Black Stains on Hot Dip Galvanized Zinc Coating

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Q. 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

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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, New Jersey

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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

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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

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