Pitting Copper Pipe

[lbp plg & htg]

So here's the story.

Asked by a friend to fix a leak at his house where water had appeared to be coming through light fitting downstairs - simple?

Torch shone through spotlight hole to see pin hole in pipe upstairs, up onto landing and pipeboard infront of cylinder cupboard removed to enable access. Under the pipeboard I have access to majority of primary and secondary pipework - all in copper. Using mirror I can see from underside that the hot water pipe is the only pipe to have pitted - cold and flow/ return all ok where access was made.

Problem is this hot pipe is pitted all the way I can see - section cut out about 20" long and there must have been around 15-20 holes formed & forming all along the bottom of the pipe. Advised him he needs to speak to his insurance company as next course of action.

My need to know is what causes this? Why just the hot water pipe? Will this spread to all other pipework? Anyone with experience in this I would be grateful for feedback.

Property is 15-20yrs old, standard boiler and hot water tank.

 

[stevetheplumber]

it could be pinholing due to flux left on surface

 

[lbp plg & htg]

Thought about that but why just the hot water pipe? Also pitting along visible pipework about 2m long even where theres no solder.

 

[Best]

I have seen this twice in recent year on two different homes. One was on a copper pipe feeding the cold tank water to a toilet & the other was on a cylinder vent pipe in a roofspace (& I think there is another pipe the same). The vent pipe was run up in an angle & the pitting- shown as white dots outside & dots green corrosion inside, was on the bottom part of the angled pipe.
Both houses the 15 mm pipes were Black Label British copper & were in the region of 20 + years. None was due to the flux.

 

[JCplumb]

You get more corrosion on the pipes around where the feed pipe is teed in from the f/e tank. The last time I saw this I checked out the f/e tank and it was about 2 inches deep in rust and suchlike. Did they happen to have a galvy f/e tank? That would be my first guess.
Just out of interest, does anyone know what metals would make copper the annode? I'm sure lead is one, not sure what others though and tbh it's something I should know.