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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Chrome-free galvanizing gets black spots
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.
- Gunsan, South Korea
July 20, 2021
A. Any white rust prevention system (Chrome, polymer etc) is only a temporary system, and will not last for more than a few weeks.
If you wish to store galvanized products for longer then you need to either store them dry, or separate the items.
Be aware that how you separate them could also cause marks. For example some timbers leave sap marks.
And also remember that galvanizing is not an aesthetic finish, it's a functional one. It is there for corrosion protection. No-one should claim that galvanizing looks beautiful, but they can claim that it looks better than rusty steel.
Geoff Crowley, galvanizing consultant
Crithwood Ltd.
Bathgate, Scotland, UK
August 10, 2021
Thank you for answer. Consumers here seem to be more sensitive to sunspots they see for the first time than to the effect of improved white rust.
- Gunsan, South Korea
August 23, 2021
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