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Is it Zinc Oxide or White Rust on Galvanize?

 

Hi,

What is the actual term for the white powder produced from hot-dipped galvanised? It leaves a black stain when the powder is wiped off. Some call it white rust, too much of dichromate powder, zinc oxide....etc. If it is really zinc oxide wouldn't it mean that a potential of failure for the zinc to protect the base metal (probably over time)?

I have this problem whenever I galvanise my lipped-channels. According to my galvaniser, it is something they can't control or avoid. Then, they tell me it is the way I bundle them that is wrong (which is bundled the same way as other suppliers). How true is this and what are my arguing points? 'Cos I have tried others and they don't give me this problem at all.

Thank you all.

Jeremy Yap
- Selangor, Malaysia

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What you are seeing is indeed zinc oxide, which is also referred to as "white rust." It is not truly "rust" in the traditional sense as would be experienced with bare, uncoated steel. If your product is bundled wet, or in the presence of high atmospheric moisture (humidity, condensation) then this can be quite common, especially if you are seeing black after wiping/cleaning.

It doesn't appreciably reduce the protection that the zinc provides to your coated product, but is a cosmetic problem. Chromium passivation of the surface after zinc coating will help prevent this, but it sounds like you've got a moisture problem.

Karl A. Hermann
- Fort Wayne, Indiana