The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Zinc carbonate formation and corrosion resistance of galvanized steel

1995

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

---

Hi, Gordon. 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 "wet 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 much more corrosion resistant skin which includes zinc carbonates. While zinc provides corrosion resistance to steel, what slows the corrosion of the zinc itself is these tight, tenacious, zinc carbonate based corrosion products.

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.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004