Summary for anodized aluminum corners Discussion
black spots on anodized aluminum corners are discussed as a likely current-distribution issue rather than a simple thickness problem. Roberto Flores reports 6061 aluminum parts with shadows or black spots, mostly on the bottom plate corners of a box-shaped chamber, and notes that thickness readings in good and shadowed areas are similar. Ted Mooney replies that anodizing usually has good throwing power, so the darkening is hard to explain by coating thickness alone. He says he has not personally seen a part of that size and complexity anodized, but the corner darkening still seems most likely tied to current distribution. His practical suggestion is to improve the shape of the internal auxiliary cathode so it better conforms to the part, possibly ending in an X shape reaching toward the four corners of the bottom plate to help even out the anodizing.
Q. We have 6061 aluminum product presenting shadows or black spots areas, most commonly on the bottom plate of product, in the corners of it.
Product is previously glass beaded, but the defect is visible only after anodize process
Roberto Flores
Employee – Monterrey, Mexico
June 10, 2025
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A. Hi Roberto
Unless I’m misunderstanding the pics, that appears to be a fairly complicated box-shaped weldment.
The first question is whether all of the pieces of the weldment are the same (6061) alloy — if not, that can be a serious problem.
Although anodizing has quite good throwing power, it is not unlimited, so I think you should measure the thickness of the dark areas as well as a few other spots. Pictures of the whole component, including how it is racked/fixtured would be very helpful because it is easy to guess wrong about such things, and entrapped air or carried over solution can cause problems. Are there any auxiliary cathodes reaching into rhat box? I have no sense no scale unfortunately.
The first pic looks like you are shining a light into the box?
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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Q. Hi Ted
As you well mention, it is the bottom of a box-shaped chamber. The alloy material is the same on top and side walls of it.
We measured thickness of the surface on good finished areas and “shadowed” areas but values are quite similar. Roughness was measured but our instrument has no capacity to read too-rough areas, beyond 180 RMS.
I am attaching pictures of macro zoom of the affected area and overall product.
Much appreciated your time and kind support
Roberto Flores
Employee – Monterrey, Mexico
June 12, 2025
A. Hi again Roberto,
Presumably you tilt the bottom end of the box up after you remove it from each tank to assist in drainage?
If I understand your pics I have to acknowledge that I haven’t personally seen a part of that size and complexity anodized. Still, it seems hard to attribute the darkening in the corners to anything other than the current distribution, suggesting that improving the shape of the internal cathode to better comply with the item you are anodizing should help. If the auxiliary cathode were to end in sort of an X shape reaching towards the four corners of the bottom plate I think it would help.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.