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topic 12096

Anodize voltage for 7075 aluminum


I work in the aerospace industry, and I have a problem with a couple of parts. They are 7075, and they were anodized to type 1 as opposed to type 1b (40 volts as opposed to 22 volts). Did the 40 volts damage the parts, or can they be stripped and anodized properly?

Ryan Anderson
aerospace - Halifax, N.S., Canada


Although I have seen Mil-A-8625 [link is to free spec at Defense Logistics Agency,], I don't think it explains the origin or reason for Type 1b coatings. I do understand from the AESF intensive training course that cast alloys tend to generate a higher current density than wrought alloys at the same voltage and may tend towards burning. So I am guessing that you are working with a cast alloy for which Type 1b has been specified to minimize the chance of burining. I don't immediately see any reason that the coating could not be stripped and reapplied, as long as the part has not been damaged; but if the coating is not burned, why strip the coating?

Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


We are contractually obligated to anodize to MIL-A-8625. It states that any material with nominal alloying elements in excess of 7.5% shall not receive type 1 coatings (40 volts). We have to anodize the parts using 22 volts, it's just a question of wether or not the part was damaged by the original 40 volt supply. I read that cast alloys are damaged by 40 volts, and I've also read that 7075 material fails to develop a coating at 40 volts, so I've sent the parts to be stripped and re-anodized at 22 volts. I've still got time to scrap the piece if I find anything further to suggest that the material was affected.

Ryan Anderson
aerospace - Halifax, N.S., Canada


Just a note to say check the sizes when they come back as the strip and re-process will have taken some of the base metal away.

Martin Trigg-Hogarth
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England

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