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Letter 720
"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
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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.
+
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
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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,
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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
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