Technical Topics - "Cost of Corrosion"
by the Technical Secretary, Douglas Mills
Chemical analysis is an important part of corrosion investigations. This is true not just of failure analysis but also in research. In relation to the former, my main period doing this was when I worked for the Metals Users Consultancy Service (part of the BNF Metals Technology Centre) in Wantage back in the 1970s. Many "failures" (70%?) had corrosion as part of the cause. Semi-micro-chemical analysis (really spot analysis - Feigl's Inorganic Spot analysis was the Bible) conducted by Asit was the first line of attack. If the failure was a copper pipe then a common test was to find out if ammonia was present. However inter-alia Asit could detect, zinc, copper, iron, carbonate, sulphate, chloride, phosphate, and to at least some degree, tell how much was there. He could tell you quickly whether a chromate film was present using a chemical called dephenyl-carbazide. The EDAX facility on the SEM was another useful tool, run by Cedric Thornton. He created many attractive EDAX element maps showing the distribution of particular elements in a smallarea maybe 10umx10um. X-Ray Diffraction helped to identify the actual compound that was present (Charlie Purnell ran this machine). If there was a need to check the composition of the metal itself eg whether an aluminium alloy was HE 30 or H6, a sample was given to George English who did spectrographic analysis. This method could detect (all?) elements accurately and quantitatively down to about 0.005% in some cases. An investigation where at least three techniques were used was into the failure of Nichrome (SONiZOCr) air cleaner wires. George confirmed the material was as it should be, Cedric showed that the corrosion product(CP) was almost entirely nickel and then Charlie showed that the CP was an uncommon compound of nickel and consistent with an unusual form of attack called "Green Rot" which arises only under alternating oxidizing and reducing conditions. Later while working for the Nuclear Power Company, an example where chemical analysis was crucial was in determining the extent of rebuild of the Hunterston Nuclear Reactor after the sea water flooding incident (alluded to in an earlier column). This was decided based on the chloride level being below a certain level. Turning to chemical analysis in research, during a large anti-corrosive paints project at the PRA, AAS was used to find out whether the amount of ferrous iron present under a paint coating after exposure to chloride solution related to the measured polarization resistance (Ret). In paints with inert pigments iron levels were in agreement with the Ret. But with active pigments e.g. lead or chromate containing, they were much lower. Later still in research at Sheffield University with Bob Akid looking at galvanic corrosion between aluminium and iron, AAS was again used to measure the amount of aluminium in solution. This was to show that the value quantitatively related to the SRET maps which we were generating (paper referenced in the Jan/ Feb 08 CM). These days this sort of aqueous analysis would most likely be conducted by ICP (Inductively Coupled Plasma) or using Ion Chromatography (eg Dionex). However EDAX analysis is still popular and will shortly be used in a current project. This leads me to the pictures. Last months issue was all about Women in Corrosion and I would like to retrospectively add my tribute to this with two women one of whom I have recently collaborated with and the other of whom is currently working in my laboratory. Hilbert worked at DTU as a corrosion scientist and has had a couple of articles published in CM, the last one involving analysis of Euro coins and the dangers associated with nickel dissolution. Kasia from Poland is working in Northampton and will shortly be doing some EDAX analysis work on steel surfaces in connection with her project on the effect of different surface preparations on the subsequent performance of organic coatings on steel. I'd like to finish with a plug for Eurocorr 08 in Edinburgh 7th-11th (further details can be found in the four page pull-out in this issue). In my opinion these EuroCorr conferences are excellent and here we have one in UK for the first time for 8 years and you should really go along!. All aspects of corrosion will be covered (see the programme on the web site www. EuroCorr.org). Even if you just go for a day, it is great chance to meet and talk to like-minded people and listen to papers on your particular topic. You also get the CD Rom which has a good search engine. So hope to see as many people as possible there! As usual any comments please send to douglas@harrbridge.freeserve.co.uk