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Etching aluminum skin

We etch the aluminum skin for the purpose of cleaning and meeting our specifications by using nitric acid. Today for the first time it appears that the skins have a colour transformation in some parts. There were a non consistently greenish colour shows on the skin surface.
I was wondering if anyone knows what could it related to?

Salar Modaresi
composites - UK
2007

First of two simultaneous responses --

Nitric will etch high alloy aluminum quite well, but poorly on low alloy. The main metal in the alloy is frequently copper. I will offer a guess that you have had a chloride buildup and have deposited a very thin layer of copper chloride. The other possibility is a huge copper buildup in the nitric and an ultra thin layer of copper immersion deposited. Try new etch solution and see what happens.

James Watts
- Navarre, Florida
2007

Second of two simultaneous responses --

Alloy? The 2000 series alloys contain copper which migrates to the grain boundaries with time and/or heat.
copper nitrate [affil links] is green.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007

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