Black Anodize over 6061-T6 Al - Defect Question

[kathadin]

I have a part that is 6061-T6 Al w/ Type II Black Anodize. A good part of the anodized surface (one large continuous area, no specific pattern or shape) is either a SLIGHTLY lighter shade of black, or less glossy, hard to tell. Thought it was residue, but nothing seems to take it off - DI water, IPA, etc. I noticed DI water beads up on the "good" portion, but wets to the "bad" portion. Any ideas on what this is, how it may of happened, and if the part is any good functionally (if not aesthetically)?

 

[kenvlach]

Lots of variables: the initial surface finish, chemical pretreatment (cleaning, etching, deox & all the rinses), anodizing thickness, dyeing, sealing... The larger the part, the more attention must be paid to the process details to ensure uniformity. In pretreatments, parts coming out of heated solutions must be transferred rapidly to the rinse or spray rinsed while withdrawing to avoid solution drying on the surface or continued etching. In the anodizing, multiple rack points must be used to ensure enough current flow throughout the part. With large sheet metal parts, the thin metal may heat up from current flow. Large parts also can affect the flow of the agitated electrolyte – leads to variation in surface temperature. The cathode-to-anode area ratio & spacing matter. With large parts, the bottom, sides & top of the part may see greater current densities since more and closer cathodes available.

With really large parts – longer than the dye and seal tanks, anodizers double-dip, i.e., immerse one end at a time. Can be done right, but an obvious chance for variation.

As to your part, the slick areas seem properly sealed. The wettable areas may be poor anodize (more porous) formed at higher T. If so, do some scratch tests to see if softer anodize there. Good anodize can scratch poor anodize.
Wetability can also indicate undersealing, or oversealing with smut formation. Smut is unacceptable on delivered parts – the anodizer needs to wipe off. Undersealing, especially, lessens corrosion resistance.

Do you have a thickness tester for anodize (eddy current type)? Thinner anodize absorbs less dye. If you can demonstrate thinner or softer anodize in the lighter areas, you have a legitimate claim.

Hope this helps. More info is needed to be anymore definitive.
Ken

 

[FinishingTalkdotcom]

Also need to check substrate. If any alloying metals are present in this area, can affect appearance of anodize coating.