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I have a problem that has surfaced on some Anodized parts I'd like your opinion on. The parts are made from Al. 7075 T 7351, they are chem filmed, masked and then Anodized type II. We've found that they have a "smutty" leopard pattern that appears to run with the material grain and does not rub off like something on top of the finish. We've been told, the problem is material surface crystallization that may be caused by contamination in the material, un-even heat introduced during the machining process or something introduced during the Anodizing process.
We've also been told the problem does not negatively impact the integrity of the part or the finish and is not a rejectable problem.
Additionally, we've been told that if we remove a small amount of surface material (approximately .005) that the problem will most likely be gone.
Do you have any thoughts or experience with anything like this ? What's your take on the advise we received ?
We plan to glass bead blast the surfaces, strip and re-Chem Film and Anodize. Does this sound like a viable re-work ?
I have seen this caused by uneven cooling at the extrusion operation, when the parts are touching each other. Aparently silicon in the alloy migrates to the areas that remain hot longer, showing up as smut during chemical treatment.
Good luck,Mike McDonald
mack products - Jefferson, Wisconsin