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Cracking of thick vs thin anodized 6061

We have been using anodized 6061 parts in a high temperature (<300 °C) vacuum chamber environment successfully for some time. Our observations are that thin (0.5mil) Type II1/2 coatings survive well, but thicker coatings tend to spall. We also see that Type III coatings survive well both thin (0.5mil) and thick (2 mils), although the thicker coatings tend to crack more than the thin coatings. Why do the thicker coatings crack more than the thin coatings? Also why does Type III resist cracking better than Type II1/2?

Janine Kardokus
- Santa Clara, California, USA
2002

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2002

First see my answer to letter above number 41531. Anodizing is aluminum oxide, very hard (grinding wheels are made of aluminum oxide), very brittle, and can withstand no flexing. Type III can withstand even less flexing than Type II, but in your case, apparently the strength of the heavier coating is withstanding the flexing more than the strength of the thinner coating.

Also, you did not mention shape, again, on an inside curve the anodizing builds out at a 90 degree angle and at some thickness (depending on several other variables) the perpendicular planes crash into each other and "CRACK". Inside curve coatings must be thin to avoid noticeable cracking.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina