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White Stuff on Ductwork

Terence McCann

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Anyone know what the white stuff on the duct work is or how it got there? It's similar to the chalking you get on paint that has been out in the sun to long.

I was thinking the ducts were sweating but it's a stretch. It's located mainly on the supply ducts in the basement. The returns are pretty clean but still a bit of the white stuff here and there.

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Reaction of the zinc in the galvanizing to some sort of VOC in the air of the basement.



Why just the supply ducts then Mike?

The guy has a pretty big workbench and likes to tinker. I have to pickup the Radon monitor on Wednesday so I'll poke around a bit to see if there might be chemicals he has been working with.

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The white stuff is zinc hydroxide aka "white rust". The most common reason is that the ducts were stored in a wet area before installation.

I've read that the white coating is not a concern as it is the formation of a protective layer. Other stuff I've read about galvanized coated structural steel sez that white rust is bad, indicating the galvanized coating has failed and there is no longer any protection.

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I agree with Bill, that the pipes had the coating before they were installed. I would have called it oxidation, probably from exposure to the weather in the lumberyard. The duct tape looks like it has been applied over the coating.

I wouldn't even mention it in a report.

The cheap plastic duct tape I will mention verbally, replace it with metal foil tape.

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