| |
Letter 0009
Corrosion of Hot Dipped Zinc Coating
-----
We have a number of hot dipped galvanized panels that were stored
improperly and have developed "wet storage stain". Most areas are
whitish in color, which I understand is zinc hydroxide, but the real
question is what is the black staining? Chemical analysis
reveals only zinc and oxygen. I suspected iron would be found
incorporated into the black areas, but none was found? Anyone
have any ideas? Aloha,
Bruce E. Liebert
University of Hawaii
-----
Aloha, Bruce.
It is believable that you are detecting no iron. While iron-zinc
compounds are formed at the interface, they do not extend all the way
through the coating; rather the surface of the hot dipped coating is
nearly pure zinc. Further, as noted in ASM International's
Volume 5: Surface Engineering,
the white corrosion products are about 500 times as voluminous as the
zinc metal, and sometimes the damage to the coating is much less
severe than it looks; so it is possible that you still are seeing
only zinc even after what looks like significant corrosion activity.
It is desirable, and often doable, to remove 'wet storage stains.'
Consult the above-mentioned text or the booklet "Painting Galvanized
Structural Steel" by the Zinc Institute and the American Hot Dip
Galvanizers Association for guidance.
I don't know what the black stains are, and hopefully a reader who
is more knowledgeable than me will chime in. But in the meanwhile, is
it possible that they are a mildew? I'm no lab technician, but if
wiping a test piece with bleach doesn't remove the stain, it would at
least prove me wrong.
|
|

Ted Mooney, P.E.
finishing.com
Brick, NJ
|
|
-----
Studies of the corrosion of HDG substrates in automotive
applications have shown that typical corrosion products are
zinc hydroxychloride and zinc hydroxysulfate. Now, your
reader didn't find any chloride or sulfate in his analysis,
but then he didn't intend to expose his HDG panels to
corrosive environment, either. without knowing the nature of
the atmosphere where the panels were stored, it's not easy
to determine the nature of the corrosion product. Happy
hunting! -- GG
George Gorecki
- Naperville, Illinois
-----
The black stains "always" appear on galvanized surfaces
before the white corrosion products form, although many
people do not always see them. Often the black spots are
only visible if one looks "into" the surface rather than
"at" the surface. The black stains appear to be sites for
the white corrosion products.
Lindsay Williams did some work at least 15 years ago at
Materials Research Laboratories in Victoria, Australia on
the effect of impurities on the corrosion resistance of zinc
coatings. He reported this phenomena. I do not have any
references to his work unfortunately. I do recall that some
of his papers were discussed in the annual
"Metal Finishing" "Technical
Developments for 19..". Perhaps Bruce Liebert can take it
from here.
If the corroded zinc coating is to be coated with an
organic material, it is essential that these corrosion
products are removed prior to coating, otherwise the
corrosion will continue beneath the coating. An etch in 5%
phosphoric acid should work. If the zinc surface is to be
powder coated, this etch should be followed by effective
cold water rinsing, grain refinement, zinc phosphating and
drying prior to coating. I understand that a chromate
conversion coating following the etch can be effective.
Hope this assists, Kind regards, Mario
|
|
|
Mario Pennisi
Mining & Metallurgical Engineering, The University of Queensland,
Brisbane, Australia
1 2
Dear Reader: please choose what you want to do.
- I want to post a
question on a different
subject.
-
- I want to answer this question publicly
-
- This letter is within a
monitored
forum, very much "alive" here in 2008. If you spot any broken
links or obsolete info, please advise!
 |