Technical Topics No 24:
Atmospheric Corrosion
by Technical Secretary, Douglas J Mills

First TT of the New Year and I have decided it is time to reveal another interest of mine …. the weather. I measure pressure, temperature and (along with my brother and cousin who live in different parts of the country to myself) keep rainfall records. But I have never attempted prediction (beyond next 24hr anyway!) Safer! (Perhaps it is the same with corrosion?). The year has started off a bit nippy. But the day that I write this, the mercury has hit a balmy 10°C. To remind us of the winter of 09/10 though, I include a picture of a frozen lake where the cracking is very similar to that which I saw a few years back while looking down a microscope at a metal sample that had undergone stress corrosion.

No question that weather plays a big part in corrosion, and sometimes unexpectedly. Many of us can remember the AA survey back in the 70s that showed the wettest parts of the country cars had the least corrosion (can you think of the explanation?). I have not done any actual experimental work in the field of atmospheric corrosion of uncoated metal but have come into contact with it from time to time. I did a consultancy on the effect of carbon fuel cycles back in the early 90s –this was aimed at calculating the “impacts” from the atmosphere on a range of materials. To predict the long term effects we relied quite heavily on the modeling work of a Dr Kucera who had produced a number of useful predictive equations. Going back a bit further, Ulick Evans was working on atmospheric corrosion in the lab in Cambridge during my time there (the actual experimental work was done by Bert Taylor but URE dropped in now and again). He was developing theories to try to understand the unexpectedly high rates of atmospheric corrosion of steel in sulphate atmospheres. He explained this neatly by a self-catalysing regenerating mechanism to drive the corrosion cell (that work got referenced in the “Impacts” work twenty years later). I rather envy atmospheric corrosion researchers as, to examine their samples, they have to make trips to the sea side (which I love being at) and/or to exotic places like Florida! But prediction is tricky. Very recently I reviewed a paper from the Heritage session of Eurocorr 2009 (in Nice) in which nominally identical lead statues in the grounds of a Palace in Queluz had corroded in markedly different ways. This was attributed in part to differing microenvironments. Also using historical relationships is fraught with difficulty as the environment can change over time and many metals are very sensitive to just small variations.

In recent times the atmosphere in Western countries has generally got more benign in terms of chemicals (sulphur etc) but there is more CO2 and the overall temperature is higher. So maybe the overall corrosion rate is not changing? There is also the possibility of doing accelerated tests in the lab In fact going back to the Cambridge days, I have a memory of John Hickling working with Dr J Chilton developing an atmospheric corrosion cabinet. Certainly designing and operating such cabinets is a challenge as you have to control the concentrations of aggressive species at very low levels if they are to simulate actual atmospheres.

When it comes to my speciality area of paint coatings, obviously the selection of the coating system will depend on what sort of atmosphere is expected in terms of aggressivity. There are five levels of classification from the relatively benign, dry indoor atmosphere (C1), up to a quite aggressive marine atmosphere (C5).

Recently at Eurocorr, Christofer Leygraf from the Royal Institute of Technology in Sweden gave a review lecture on atmospheric
corrosion. In terms of experimental approaches to the subject, apart from the already mentioned field exposures and lab exposures (with in-situ analysis), he discussed exposure in ultra-high vacuum conditions using zinc in carboxylic acids as a model system. He emphasised that Corrosion and Run-Off are different processes that occur at different rates (the latter relates to possible damage to the environment e.g. causing aquatic toxicity), the former to the loss of structural integrity. He also pointed out that nano-sized particulate matter was often a product of atmospheric corrosion and its effect on human physiology was largely unknown.

In relation to the last point most people are familiar with weathering steel and I include a picture of such a steel at a Cycle Hire station in the middle of the Peak District. When weathering steels were first used they left nasty stains - less of problem now as they are mostly aged
(weathered!) before use outside.

Moving on to other matters there is an article about CPD in this issue written in the main by the Science Council. This area is controversial and there are arguments in favour and against the 1 year validation that the SC proposes to instigate. In the next issue of CM, we may well run a short article giving the counter arguments and hopefully that will encourage you, the membership, to debate the area.

There is also the CED day meeting coming up in Warrington on Thursday April 29th. Loosely themed around Corrosion Monitoring, I myself will give a talk on Monitoring of coating systems. Hope to see some of you there!
For further details see CED on web page.
Any comments as usual please send them to:
Douglas@harrbridge.freeserve.co.uk

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