Summary for white spotting galvanized sheet metal Discussion
white spotting of galvanized sheet metal is most likely wet storage stain or zinc oxide buildup, according to Ted Mooney, P.E. He explains that galvanized steel forms a protective zinc-oxide layer, but if that layer is repeatedly broken or the metal is stored where it cannot breathe, white corrosion can build up and keep returning. He notes that if the corrosion is on the side without insulation, the insulation is not the cause. For parts that must be used, he recommends mechanically removing all corrosion, wiping with a stabilizing solution, and then painting soon afterward with a coating made for galvanized surfaces. In the follow-up, Al Harding says the parts are Tomahawk live box traps made from 18-gauge galvanized wire, and Ted suggests storing them on cheap untreated wood so the bottoms can breathe, while avoiding pressure-treated wood because its copper can accelerate corrosion.
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?
Abbie Tremain
construction – Marietta, GA, USA
2004
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.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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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.
Jeff Watson
– Pearland, Texas
2004
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?
Ahmed Fahmy
Q.C. Engineer in Siac CO. – Cairo, Egypt
2006
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.
Al Harding
– Deplanes, Illinois
April 20, 2012
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
Need quick confidential answers? $25
Need project assistance? $100/hr.
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,
Al Harding [returning]
– Desplaines, Illinois
June 26, 2012
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
Need quick confidential answers? $25
Need project assistance? $100/hr.
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