Pitting along copper tube seam

Did you carry out metallography? Where are the pittings precisely: In the heat affected zone odr exactly at the border? Is there a hardness profile across the weld available?
Michael Dahms

The water may be demineralised but that doesn't mean its 100% pure. Surprisingly low levels of certain contaminants can cause pitting especially in regions where metal has been under mechanical or heat stress. Is there any way you could apply a heat stable paint on the inside surface of the pipes?
Giz
Phil Smith wrote:

Phil,
What is your water chemistry?
Dissolved Oxygen?
Carbon Dioxide
pH
Conductivity
Alkilinity as Calcium Carbonate?
Ed

Phil:
You might start by looking at the Copper Development Association (CDA) web site, Get a cup of coffee or tea to sip, because it loads slowly these days. There are sections on Copper Nickel in general and on marine applications. Also look at the Nickel Institute web site, since they also cover cupronickel.
A good starting point at CDA is: http://64.90.169.191/applications/cuni/default.htm It has a couple articles. The DKI pamphlet says resistance welding should not be a problem. The Corrosion 2000 article by Powell & Michel talks a lot about film formation. If your weld has a funny film, then it could pit.
What have you got for flow velocity, sulfide content, and other environmental parameters?
---------------------- Pittsburgh Pete