The Corrosion Engineering Division recently held its third Working Day since its relaunch under the chairmanship of Nick Smart a couple of years ago (the previous two were in Birmingham (October 2008) and in Buxton (April 2009) and both were reported in CM). In my view these meetings are getting better and better!

This one took place in Birchwood Park in Warrington (where Serco are based and where this author worked for a couple of years back in the 70’s so visiting the site was quite nostalgic although a lot had changed, mostly for the better). It was attended by well over 50 people including a contingent of students from The University of Manchester. The working party meetings (Coatings, CP, Monitoring, Nuclear, Oil field chemicals, Concrete, Water treatment) were given a little more time to conduct their business (although they probably still claim they did not have enough time!)

The main theme was Corrosion Monitoring and three very good lectures were delivered. I say that even though one of them (on monitoring coated substrates) was given by me! The others were by Gareth Hinds on Oil and Gas and by Colin Britton. The latter was a nice overview by the doyen in the field (Colin contributed to the Hoar Report back in 1971!). Anyway we hope to put all three lectures up on the CED website. There was also a good lunch and a fascinating tour of the Serco labs.

It is to be hoped that these CED meetings will become an annual event (similar to the EFC working party meetings) equivalent to the old Spring work week (these days people can give up one day - in the 80s they could give up a week!). Anyway at this “Spring work day” I met someone who works for a company that specialises in providing equipment to apply Anodic protection. So that catalysed me into thinking this area could be the subject of my next TT. It actually took me back to an early corrosion lecture given by John Chilton in the Dept at Cambridge in about 1969 that struck me forcibly. He told us about how by driving the potential positive (in an environment which was oxidizing!) one could passivate the metal. This was particularly applicable to plain carbon or low alloys steels in sulphuric acid. When concentrated this is sufficiently oxidising to passivate iron without any applied potential. But by connecting the vessel to the positive terminal of a battery it can be made to anodically protect even when more dilute. Recent information received from my CED workday contact demonstrated to me that successful application of AP is quite subtle and relies strongly on good controlling equipment. It works in several solutions eg alkali (found in paper industries where sulfide is also common). However with too much chloride around (or other depassivating agents) it will not work. If it goes wrong (power cut or whatever) the consequences can be catastrophic (curve flips from passive to active -- corrosion rate much higher than it would be without the application of the AP in the first place). So not a method for the faint hearted! Also if the structures are big the currents that might be needed could be very large (100s of amps). Nonetheless it is worth it if you can cut the corrosion rate down e.g. from 50 thou per year to just a few thou (this example came from a paper by Ian Munro where AP had been applied  to vessels being used for processing in the paper industry).

This leads on to a hobby horse of mine relating to my interest in the mechanism of protection by organic coatings. Can passivation occur naturally (under fairly specific conditions) by connecting to a high potential surface? In Cathodic Protection you can connect to another active metal which will drive the potential of the structure DOWN into the regime where the metal you are trying to protect is relatively immune. Can you do the same with AP? i.e. drive the potential of the metal UP by connecting to another metal. The received wisdom is it doesn’t work too well e.g. connecting copper to iron even in an oxidizing environment without supplying an independent source of positive current would quite likely lead to increased attack on the iron. But supposing one connected some passive steel to a bit of active steel via a large resistance as might occur under a paint coasting. Assuming an oxidizing agent was present (eg from inhibitive pigment in the film or even just oxygen itself) maybe the potential of the active part would be driven up and it WOULD anodically passivate.

It does appear that something like this can happen because if you apply a coating to steel, expose it to solution and measure the potential then, assuming it is a decent coating, the potential tends to be in the passive region. If you then remove the coating and expose the metal to a mildly corrosive environment it takes longer to break down. These ideas may be a bit controversial. But they do seem to relate to the general concept of anodic protection i.e. the bringing of the metal into the regime where it is passive rather than active. The SKP is the technique that could help us move this forward.

If you have any thoughts on this or other matters as usual the e-mail address is Douglas@harrbridge.freeserve.co.uk

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