On 12/03/2019 06:47, misterroy wrote:
On Monday, March 11, 2019 at 8:33:38 PM UTC, newshound wrote:
On 11/03/2019 17:37, misterroy wrote:
What could have caused the heating pipe to corrode so badly that it failed? It was next to an external wall and lagged. The are a couple of lengths of the pipe lying outside the building unused for the same 6 years, other than the threaded ends, they look new.
image https://drive.google.com/open?id=1o8...cqBdRGYatmQb6R

Do you, by any chance, have an electro-osmotic damp proof course fitted?

I have had two instances of *earthed* components in contact with damp
masonry corroding in a similarly dramatic fashion. One was a galvanised
back-box (which I replaced by setting-in a plastic one), the other was
the screws and after that four six inch lengths of 316 stainless steel
6mm studding which I set into the rubble stone wall to support radiator
brackets.

I havn't had any trouble with other galvanised back boxes, but this one
was in a bit of wall which the DPC was spectacularly failing to protect.

However given the distribution of corrosion in your case I would be more
inclined to associate it with condensation within the insulation,
especially in the presence of salts.

Does the earthing cancel out the benefit of galvanising the steel, giving the electrons somewhere else to go?

I was going to say no, that earthing is necessary to close the circuit.
Making the DPC conductor positive steals electrons from the piece of
metal, turning them into ions which are then mobile instead of being
part of the structure. But on reflection I think you are saying the same
thing.

It's electroplating in reverse. Similar to the way that sacrificial
anodes are used to prevent corrosion of marine hulls, etc.