Pitting corrosion - outside of domestic CH pipe

[mobley]

Hi there,

can anyone offer any suggestions about possible causes behind a recent failure of 15mm copper in domestic CH duty? Piping was installed 15 years ago in an extension. Floor structure is concrete slab sealed with bitumen, pipework sleeved in polythene / fibre insulation and resting on concrete/bitumen. 50mm polystyrene insulation slabs laid on the bitumen, cut into sections to fit around the pipework. Chipboard on top of polystyrene to complete the floor.

Pinhole failure in one of the straight legs, around 50cm from nearest join and on the top surface of the pipe as fitted. A section of pipe around 20cm either side of the failure shows similar pitting, distributed around the circumference. Odd thing about the pitting / failure is that it's clearly from the outside in, the internal surface is perfect. Further along the same pipe run the copper is completely sound.

Here's a picture of a few sections I cut out, cleaned up with wire wool. You can see the pinhole failure on the bottom section


Can find very little about pitting corrosion from the outside, am keen to understand the possible causes to avoid a repeat / assess the risk elsewhere.

Mike

 

[Last Plumber]

I can't open the picture.

You can get impurities in copper and it could be a one off but you can also get corrosion from lots of other sources too.
Was it in direct contact with anything like
Concrete or another metal ?

 

[steadyon]

If the polythene / fibre insulation was the typical "hessian on a roll", then the polythene is not continuous, but is perforated. This can let the strong alkalis associated with damp / wet cement get at and attack the copper.

 

[mobley]

Thanks for the reply. Corroded section was sleeved in insulation and effectively just above the bitumen sealed concrete slab. Pipework is resting on the bitumen in a few places but is mostly suspended just above it by e.g. radiator connections. There was nothing else near it apart from polystyrene insulation to the sides and chipboard over the top.

 

[mobley]

Thanks for the reply - yes it's exactly that sort of insulation with perforated polythene lining. I'd have understood the problem if the pipes had been laid in wet concrete or screed, but they were put in on top of the sealed slab. The slab was poured and left for at least a month or two while the extension was built around it; after the structure was weatherproof it had a few weeks to dry out before the bituminous sealant was painted over the whole slab. The sealant is mostly still intact now, including under where the the leaking pipe was. The damaged pipe / insulation wasn't in direct contact with the bitumen as it was suspended at this point by the nearby radiator connection. So I guess there are plenty of things around that could damage the pipe, maybe more likely it happened before it was installed and wasn't spotted?

 

[Stigster]

The external corrosion/pitting you show in your picture is reminiscent of galvanic action/electrolytic action.

This link shows copper tube that has been determined to have suffered this type of corrosion and it looks a lot like yours.

Corrosion of Copper Water Pipe Examination

 

[lame plumber]

anything laid in or near concrete needs either denso taping yeuch or use copper sleeved in plastic, anything else is pitta and will suffer over time

 

[Chalked]

Do you have an electric damp proof system? The one where they burry a wire in the wall?

 

[mobley]

Thanks all for suggestions and advice. There's no electric damp proofing installed and yes the pictures of stray current corrosion do look quite similar. Unfortunately the lab report doesn't make any guesses as to how it might have happened in their case either!