7075-T6 Chromate Conversion

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We are processing 7075-t6 impacted parts. On the machied surface we are seeing a growth. White spots will appear after cleaning. THese areas can be wiped off only to leave a very dark spot on the part. We have tried 3 different cleaners. Alkaline NON Etch, Alkaline Etch, and a solvent based cleaner. We are still seeing this problem. The parts are all from the same batch and heat lot, but one part may run fine while the next exibits the measels. any thoughts

Robert Davis
Job SHop - Texarkana, Texas, USA

First of three simultaneous responses -- +++++++

When 7075 is very old and/or when 7075 has been heated, the zinc goes to the grain boundaries, then when you anodize the zinc plates away and leaves a pit. There is nothing can can do about the particular batch of 7075 your client is using. In the case of the zinc laying just under the surface, which surface has been completely converted to aluminum oxide, then you may save a few pits by ramping up slower.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note:
   Mr. Probert is the
   author of Aluminum How-To / Aluminio El Como

Second of three simultaneous responses -- +++++++

Alkalinity brings out the worst in 7075. It removes aluminum and leaves a surface enriched in Cu, Fe, Mg, Zn, etc. Acid etch if necessary, use a nitric acid-based deox/desmut and rinse in DI water. See also Letter #45714.

Ken Vlach
- Goleta, California

Third of three simultaneous responses -- +++++++

You do not say anything about a smut remover or a deoxidizer. What do you use?

James Watts
- Navarre, Florida

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Our process is as follows:
Clean
Rinse
Rinse
Deox
Rinse
Chromate Conversion
Rinse
Rinse

THe cleaner is heated to 160 degrees. Also the Deox is a Proprietary blend of nitric and sulfuric acid. When viewed under magnification. The parts almost appear to have craters in the metal.

Robert Davis
Commercial Manufacturing Company - Texarkana, TX