Letter 0009

Corrosion of Hot Dipped Zinc Coating

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

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

Jörg Martin
- Germany

Black staining may be caused by friction of the surface!

Wolfgang Schwarz
- Germany

According to the AGA white rust though 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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