Passivation of Polished Aluminum

A. Dear sir.

best for aluminium is anodising only . you can choose green / blue/ or choice of your colour.after anodising it can be sealed by solutionsing at it will protect a corrosive resistance.

with warm regards,

A. This is the old problem of the term "passivation" used for multiple processes.

If the concern is surface contaminant iron turning to rust, then a stainless steel type passivation (nitric or citric based) is what will remove that iron for you.

If the concern is to protect the aluminum from something along the lines of chloride corrosion, some sort of coating or anodizing is probably best.

Of course normal oxidation on the surface is something you DO want, aluminum naturally forms an aluminum oxide layer.

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

(note: this book is two volumes)

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A. Hi Leticia. This issue is easier to understand if we postpone the discussion of aluminum for a couple of sentences and think about steel first. We all know from a lifetime of experience that unprotected steel will soon start rusting and just keep rusting. The reason it keeps rusting is that the rust is loose, fluffy, and hygroscopic -- the rust contributes nothing at all towards sealing the underlying steel away from the environment. On the other hand if you paint or lacquer steel, it helps keep water and oxygen away from the underlying steel and it can slow the corrosion.

Now, back to aluminum. Aluminum is a very active metal that reacts instantly with oxygen to produce an aluminum oxide white rust. In an ideal situation that aluminum oxide would be tight, adherent, and waterproof and would shield the underlying aluminum from continuing corrosion much like a paint or lacquer would; in a less ideal situation the aluminum oxide would be porous and hygroscopic, and ineffective in shielding the underlying aluminum from the environment.

Anodizing is a process of very carefully engineering and controlling the formation of that aluminum oxide in order to make it tight, adherent, and waterproof. If we simply put bare aluminum out into the environment and let the oxide form as it will, what usually happens is, depending on the grade of aluminum, the localized corrosion forces, etc., ugly white corrosion and pitting build up. Aluminum needs to be protected from the environment with anodizing, or chromating, or chromating followed by painting or clearcoating. Good luck.

Regards,