Technical Topics: Corrosion of Copper and its Alloys Particularly in Plumbing Systems

The main theme of this month's CM is cathodic protection. My own recent foray into this area was covered a couple of articles ago. I'll try and work in a bit about CP this time but mainly I want to talk about copper. This was the first metal whose corrosion properties I got to know intimately (during my time at the BNF in the 70s). As most people know this metal is relatively noble (Eo of +0.34V), it is a good cathode (hence need to beware of joining it to other, more active metals) and is relatively easy to extract from its ore (used to be found native).



Early on I was given a copper spike to examine brought up from the sea bed after Sir Cloudesley Shovell's ill fated fleet sank of the Scilly Isles in 1707. Despite being submerged for over 250 years in sea water this was virtually uncorroded as was a pair of brass dividers whose steel points had gone long before (possibly an example of adventitious CP - some hundred years before Humphrey Davy "invented" it as a practical way of preventing corrosion!).

Although these examples of corrosion resistance were impressive, nonetheless into the BNF poured a constant stream of copper (and brass) failures including hot water cylinders, copper pipes from both cold and hot water installations and dezincified brass fittings. Back then a variety of mechanisms were invoked. Some of these are much less common these days eg. the notorious type I pitting of copper pipe due to carbon films has been almost eliminated and even the classic type 2 pitting seen in soft, hot water is quite rare. However, other failure modes continue to cause trouble, eg. microbially induced (MIC) pitting and erosion corrosion: general corrosion of copper is also not uncommon resulting in blue water which can affect taste and cause staining problems (see photos). The latter relates to copper's need to form a uniform stable protective film - normally copper hydroxy-carbonate above copper oxide. A number of factors affect whether this film forms satisfactorily, one being water stagnation.

So there could be two identical houses built at the same time; one may exhibit blue water corrosion whereas its neighbour gives no trouble. Sometimes this relates to how soon after completion the house has been occupied. If water has been left for some time in the pipework, the formation of the protective film, as indicated above, is hindered (probably because this film needs oxygen and CO,, both of which get rapidly depleted).

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Regarding hot water cylinders a friend (KA from Huntingdon) has had to replace hers recently "due to corrosion" (unfortunately it had gone before I got to the scene- copper still has a good scrap value!). The mechanism in this case may well have been a form of type 2 pitting (CP could help here eg. fitting a magnesium anode) and this normally relates to water composition and/or wrong temperature of operation. It is often these kind of factors which cause copper and brass to fail not the poor old plumber with his "mountainous quantity of aggressive flux" or his use of a "dodgy piece" of copper.

Before I leave copper there is one other interesting aspect of this metal in relation to Corrosion Intercept Technology (CIT). This was used in the bag which contained my guitar strings (seeTT No 2).This bag is coppery in colour and I have been told that the claimed "gas neutralizing barrier" is in fact finely divided copper. Can this adsorb aggressive gases? Well copper is more prone to react with sulphur than with oxygen so it might well help remove SOZ, H2S and the like . It is a moot point whether it will adsorb oxygen and water! It is additionally claimed that a "block?" can be an effective guard against corrosion if placed in a confined space containing the item to be protected.



Very recently I had cause to investigate my mother's Clavichord which had lain undisturbed for several years. It has some 70 strings and I tried tuning them. It worked fine for the first half dozen but while tightening (only a very little!) the seventh there was a small crash and two pieces of string were suddenly present where one had been before! SEM evidence of the fracture surface shows corrosion. If I'd put CIT material inside the sound box ten years ago would it have helped? We'll never know! Meanwhile I'm going to have to go to the Early Music Shop to try and get some replacement strings!



Anyway if anything in this article struck a chord please get in touch. The e-mail address is as usual douglas@harrbridge.freeserve.co.uk

PS. Thanks are extended to Trevor and to Phil for help with this month's column and particularly to Phil (Munn) for providing the photographs. Also to Stuart for suggesting it!