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Ted Mooney, P.E. RET
Pine Beach, NJ

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White spotting of galvanized sheet metal

I have some galvanized sheet metal that has insulation between it. The sheet metal is starting to show large spots of a white powder look. When wiped off, it comes back. What is this, what causes this and how do I get rid of it?

I certainly can't criticize a design based on a sketchy one-paragraph description, but this sounds like wet storage stain which will keep coming back because the insulation doesn't let the galvanized surface breathe. See topic 09 for one discussion of the need for carbon dioxide from the air to get to this surface. A careful search of the site will reveal many more such discussions.

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If the corrosion is on the side of the metal without insulation the insulation won't have anything to do with the corrosion. Non-passivated galvanized metal will corrode like this during storage or anytime the protective zinc-oxide film is repeatedly broken by contact. What happens with galvanized metal is that the zinc coating corrodes almost instantly, forming a stable, self limiting layer that doesn't allow oxygen to get to the remaining zinc. If that layer is broken repeatedly the corrosion will build up. Zinc oxide is white, so the areas where it builds up will have that white scaly look.

If you have to use these parts, the best way to deal with them is to mechanically remove all the corrosion, then wipe the surface with a stabilizing solution and follow with paint. Make sure you select a paint made for galvanized surfaces and get the parts painted soon after cleaning.

How to remove zinc hydroxide (whitish spots) from galvanized steel sheets?
and is it able to happened for galvanized steel sheet with galvanized coat=270 g/m2 or there are no relation between the existing of whitish and the galvanized coat thickness?
what about use oil to remove whitish spots?

Q. Prevention of white build up (oxidation?) on animal box traps stored outside on metal shelving. No such build up occurs when traps are used outside.

A. Hi Al. As you see, we appended your inquiry to an existing thread which may partially answer it for you.

If these traps are galvanized (you didn't say what they are made of), the problem is probably "wet storage stain". You may be able to improve the situation by having the boxes chromate passivated, but the real problem may be that they can't "breathe" in storage.

Zinc protects steel from rusting, but it is not corrosion resistant itself. Rather, the zinc carbonate corrosion products form a very tight, hard, glassy, impervious skin that seals the item from the environment. If the box can't get dry or can't freely breathe carbon dioxide from the atmosphere, the zinc corrodes as a white oxide instead.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 26, 2012

A. Thanks for the reply. They're Tomahawk live box traps for nuisance wildlife; traps are 18 gauge galvanized steel wire. I thought of placing wood underneath when stored on what appears to be stainless steel shelving. Sorry for delay -- this is the most I've ever used email or iPad. I'm not very techno savvy! But I'm a darn good trapper. Thanks again.

Sincerely,

A. Hi again.

That's a good idea, both for electrical insulation and to let the bottoms breathe. But don't use pressure treated wood (it contains copper which accelerates corrosion of galvanized). Use cheap scraps of untreated wood, and throw it way when it gets too rotten. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 26, 2012

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