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Oxide layer stresses

If a ring, e.g. four inches in diameter (3.9 id) with a section of .05 x .5 inch, was to be manufactured from 6061T-6 and great care was taken to produce an accurate cylindrical geometry, and if a 180 degree section of this ring was then hard anodized on the outer surface only, would the ring still be as accurately cylindrical as it was before anodizing or would stresses inherent in the oxide layer distort the ring? If distorted, would the stresses be compressive or tensile? Assuming that the ring is distorted, are there techniques to minimize the effect?

Michael Davenport
manufacturing - Leander, Texas, US
2004

I would be mildly shocked if the ring was still a perfect circle after cutting it in half. I also think that it will "open" up more after it has gone thru chemical processing. You have not stated how much distortion that you can live with. The only way to find out is to try it. The internal stresses are going to be slightly different from lot to lot, so do not bet the farm on one success or one failure.

James Watts
- Navarre, Florida
2004

In retrospect, I didn't present my question well. I'm trying to learn if the anodized layer, in particular 'hard coat' contains internal stresses that will distort a delicate structure. Perhaps my example would have been more effective if I had proposed a thin, planar shape that was anodized on one face only; would stresses in the anodized surface warp the structure, making it non-planar?

Michael
- Leander, Texas
2004

OK, hard coat does not have a significant internal stress. However, etching one side of a thin plate will relieve surface stress on that side an will cause/allow a minor bowing of a thin plate. An item of significant importance is the coefficient of thermal expansion for the hard coat is roughly 1/10 of the parent alloy. This may cause problems if your allowable distortion is tiny and your temperatures more than a few degrees variable.

James Watts
- Navarre, Florida
2004

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I think that you won't have much distorion at all from the actual anodizing process..what MIGHT cause problems is the actual racking of the part. For hardcoat one needs good, solid, tight racking. So you can imagine if one clamps on either the ID, or OD of your ring, it very well could effect the concentricity (I hope I understood your question).

Marc Green
anodizer - Boise, Idaho
2004