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topic 5229

White Rust Attack! What's the Cure?


(2000)

Is my roof decaying and what can I do about it? I had a company grind down, seal and paint my 1920's galvanised iron roof. 7 months later white streaks began appearing and are are now spreading all over the roof. The paint company says its not the paint. The roofing company says it is 'white rust' which their guarantee does not cover. They cannot really explain what white rust is and what it is doing to my roof. Can anyone help with: explaining what white rust is what it is doing to my roof how urgently I should treat the problem what I should treat the problem with?

Thanks.

Judy Norton
- Johannesburg South Africa


(2000)

White rust is the common name given to the zinc compounds that are formed by corrosion of the zinc coating on your galvanized roof. What appears to be happening here is that the paint is not protecting your roof from corrosion. I would be curious to find out why the guarantee does not cover this condition.

The zinc is put on the galvanized sheet to protect the steel from corrosion and the formation of white rust is one sign that it is doing its job. In a normal atmospheric exposure, this condition may stabilize once the zinc surfaces have been coated with the corrosion products. I hope that the painters have not applied some chemical treatment that is accelerating the zinc corrosion.

larry hanke
Larry Hanke
materials testing laboratory
Minneapolis, Minnesota





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