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Problems with anodising high zinc alloy

We are trying to chromic anodise a high zinc alloy ( 6.5% Zinc). The problem is that the amps fall away to zero after 2-3 minutes, There is nothing wrong with the rectifier because there are no problems with other alloys.
Has anyone had similar problems with high zinc alloys?

Mike Channing
senior chemist - Hampshire, UK
2007

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You can anodize aluminum, you do not anodize the high percentage of non-aluminum alloying ingredients. Our Mil Spec does not allow chromic anodizing on alloys exceeding 7.5 % non-aluminum. ("unless otherwise specified") 7075 is 11.13 % non-aluminum.

Boeing, in 20-43-01,5.C.(2) says to use only 22 volts when chromic anodizing 7075-T73 and 7178.

If the metal is old or has been heat treated, then the zinc has migrated to the grain boundaries, then falls out upon being made anodic in chromic acid. Try ramping up slower, and stopping at 2 volts. Be sure the sulphate is under 500 mg/L.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007

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2007

Thanks Robert, we tried it out on one part. We stepped up the voltage slowly to 22 Volts, and left it 20 minutes longer than normal.
The result was excellent and now we can process all the other parts the same.
Thanks once again for your help.
Regards,

Mike Channing
aerospace - Titchfield, Hampshire, England