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Black Stain on Aluminum Casting after hard anodizing

Q. Hi all,

I'm having an issue on some Aluminum Castings, which have black stains appearing after hard anodising process.

Anyone has experience this? By the way, the castings have went through FAI and are all sound castings.

Thanks.

Alex Chew
Engineer - Singapore
August 21, 2008

A. Castings require different surface preparation than wrought metal. Apparently you have not reduced the amount of silicon on the surface. You anodize aluminum. You do not anodize silicon. Come back and let us know the alloy. You will need nitric and ammonium bifluoride, we will tell you how much when you say either sand cast or die cast,to reduce the silicon. And do NOT alkali etch.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

August 22, 2008

Q. We are facing Black stains problem after sulfuric acid anodizing on a part, die casting using grade AS7GO6.

We are etching with acid.

Please help

Amir Baig
- Karachi, PAKISTAN
June 27, 2012

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