"White rust" on galvanized steel

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I've recently encountered a quantity of galvanized sheet steel which has appears to have what the tin benders call "white rust" on the surface. I assume this is essentially due to oxidation of the zinc and I would think that it would be superficial. Several of the tin benders, however, claim that this spells disaster and that the useful life of the Material when exposed to atmospheric corrosion (oh, say as a roof) has been severely compromised. The material was stored outside in bundles through several rainstorms. The white deposits appear between the sheets, and where the bottom sheets were in contact with the ground. Any opinions?

Robert M. Wilson
aerospace - Wichita, Kansas
"The Opinions Expressed herein are my own and not those of my employer"

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Robert, I agree with the "tin benders", but it depends on its end use whether it has been seriously compromised or just compromised.
It is not an ideal product to work with and some one made a boo boo by letting it set unprotected. That is easy to do, as the top sheet dries out daily and doesn't "rust" while the inner sheets do. Aluminum will do the same thing. Chromate or phosphate and painting with a good paint will somewhat negate the problem. Hopefully , the responsible person learned from it.

James Watts
- Navarre, Florida

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Mr. Wilson:
A very similar question came up on this page last year. I think you will find it interesting reading. See Letter #9

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

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When one purchases galvanized steel it is either "chem treated" or non "chem treated" Chem treatment is a chromate solution applied at the mill to inhibit the formation of "white rust" It is used where the galvanized steel will not be painted afterwards. Examples include guard rails, trash cans, pails.

Non Chem treated galvanized is purchased when the parts are going to be painted. Unfortunately the chem treatment does not allow for typical painting processes. ( I understand that Oakite has a paintable passivation being evaluated by the mills for coil coating applications)

You can specify to your steel supplier the type of galvanized desired.

As far as removing the "white rust" phosphoric acid is very effective and is not too aggressive. Remember the protective zinc has been removed to some degree and a galvanic site has been created. the part should treated and painted or the damaged area cut out during fabrication.
Good Luck

Dan Zinman

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I believe that you should sort out any panels with white rust, and relegate them to the scrap heap. This is a case where preservation issues in storage have been ignored. I would not want them on my roof.

Tom Pullizzi
platronica.com
Falls Township, PA

We purchase large quantities of roll formed galvanized profiles. Can you recommend the proper method of storing this material over a period of 12 to 18 months without

(a) creating white rust on the galvanize or

(b) oxidizing the rolling oil into an insoluble goo.

We weld, fabricate and powder coat the finished products.

Rob Morgan
mfgr - Kane, Pennsylvania

December 13, 2008

Hi, Rob. I think the principal storage issue is that they be dry and be able to "breathe". My limited understanding is that a fresh supply of carbon dioxide from the air is important to make sure the surface maintains a percentage of carbonate to avoid white rust. Storing the panels touching each other, where they can't breathe, seems to be a principal cause of white rusting.

Regards,

Ted Mooney
finishing.com
Brick, New Jersey

February 16, 2009

reading your previous answers to questions on "white rust", besides proper storage, are there any other preventive measures to avoid this?

Ramon Soriano
factory manager - Saudi Arabia

April 26, 2009

how can we monitor and diagnose the white rust on galvanized sheets by chemical analysis?

abbas zare
galvanizing shop - kashan, iran