Passivation of Polished Aluminum

A. Hi Emily,

Unfortunately not. Stainless steel is a passive, inherently corrosion-resistant material, just requiring removal of any traces of iron on its surface with citric or nitric acid. Aluminum is a very active material best protected from corrosion by anodizing, chromate conversion coating, or clear coating (or paint or powder coating if you don't need to retain the grey metallic look)..

Luck & Regards,


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

A. Emily,
"Passivation" generally means different things for different metals.

The stainless-steel style passivation treatment is merely the removal of iron from the surface, so it's applicable to non-ferrous metals when iron contamination is involved, but otherwise it's not applicable.

The natural passive aluminum oxide surface layer forms with no intervention, you don't have to do anything.

Surface protection that's better than the natural oxide layer would involve one of the processes Ted mentioned.

A. Hi Mildred.
The usual passivation treatment for aluminum is chromate conversion coating, which shouldn't be heated above 140 °F in your drying process. Anodizing might be ideal, but maybe not. If you can't tell us what the fill material is, it might be best to privately retain a confidential consultant because the answers you receive here might be misleading when we don't know the actual situation and are just conjecturing.

Good luck.

A. It is always difficult to know the meaning of a one-word finishing term like 'passivate', which is prone to be used as slang, when it is offered out of context; the more so if a language translation occurred somewhere. My guess is it is referring to chromate conversion coating of the aluminum, but you really need to find out from the customer what they want.

Luck and regards,

A. Hello, Jie. The word "passivate" is troublesome because aluminum cannot be passivated in the same sense as stainless steel; but it can be passivated in the same sense as zinc plating, i.e., with a chromate conversion coating. What exactly do you mean by passivate, and why do you need to do it?

Regards,

A. Yes, oftentimes one must be specific about what is meant by "passivation".

If the goal is simply to remove any surface contaminant iron that may have been deposited during machining and tooling, stainless steel passivation procedures apply. This can be done quite easily with citric acid.
adv. Let us know if we can help.

A. Nitric acid passivation is a process used to remove free iron from Corrosion resistant steels (read stainless steel) and is an etchant, not a passivation process for aluminum.

Try a clear chromate or trivalent chromate perMIL-DTL-5541 [⇦ this spec on DLA] F class 3.

A. For high end material, I would look at bright dip and clear anodize and sealed with DI water. You might find a suitable clear coat for it also.

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.

ASM Metal Handbook
9th Edition, Vol. 5
"Surface Cleaning, Finishing & Coating"

on eBay or Amazon
or AbeBooks

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A. Hi

I believe that James is right. Bright dipping or electropolishing followed by a very thin anodizing is the way to go. ASM's Metals Handbook Vol. 5 "Surface Cleaning, Finishing, & Coating" has interesting coverage of this topic.

Luck & Regards,


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

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

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or AbeBooks

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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,