Cathodic Protection of Coated Pipelines
It is with some trepidation that I embark on writing about the topic of Cathodic Protection as I know there are quite a large number of Institute members who make their living applying and advising on CP and I am sure they know a lot more about it than I do (or ever will!). My lack of knowledge is partly because it is not a subject area that I have (until recently anyway i.e. almost into my dotage!) done any specific research into. However, it is a subject which I have always been interested in (a really rather unique way of protecting metals "invented" by Humphrey Davy to protect ships and first used on pipelines, using zinc, in the 1930's in USA).

In a nutshell the aim is to lower the voltage/ potential of metal to below a value where it no longer has any thermodynamic tendency to corrode. This can be done by attaching the metal structure to the negative terminal of a battery (but beware, I tell my students, it will only work if you have conducting environment i.e. you need to run your car through sea water!) or by attaching reactive anodes e.g. zinc to the structure e.g. hull of a ship. The latter don't always work! Thus in the 1970's while working at the BNF Metals Technology Centre, I was asked to investigate some copper cylinders where the central magnesium anode had not "done its stuff" properly resulting in failure. This was attributed to the magnesium anode passivating i.e. its potential became higher than that required to protect.

In relation to coatings while working at the Paint Research Association I was asked to investigate the Cathodic Disbondment test facility (the alkali produced under CP can cause loss of adhesion and this needs to be tested for). The lab was in a very remote corner of the PRA - latter was a bit of a rabbit warren - it took three goes to find it! When I did, I found several BBC computers, lots of little potentiostats, many, many wires connected apparently randomly to electrodes and lots of pieces of painted metal with a central hole in the paint.

The man who ran the lab had left several years earlier and I'm afraid my efforts to rejuvenate it and offer BS 3900 part 16 (?) never really got off the ground. So it was really only during this last year that I got into the subject in a more organised way when I supervised a project at Northumbria specifically on Cathodic Protection of Pipelines. Certainly it has opened my eyes to the complexities inherent in controlling and monitoring CP. Coating of course provides the primary protection and this can be cold wrapped tape, 2 or 3 layer PE or various epoxy - and/or urethane paint coatings.

However, because the coating may become damaged, e.g. during construction, 3rd party damage in use or because of ageing of the coating such that it's resistance properties diminish, cathodic protection is needed as back up. The challenge is to monitor effectively the CP. In Finland about half of CP stations are remotely monitored. However, manual measurements are made at the other half with test posts approx. every 1km and every point measured annually with bi-monthly measurements made at selected points. CIPS (Close Interval Potential Survey) is commonly used.

In the case of the Northumbria project the company which the student was working with wanted to operate and understand a device known as a Pipeline Current Mapper. However, before starting on that we felt we needed to do some lab investigations with typical coated pipes to try to understand a bit better what was going on (Fig. 1). We did these experiments in sand and sea water with wrapped or coated pipe and with impressed current or magnesium anodes. One of the more entertaining things we discovered was that magnesium anodes do not work too well in the equivalent of sea water - after just a few days the anode had virtually disappeared! (Fig. 2). Luckily it was a rather a different story in soil or sand (the latter was recommended as a more controllable environment in which to do lab experiments). This work is still ongoing so watch this space - there may be more to report next time! Regarding the photo that heads this column this is a new high definition one taken this year (sadly the guitar is missing!) Anyway I may well return to the subject of CP in a future column so if there are any information, stories, questions, anomalies that readers would like to share please get in touch. My contact details are as usual: douglas@harrbridge.freeserve.co.uk
Thanks are due to Janne Lumme and Paul Bailes for assistance with this month's column