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
Accumulation of moisture inside the lagging plus warmth from lagging equals accelerated oxidation?
Most likely significant electrolysis due to a f***ed mains earth system and bonding.
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.
water and air usually or maybe the plating was naff? Brian
It is actually quite surprising how slowly iron corrodes under normal but adverse conditions. The upper bound on metal loss rate is usually assumed to be 0.1 mm / year even in marine environments. You can get
*pitting* at much greater rates than this, which is why car bodywork was so dreadful a few decades ago before manufacturers bothered properly.But 0.1 mm/year explains why Victorian Piers and lock gate mechanisms are still there, even without repainting.
Does the earthing cancel out the benefit of galvanising the steel, giving the electrons somewhere else to go?
If it is attached to copper pipe elsewhere in the system, zinc and copper are perfect for galvanic action. Depends too on the quality of the water in the system. Inhibitors are available.
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.
No, galvanising allows even thinner sections (corrugated iron, for example) to survive for decades.
Is there some way to stop this corrosion in a CH system using copper pipes & a galvanised steel cylinder/tank?
NT
The reason why galv water tanks rusted was the copper pipes they were attached to. Now we have plastic tanks.
Generally, any inhibitor in the system won't be in the header tank, just in the pipework. Obviously, a problem if it's a galv.steel tank.
But it protects steel radiators in a copper pipe system. (Somewhat) Another reason to use plastic pipe instead of copper.
The way galv tanks are protected is with a magnesium "sacrificial anode" Has to be replaced every few years as it "rots" away instead of the tank.
You normally pour the inhibitor into the header tank from where it circulates round the system.
You won't get much in the system if you do that. The system has to be partially drained and then the inhibitor added to get in in the pipework/radiators.
Or if no header tank, the highest rad after you have de pressurised the system a bit and let some air in.
That goes without saying. and you will stll have some inhibitot in the header tank
thanks, useful. Galv tanks are so much cheaper than copper.
NT
Cast iron and "ordinary" steels (like anchor chain, for example) have very similar corrosion rates in marine environments.
There's the cost of the sacrificial anode replacements to consider too.
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