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Corrosion

I have a question of corrosion in a residential application. A copper water pipe is being supported by a galvanized steel bracket. It is located in a basement, and there is evidence of a high moisture situation, and possibly the pipe is leaking. The pipe has some kind of "white powdery" corrosion where the bracket holds the pipe. Is this galvanism corrosion or just rust from the steel?

Adam Sommer
- St. Louis, Missouri, USA
2002

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Galvanizing is a hot-dipped zinc coating. The corrosion products of zinc are usually white and powdery and rather voluminous. So it is possible that that is what you are seeing. But it's also possible that you are seeing evaporated water 'hardness', i.e., calcium carbonate and other minerals that are in the water in small quantity, but are left behind when the water evaporates (usually this will be in the form of miniature stalactites).

You have the ingredients for galvanic corrosion, whereby the corrosion of the zinc will be accelerated because of the contact with a large area of copper. But whatever you can do to keep the area dry is probably more important, all around, than worrying about the accelerated corrosion of the zinc due to contact with copper.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002