Technical Topics: Electrochemical Measurements - Are They Any Use?

It is New Year again and it's TT No.10 (double figures!) Have I really written nine previous TT articles? Will I be able to keep it up? It is lucky corrosion is a big subject and has a number of ramifications. In fact for me that is part of the appeal of corrosion - the way it covers physics, chemistry, biology, all the different types of engineering etc. Anyway, there was quite a bit of interest in the last article on copper corrosion and I will return to that in a future article.

This month I would like to talk about Electrochemical Measurements, and particularly those designed to monitor corrosion rate (and also the protective ability of a coating system). It would seem natural for corrosion to be monitored using electrical measurements because, after all, corrosion activity involves voltages, an anode and a cathode and it is the corrosion current flow so generated that characterises the rate of metal loss. Also (and I particularly like this aspect), electrochemical techniques can give you a numerical indication of the rate of corrosion activity. Visual observations, such as might be made eg. after an accelerated test of some sort, are all very well but don't give you an actual value. A simple potential measurement can be very useful, but even that only provides a qualitative indication of behaviour, whereas polarisation resistance (LPR), impedance (EIS), or electrochemical noise (EN) measurements ARE quantitative, There have been numerous examples of the use of these methods in the laboratory and in the field, and the subject is very popular within "academiics research". Every three years an EMCR (Electrochemical Methods In Corrosion Research) conference is held - most recent one in June 2006 in Dourdan, near Paris. Even within the smaller field of Organic Coatings research there is an electrochemical techniques (AETOC) conference every eighteen months (the next one is to be held in Vigo, Spain in April of this year). However the degree of use of electrochemical measurements in industry still tends to be patchy. A degree of suspicion can surround these methods and (particularly in the coatings field!) it can be a struggle to get industrial users to "take them on board"(at least that has been my experience, people like Bill Cox may disagree!).

Lab assessment protective coatings using DC and EN
Why might this be the case? Well there can be a "Black Box" element to the use of more advanced electrochemical techniques. Are we really measuring what we think we are measuring, etc? This is quite a valid concern. Early in my career in researches at Trent Polytechnic, we were trying to determine the corrosion rate of stainless steel in a microbiologically active environment using polarisation resistance (LPR). Results were indicating that the corrosion rate was such that the stainless steel specimen should be disappearing, if not overnight, then at least relatively quickly. Instead, it was obvious that the samples were hardly corroding at all! So what was giving the apparently high corrosion current? Luckily I had attended a lecture given just before by Tony Richardson and he had made us aware that an adventitious redox reaction could produce a spurious result and that was what was happening in this case. That process was going on 30x faster than the corrosion action on the steel! So electrochemical rate measurements are best used on a system that is relatively well characterised and where the chemistry is known. There is also the need to calibrate the equipment. It is perfectly reasonable to do this (after all, to make thickness measurements, for example on coatings, you need to do calibration). This is not too difficult in the case of ac impedance (using dummy circuits) but can be more difficult in other cases, e.g. electrochemical noise. The vendors of such equipment don't often sell you a standard calibration box with which to check your equipment unless you ask for it!

Another aspect of electrochemical techniques that can deter people from using them is their perceived complexity, made worse sometimes by the obscurity of accompanying technical literature. I received something recently from a company offering corrosion rate measuring equipment that stated: "Datasheets are enclosed for the multivariable transmitter that will monitor general corrosion, localized corrosion (pitting), and conductivity through a 4-20 mA signal and HART® protocol in real time. The corrosion rate or a pitting factor is configured as the primary variable using a standard 2-wire 4-20 mA output. The remaining outputs are configured as secondary and tertiary HART variables." Although I have been working in the field myself for a quarter of a century (or perhaps because of that!), I find it difficult to work out from the above description what exactly the machine does and how it does it! (My apologies to the writer if he/she is reading this!). The "pitting factor" suggests electrochemical noise (EN) is involved - a method I have become familiar with in recent years. This is a relatively "young" method and I admit that development has been slow and difficult at times. However, there is progress! The sort of mistake we made with the stainless steel that was illustrated earlier is less likely to occur these days. So overall I'd say if you are out there in Industry and have a chance to use electrochemical methods in 2007 then "Go for it". If you are new to the field there are plenty of specialists in ICorr who would be pleased to help! And as usual, if anyone would like to raise any points with me about this or anything else on the technical side, my e-mail address is Douglas@Harrbridge.freeserve.co.uk Thanks are due to Bill Cox for checking over this article. High temperature data plot using EN (Courtesy Bill Cox) Thanks are due to Bill Cox for checking over this article.