Accelerated Testing particularly of Coatings

In this my third column I thought I'd write about a subject that is dear to my heart which is accelerated testing.

What is the best method of finding out in the short term how something is going to perform over very many years?

This actually relates back to the transport and storage issue I talked about last time, particularly containers for the underground storage of nuclear waste. I worked in the nuclear industry for three years and enjoyed it. (OK, some people have opined .particularly young Becky who I met in America in 1995, that there is strong evidence that crawling around inside a nuclear reactor immediately after it had been working, as I did at Hunterston B (AGR) in 1979, changed me forever, but how could she know what I'd have been like if I hadn't!)

Overall I was very impressed with their efficiency and minimal environmental impact both visually (they are small) and pollution wise . So certainly if the nuclear option (fission that is, rather than fusion, which seems to be to be no nearer today in producing commercial amounts of electricity that it was when is I started work 30 years ago) is to stand a chance of winning public support, we must gain understanding in how best to do accelerated testing for materials for underground storage.

Most of my work, however, has been concerned with more prosaic testing particularly of anti-corrosive paint coatings.

For this it appears that Industry still loves best the hot salt spray test. This is where you bash the sample at 35°C (or a higher temperature) continuously for 1000 or 1500 hours with 5% salt, a situation that in 90% (even 99%?) of cases bears no resemblance to service conditions.

Actually I don't have much problem with the hot bit (some people get very excited if you run a test above "normal temperatures" (particularly above Tg) but as long as you don't go too high, I believe raising temperature accelerates the test quite effectively. This is because chemical processes which are involved in degradation are generally accelerated 2x by every 10°C rise in temperature and ionic conductivity (the lack of which is the property which results in most paints being effective) can be accelerated by up to 10x per 10°C rise (if you don't believe me, read my PhD thesis!) . However, better than continuous hot is cycling between hot and cold and many commercial cabinets allow this. Using very strong salt solutions is definitely unfair in most cases - I prefer using medium strength solutions eg diluted Harrison's (0.35% ammonium sulphate and 0. 05% sodium chloride) for cabinet testing although we also use 0.5% NaCI or 0.5% sodium sulphate or 3% NaCI (equivalent to sea water) for immersion tests.

There are two other thorny issues though about cabinet accelerated testing of paints. One is the scribe. Important as this is, it is not standardised (although one IS required to say how the scribe was made - scissors , razor blade, scalpel , blunt end of screwdriver or whatever). I personally liked the Giodarno Scribing machine I used in the USA (in operation reminiscent of a scene from a James Bond movie) or the Mabbutt scriber (yes it is the same man as our Hon Sec!) and/or scribing using a laser (It is interesting to note that accelerated testing of metallic coatings also uses continuous salt spray but NEVER specifies a scribe!) Second is the way of assessing the samples after the end of the test. In this day-and-age visual inspection seems a bit "stone age" ! Yet this is still virtually the only method employed and it is essentially qualitative. Although some measurements can be made eg spread from the scribe, to get a number the author would advocate electrochemical testing. I think I'll leave a discussion of this for another column!

As usual if anyone who wants to share anything on what I've written above please contact me t the following address Douglas@harrbridge.freeserve.co.uk . (Thanks are due to the several people who did this last time - the more the merrier!)

Dr Douglas Mills,
Technical Secretary