimized, or is it so prevalent is because of the large grain size?

Bob Smith
- Cambridge, Ontario, Canada
2003

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2003

Hard Anodizing forms perpendicular to the surface, therefore, on a round rod, as the coating gets thicker the outside of the coating appears to separate as the perpendicular planes get further apart as the diameter increases.

That said, heavy thicknesses sometimes crack. Coatings formed at too high current density appear to crack. High alloys like 2000's and 7000's appear to crack from the copper or zinc in the surface (you only anodize aluminum).

If the temperature is too low for the current density, the coating will crack.

If you need heavy thickness on a round rod, try higher temperature with the glycolic type additive.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina