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How to prevent discoloration on aluminium surface

I hope someone can help

We etch (degrease) our aluminium products of 7 series in caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] (50g/L @ temp. 55C)for 2~3 mins and then put it into chemical consisting of Potassium Dichromate (50g/L) and sulfuric Acid (10ml/L)for removing ashes. We found some discoloration (cloudiness) occur on the surface of the product. Does anyone teach me how to avoid this discoloration happened.

Thank you.

Jarrico
- ssu, pk, Malaysia
2003

You need to dip into 10% nitric acid desmut solution at room temperature to remove the gray smut. If the AL alloy contains lots of silicon, you need to put fluoride ion into the desmut solution to remove the silicon.

S. Y. Yuen
- Hong Kong, China
2003

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