Anodizing aluminum with formic or citric acid

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.

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby

(note: this book is two volumes)

on eBay or

AbeBooks

or Amazon

(affil links)

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 ⇦ this on eBay or Amazon [affil links] , 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.