Before we start just let me make a plug for the Correx conference taking place in Birmingham from Oct 27th - 29th (see other publicity in this issue). One of the sessions is on coatings and the other on CP, both areas dear to the heart of many (over half) our  members and both of which get some coverage within this article. The conference is very reasonably priced and the CED workgroups hope to meet there.

Anyway in this months TT, I am returning to the topic of bimetallic corrosion, a subject area that constitutes the greatest proportion of the technical queries that I receive.

We will start by reminding ourselves how devastating this can be with an example of a gantry sign sent to me eighteen months ago by Rob Poulton and which I have already published in a article on aluminium.

This picture shows aluminium bars welded to aluminium matrix frame with plastic/pvc sheet on front of the sign attached with galvanised bolts. Not nice at all! But this months’s article has really been inspired by my listening recently to a couple of papers about use of powdered magnesium as a cathodically protecting pigment for aluminium at the Advances in Corrosion Protection by Organic Coatings Conference in Cambridge.

The story in a nutshell is that for protection of their aluminium alloys the American Air Force relied heavily on the use of chromate pigment and passivation treatment. Both very effective- but no longer allowed! So the group at North Dakota State University have been working on an alternative based on magnesium pigment dispersed in an epoxy binder. This has now has reached a state where it seems to work pretty well in the lab and is going to tried out on an actual aeroplane. It is the theory behind the use of this pigment that interests me.

Both talks used the term cathodic protection to describe the action of the magnesium (at least in the early stages of its operation- the potential does rise with time - see graph) This was based on the observation that the potential taken up by the system was below the galvanic potential taken up by the bare aluminium alloy (typically 2024 or 7075) in, say, 0.1M NaCl. Thus if the latter had a potential of say -0.6V (SHE), the potential of the system (coated with the magnesium containing paint) might be -1.0

Now there is problem with using the term cathodic protection to describe this situation. This is because the thermodynamic potential of aluminium that you would need to get below to achieve true CP is -1.67 volt. This would be impractical with magnesium (and indeed even impractical using impressed current CP as it would generate vast quantities of hydrogen). The concrete people have encountered this before in applying CP to protect steel rebar. They have found that quite effective prevention can be achieved by getting the potential into the region where it is below the natural potential but still above the true protection potential (in iron’s case maybe between -0.5 and -0.3V). Note applying any amount of negative voltage tends to drive positive ions towards the metal surface and negative ions away thus reducing chloride and generating alkali. In iron’s case this would induce some level of passivation because magnetite is stable (work by the late Geraint Thomas confirmed this).

This brings me to a related observation that I made when testing zinc dust loaded paints on steel in sea water. As with the magnesium “paint”, the potential rose within a week or so to above the protection potential of the steel. (In fact by the end of the test the potential was close to the unprotected steel value). But although there was some white zinc corrosion product observed on the outside, there was no attack on the steel at all (note this paint system was too porous (confirmed by Resistance measurements using EIS and ENM) to be working by resistance inhibition). In this case the zinc ions may well be contributing to the inhibition.

Similarly in the case of magnesium on aluminium a magnesium hydroxide film may well be being laid down Of course the reason that the aluminium alloy itself has a much higher voltage (nearly a volt higher than expected) is because of the excellent aluminum oxide film. But in certain environments (eg the M-way gantry sign) that oxide can (and does) break down (mercury compounds cause this ). I wonder what value of potential would have been needed to effectively protect the gantry sign? Quite possibly well below that which could have been achieved by just connecting it to magnesium.

So like many corrosion situations one needs to know all the facts before being able to predict what will happen. That is what keeps the subject fascinating. Returning to the example in the last TT of selective corrosion, this has generated interest with one correspondent sending in a very good example of selective corrosion and another proffering an alternative (and no doubt better!) explanation for what happened to the zinc coated barbed wire. I will return to those next month. Any comments, as usual, please contact Douglas@harrbridge.freeserve.co.uk

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