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White Spots on Coated Aluminum

I am experiencing white spots on coated 5182 that do not show up until after the coating process. Based on the repeat pattern, mirror image, and fade pattern, it is apparently being generated at one of the intermediate processes when the metal is bare, but we have not been able to find a bare sample. These spots do not wipe off. We have not been able to determine the exact cause, whether the aluminum is oxidizing, etc. The particular piece of equipment supposedly causing these spots is only a trim operation and no lube or cleaning agent is used at this operation. It has become a major cause of rework and any input as to the possible source of these spots would be greatly appreciated.

Britton L. Snow
a metal warehouse - Muscle Shoals, AL, US
2003

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First read the "White Spots" paper posted on finishing dot com. Then consider that the magnesium bearing alloys frequently have hunks of exposed magnesium and you do not anodize magnesium, you anodize only aluminum. Then go look for chlorides in the system, then consider the dissolved aluminum in the anodizing tank, finally try dipping in 5% nitric acid before dyeing and/or sealing.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003