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Black spots on aluminum after etching

I am a student who's doing my final year project on etching aluminum. I've come across a problem whereby when etching aluminum 5052 sample in Sulfuric acid, flame coat it and baked the sample in oven for a few hours under the temperature of 150 °C, black spots began to appear on the surface of the sample. I really would like to know why that black spots appeared? from observations the black spots appeared because of the etching process. for your info, when using Aluminum 6061 sample, the problem did not occur.

Monica Lim
student - Bayan Lepas, Penang, Malaysia
2004

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I can't be sure about what you are describing, but the black areas sound like copper. After etching, especially a high copper alloy, you have removed aluminum from the surface but left behind copper and other materials. It is necessary to then desmut in nitric acid or a proprietary formula to remove the copper.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004