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topic 53050

Passivation of Polished Aluminum

A discussion started in 2009 but continuing through 2018

September 8, 2009

Q. I need polished aluminum sheet that will not oxidize and will maintain an emissivity of .05 over time. I believe passivation in nitric acid will do this but want to make sure I am going the right way and to find someone that can do this. Any suggestions are welcome.

Mark DiPietro
Design Engineer - Campbell, California

simultaneous September 10, 2009

probert book
Aluminum How-To

"The Chromating - Anodizing - Hardcoating Handbook"
by Robert Probert

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 per Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,] F class 3.

Bill Grayson
Metal Finishing - San Jose, California

September 10, 2009

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.

James Watts
- Navarre, Florida

September 14, 2009

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,

- Banglore, India

September 15, 2009

Q. Alloy 1100-0 seems like it might be my best choice. It is resistant to corrosion and has very low emissivity. What would be issues with this?

Mark DiPietro [returning]
- Campbell, California

September 17, 2009

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.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

Why does bare aluminum continue to corrode?

October 2, 2018

Q. Dear Sir or Madam, If aluminum naturally produces an aluminum oxide coat, why does it keep producing the white corrosion afterwards? How can I prevent for this to happen?

Leticia Vargas
Automotive - Novi, Michigan, USA

October 2018

"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon

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.


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Ted Mooney, P.E. RET
Pine Beach, New Jersey
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