My name is Matthew Patterson, a manufacturing planner in aerospace and a first time poster on this forum and so far I am impressed by the help of other people I’ve read on other posts. So I’m going to try my luck here.

I’ll give you what I’m working with:

Material: Al 6013 sheet metal in T6 condition (received in T4) AMS4347 .040" Thick.

Process: Chromic Acid anodizing (Type I class 1) using 40 volt processing and hot DI water sealing.

Problem: randomly recieving parts in post-anodic condition with dark staining on the material which is transposed (mirrored) to the opposite side of the sheet.

Affected areas contain a slightly higher concentration of copper and slighly lower hardness value (both still well within AMS4347 specification).

Normal piece exhibited Cu. Content of .917/.927%.
Dark area exhibited Cu. Content of 1.04/1.05%.

I attached a few pictures but it is difficult to photograph the staining condition. The camera we use does not take well to anodic coatings. Please look carefully to find the random areas of dark discolouration as they are much more apparently in person.

As aluminum sheeting is not porus I cannot see any chemicals able to penetrate through the material and re-processing the parts does not eliminate the stains.

If anyone has any ideas as to what is causing this I would be most appreciative. If any clarification or if more information is needed please let me know.

I appreciate everyones time!

Matthew Patterson

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