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Phosphoric acid anodize, why can't I rework more than once?

Our specification for Phosphoric acid anodizing in preparation for adhesive bonding is similar to a large aerospace manufacturer's. Both indicate that items can not be stripped and re-anodized more than once (regardless if it is clad or bare aluminum). Why is this? Do additional rework cycles adversely affect the bonding characteristics of the anodize (and how)?

Terry L. Warner
- Elmira, New York

I think the reason would be primarily dimensional, Terry.. Remember that anodizing is not like painting or plating where material is added to the surface. Rather, in anodizing the anodized film is formed from the aluminum substrate; when you strip it you are left with thinner aluminum than you started with. If you built and stripped two anodized films, then built a third film, the remaining base metal would probably be too thin.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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