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Letter 0009

Corrosion Black Stains on Hot Dipped Galvanized Zinc Coating

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Some time ago, I asked whether or not anyone had any idea what was responsible for the black storage stains on galvanized sheets.   From the responses I have read, there does not seem to be an answer. The question of mildew is interesting, but since the black stain is so resistant to chemical attack, I would tend to discount that possibility.   Therefore, this question remains unanswered; however, we intend to look into this matter further.

Bruce E. Liebert
University of Hawaii

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Aloha, best greetings from Germany. My name is Jörg Martin and I have a solution to your problem.   When you put a zinc surface on Steel material you definitely have an activated skin. Dip your part in a 1,5 % HNO3 and look at the Surface. When you see a reaction then you have "reaction stains".   You must passivate the surface with yellow or transparent chromate and you have no more black stains.   I hope that is a solution.   Best regards from the cold of Germany .  

Jörg Martin
- Germany

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Black staining may be caused by friction of the surface!

Wolfgang Schwarz
- Germany

Thanks Wolfgang. I believe that is sometimes called "fretting corrosion" if Bruce or the readers which to search the site for "fretting corrosion".

Regards,


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

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According to the AGA, white rust although usually white to grey can turn black in advanced stages. White rust is caused by the lack of carbon dioxide flowing across the surface of galvanized material. The desired corrosion product on galvanized steel is zinc carbonate. White rust is zinc hydroxide that has been inhibited form evolving to zinc carbonate due to the lack of free flowing air on the galvanized surface.

Mike Kern
- Plymouth, Indiana, USA

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