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Possible galvanic action in water pipes

2005

We have galvanised steel feed pipes from galvanised roof tanks to flats which internally contain galvanised steel, and/or black iron, stainless steel, and copper pipes to various
bath/basin/sink taps.

Rust appears intermittently in some, but NOT ALL flats with similar pipe configuration; one in particular, where the above ground pipes and calorifier in the flat are in copper has particularly bad rust from all hot taps. Another flat immediately above this, with the same copper configuration, has NO rust. There appears to be little or no rust in the cold water which is delivered to flats by the exact same medium.

Can you please advise (A) what is likely to be occurring, (B) the likely cause and (C) the method of cure, bearing in mind that there is no offtake where the water enters the flats and it cannot therefore be tested. Most of the pipework in the flats is buried in floor screed.

John Defty
Cedar Grange (Poole) Management Co. Ltd., Freehold Property Management. - POOLE, Dorset, United Kingdom

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Pipe corrosion is a common problem, especially when there are two or more different metals in electrical contact with each other. This type of corrosion is caused by a galvanic couple being set up between the different metals, with the more electronegative one acting as the anode and the electropositive one the cathode. The anode dissolves or corrodes. The important features of such failure are that the two pipes are in electrical contact and the water has adequate "corrosive characteristics"; that is, the water will attack the anodic pipe. One reason for the variability in the extent of corrosion could be that there is no electrical contact between the two types of pipe; this is one simple way of minimising the corrosion effects - just put in a plastic connector. It is also possible that the sequence in which the pipes are connectedc could influence the extent of corrosion; putting the most electronegative metal in direct contact with the most electropositive will result in an increased chance of failure. There are two types of copper pipe corrosion found in the UK, type 1 and type 2. Type 1 is only found in cold water systems and type 2 is in hot water systems. These will be well known to the water supplier, so it would be a good idea to ask them whether the water supply supports this (or any other) type of corrosion. "Rust" is often used as a general term to describe the formation of corrosion products on metals, but if by "rust" you mean the brown stuff we normally see on iron and steel it suggests the galvanised pipes and/or black iron are being attacked. This could be due to a miriad of reasons, including a galvanic couple between the iron containing pipe and a more electropositive pipe, puncturing by a nail or tack, erosion corrosion, etc. If the galvanised pipes are zinc coated internally, it would suggest the zinc has been removed, probably by erosion, to expose the iron, which itself is being removed by the eroding material. You do not say whether there have been any failures in the pipework, so I supsect the only current symptoms are associated with dirty water. If there isa afilure, you will obviously have to change the pipe, but since it is embedded in concrete, this will not be a simple job! If you have any pictures of the corrosion products and/or failed pipes, that would be very useful. It will also be a good idea to ask the water supplier for further advice.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2005

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2006

GALVANISED SEAMLESS STEEL PIPES FOR STREET LIGHTENING LEADCONDUIT. DOES SUCH A MATERIAL RESIST AGAINST CORROSION IN TERMS OF TIME?
WHAT DO I HAVE TO DO IN ORDER TO BE SURE THAT IT WILL LAST?
I COULD USE A HARD PVC TUBE, INSTEAD. BUT THE CLIENT WANTS STEEL PIPES.

FILIO SAVVA
AKTOR SA - ATHENS, GREECE