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Anodization being stripped from enclosure body but not end caps

Anodization being stripped from enclosure body but not end caps

(OP)
A device recently came back to our repair department with anodization missing off the enclosure, as shown in the photo below. On the right is a new enclosure, on the left is the enclosure in question.



It almost looks like part of the coating was wiped off with someones fingers, given the fingerprint like smudged areas. This made me initially suspect some chemical on the handlers hands... However, the valleys between the fins are also missing their coating, ruling out this theory.

If the process wasn't "applied" to the surface, it should have affected the entire enclosure. However, the front and rear end caps, supposedly anodized in a identical manner to the body, are completely fine.



The different parts are made by different suppliers, so right now I am suspecting a manufacturing defect. Am I on the right track? Does anyone with more experience with anodization know of a process that could do this? Just today I heard that another unit is coming back with the body almost stripped to completely bare aluminum... I'm hoping to get out in front of this problem, but I just don't have a lot of materials science experience.

Thanks in advance!

RE: Anodization being stripped from enclosure body but not end caps

Anodization is an electrolytic process which uses an electrical current to convert the surface of the aluminum to a protective aluminum oxide. Color is created with a dye that is absorbed into the pores of the aluminum oxide. It looks like a highly alkaline cleaner was used to "degrease" the part. This dissolved the anodizing but was not left on long enough enough to completely remove it. So you are seeing variations in the residual thickness of the anodize. The tips of the fins have a thick anodize because that is where current will concentrate, while shielding the troughs, which get a thin anodize. Oils in fingerprints will protect the anodize from the cleaner for a short time.

Also, sealants may be used on anodized parts to provide more corrosion protection. This, as well as simple thickness differences may explain the differences between parts.

That is my guess, anyway.

RE: Anodization being stripped from enclosure body but not end caps

(OP)
Thank you for that info! Unfortunately the equipment these devices are installed on moves all of the country and is commonly serviced by shops with no relationship to the main maintenance facility... tracking down whether a cleaner was used will be difficult. Do you know whether a chemical test of the surface could detect the presence of alkaline substances?

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