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-----Cloudy surface after clarification in nitric acid desmut
Q. Hi! Please tell me why a cloudy surface is obtained after clarification in nitric acid. I do bright color anodizing, but it turns out to be matte.
Andrey- Sarapul, Russia
October 6, 2025
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A. Hi Andrey.
I personally have not heard the word 'clarification' in this context. But you are probably referring to immersing the parts into dilute (1/2% to 1%) nitric acid as a way of driving out the sulfuric acid after anodizing and before dyeing?
I am surprised that this would cause a cloudy surface, and am not really confident that this is what is causing the problem of lack of brightness. Can you tell us what alloy you are anodizing, and about your etch bath (if applicable) and your non-etching caustic cleaner?
If other readers are familiar with a nitric acid dip after anodizing causing loss of brightness, please inform me as well as Andrey.
Luck & Regards,
Ted Mooney, P.E. RET
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Q. Hi Ted .
Probably, it would be correct not to lighten but to deoxidize to remove foreign metal impurities from the surface of aluminum. After this procedure, the aluminum becomes cloudy. I eliminated etching entirely to achieve a glossy finish.
- Sarapul, Russia
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A. Hi again.
Please tell us the alloy, the concentration of the nitric acid, and the immersion time.
Nitric acid is an appropriate deoxidize/desmut for only some alloys. Removing the copper but not the silicon if present, for example, will offer a poor surface for anodizing.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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Q. [illegible]31T analog 6063, nitric 15-20%,
time 2 minutes.
- Sarapul, Russia
October 14, 2025
A. Hi again, Andrey.
If it comes our of the desmut tank cloudy it will come out of the anodize tank cloudy.
• No etching or reduced etching may help.
• Although nitric acid should be a sufficient desmut, there are better desmuts.
• The quality of the alloy may not be truly equal to 6063, having more iron for example.
Hopefully a reader with lots of 6063 experience can flesh out this response further.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
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