040. Zinc carbonate for protection of galvanized surface

Letter 040

Zinc carbonate for protection of galvanized surface

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I understand that the white bloom that occasionally forms on damp galvanized steel is zinc hydroxide. I have been told the zinc can be protected from this corrosion if a layer of zinc carbonate is formed over the zinc. Do you know any thing about this mechanism - it's chemistry, how the carbonate layer can be formed, and how effective it is? Thanks for comments in advance.

Gordon J Esplin

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I think the thrust of what you have been reading is that if galvanized steel sheets are stored in a stack outside, or where they can stay moist, and they can't "breathe" because they are stacked, then they are prone to getting zinc hydroxide white rust storage stains . . .

Whereas if they are open to the carbon dioxide in the air--such as when they are actually in use mounted on a building--then there is good tendency for the formation of a more corrosion resistant skin which includes zinc carbonates.

If I have misunderstood, or if anyone has a deeper appreciation of the issue to enlighten me with, let me know please.

Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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