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"Chrome-free galvanizing gets black spots"

July 20, 2021

Q. I am an employee of a galvanized steel pipe manufacturing company. Manufactured galvanized steel pipe is being post-treated with the recently improved chrome-free coating. The problem is that white rust does not occur due to the improved corrosion resistance of chrome-free when stored outdoors, but after about two months, when you look at the product, black spots appear in the part where the products are in contact. In a high temperature and high humidity environment, as far as I know, if the corrosion resistance of the coating is insufficient, white rust occurs due to the combination of zinc and oxygen. If the corrosion resistance of the coating is good, the reaction between zinc and oxygen is blocked, resulting in black spots due to incomplete reaction. As it flows, the range of spots expands and becomes darker, and eventually white rust occurs when the coating layer disappears. I was wondering if I was doing something wrong, so I left a question like this. Please help.

61293-1

KyungKen Lee
- Gunsan, South Korea
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