Metal finishing Q&As since 1989

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Anodizing forged aluminum cast accessories

I am an anodizer from Saudi Arabia.I wanted to know whether we can anodize forged aluminium casted accessories having 10% Mg and 2% Si or vice-versa. I have noticed during etching it turns black and if you avoid etching in NaOH solution it turns black in desmut/anodize bath of sulfuric acid solution of 180-200 gms/lit concentration. Please advise.

Thanks.

Masood khan
anodizer - Riyadh, K.S.A.
2007

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Forgings are better than castings for anodizing due to a denser, more homogeneous microstructure.

The behavior suggests a 10% Si alloy. Caustic etching preferentially dissolves Al, leaving a dark, Si-rich surface. Desmut solutions should contain fluoride when high-Si alloys are processed. Turning black during caustic etching also suggests the presence of Cu, typical of many die casting alloys.

Aluminum casting alloys (especially, die casting alloys high in Cu, Fe, Mg, Si) should be etched in nitric+hydrofluoric* acid rather than NaOH solutions. Etching times should be kept short. Desmutting may not be necessary after acid etching. High Si alloys form gray anodize even with proper pretreatment.
*Ammonium bifluoride may be used in lieu of HF.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2007

My question is can we use potassium fluoride [affil links] instead of hydrogen fluoride in the de-smut? The thing is that I have potassium fluoride but I don't have hydrogen fluoride. Thanks.

Danny Gauley
anodizer - East Angus, Quebec, Canada
March 14, 2012