The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Copper pipe/ galvanized steel nail plate contact corrosion?

We are about to look at a section of 1/2" copper pipe that reportedly leaked due to contact with a steel "nail plate" (protective plate installed over wood framing). Is this possible? My understanding is that steel/zinc should be "sacrificial" to copper and hence, any corrosion would have affected the steel plate. I'd appreciate any thoughts.

Mark Kellogg
Materials test lab - Lafayette, California, USA
2005

It's hard to see a galvanic corrosion problem here. Even with sufficient moisture from condensate on the copper pipe it's unlikely the copper would succumb to the nail plate. It would be my thought to look for a water hammer condition causing the pipe to rub back and forth against the plate and simply wearing through at that spot.

Bob Collins
- Conroe, Texas
2005

---

A potentially perplexing problem, but there is insufficient information to make any judgment. Firstly, it is not known how the copper pipe has been used; secondly, it is not known if it has corroded or if it has been punctured by a nail or something similar. Thirdly, it is not known what the copper or galvanised steel have been exposed to. Until these factors are known, everything is speculation. However, saying that, there are a few fundamental factors; firstly, for copper to corrode it must be anodically polarised. This can only occur if there is a corresponding cathodic electrode. I agree with the original submission that galvanised steel will usually act as a sacrificial anode, thereby cathodically polarising the copper. However, under certain circumstances reverse polarisation can occur; for instance copper IUD's are made from pure copper and stainless steel. Under the correct conditions the stainless steel becomes passivated and the copper dissolves.
I would keep an open mind as to what is going on. Once the sample has been examined, then, and only then can conclusions be drawn. I would also recommend that during examination of the copper pipe special care is taken to look for (and test for) pitting corrosion. There are at least three types of pitting corrosion known in copper based water systems. These are well documented and anyone involved with this type of issue will be fully aware of their characteristics. There are also numerous other methods by which copper pipes can fail, so look for those as well. Once you have examined the failures, perhaps you will show what you find.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2005