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Anodizing aluminum with formic or citric acid

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probertEthumb Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

You'll love this book. has sold 880 copies over the years without ever a single return request :-)

Q. Hi, I would like to ask whether it is possible to anodise aluminium using less aggressive acids than sulfuric such as formic or citric.

John Payton
- Norwich, Norfolk, England

A. To me, the two nastiest acids are hydrofluoric and formic, so I would not even start to consider formic.
Less aggressive acids might be phosphoric or boric/sulfuric. Literature is available on both.

James Watts
- Navarre, Florida

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"

by Wernick, Pinner
& Sheasby

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A. Hi John. It is possible to anodize in just about any acid -- if by 'anodize' you mean make the part anodic and build some sort of film on it -- because almost any acid will conduct electricity. But the characteristics of that film will vary tremendously based on the anodizing conditions and they would not approximate the film formed in 10-12 percent sulfuric acid.

Remember that in sulfuric acid anodizing the sulfuric acid is constantly attacking and dissolving the anodized film, leaving a honeycomb type structure, and other acids may not dissolve the anodizing i a similar way.

tartaric acid [on eBay or Amazon], phosphoric acid, chromic acid and boric acid are a few of the commercially used acids, but the coating does not look or function anything like a sulfuric anodized surface.

"Aggressive" toward human biology and aggressive towards aluminum are not the same thing, and I'm not sure which you mean. Good luck.

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

Readers may wish to also see:

Topic 43230 "Alternative to sulfuric acid for anodizing aluminum"

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