Technical Topics - "Corrosion of Aluminium"
by the Technical Secretary, Douglas Mills
It's the New Year again and one is allowed, 1 think, to wax a bit philosophical. I was trying to work out why I seem to have so little time to do anything these days (including writing myTT column). Three explanations spring to mind 1) the whole universe is speeding up ie each second is now shorter than it was (the evidence for this is that my son also seems to have less time to do things compared with what I remember I had when I was his age) 2) More and more things come your way as you advance through your life and you fail to shed old things to make way for the new things 3) everything takes longer as you get older. Anyway maybe readers have there own thoughts on this! Talking of you writing in, my last two columns "Let it Rust" and "Corrosion in Concrete" have generated interest (even controversy!) such that people have felt moved to write in to the editor. Other people have written to me directly and I apologize if I have not replied (for explanation see beginning of this column!).



Anyway this month I thought I'd ruminate on a controversial metal which is pretty big business viz. aluminium. Two academics who have done a lot of work / know a lot about aluminium are George Thompson and Alison Davenport (respectively of the Universities of Manchester and Birmingham). My own experience is a lot more limited. However, I do still receive technical enquiries often relating to unexpected failures (see above, recent picture of motorway gantry sign). Why does aluminium sometimes cause problems? Well it relies on a thin, adherent, intact, stable and difficult to dissolve oxide, but underneath there is one heck of a reactive metal waiting to get out (Eo at -1.67 SHE is very low - similar metals include titanium, zirconium and chromium). If one was just using relatively pure aluminium particularly if it was anodized (pretty colours possible!) then all would probably be well. But of course for many applications alloys are used. And once you put alloying elements in the corrosion resistance (no doubt because the intact oxide film is not quite as intact as it should be) is not so high. Since important alloying elements include copper (more cathodicthen aluminium) and magnesium (more anodic than aluminium) you end up with an alloy containing loads of built-in small galvanic cells (i.e. at the 2nd phase strengthening particles). So pre-treatments are needed and/or paint coats or both. A favourite pre-treatment used to be chromating but nowadays that is not so acceptable. The EuroCorr conference that I reviewed last issue had a session on self-healing coatings and several of the papers were in the area of chromate replacements. The problem here is that none of the alternatives work so well.The great thing about chromate is that it is a strong oxidising agent, which enables it to effectively oxidize up any weaknesses in the air-formed oxide and that the reduced product is solid chromium oxide (or hydroxide) that effectively forms an additional protective film on the surface. (Of course the oxidising power of chromate is also one of the reasons why it is relatively toxic).

Stress corrosion and pitting are also common forms of attack. Several failures I investigated in my days at the British Non-Ferrous Metals Research Assocation occurred on alloys which had been in environments (e.g. containing chloride ion) which could pit and, in a couple of cases, crack the alloys. Some work ! did more recently with Bob Akid, Technical Editor of this journal illustrated the pitting effect most effectively1. We used an electrochemical Scanning Probe Technique (e.g. the Scanning Reference Electrode) which showed up the nasty pitting quite well. That work involved galvanic effects as well - a sandwich of pure aluminium clad onto aluminium alloy joined to steel. The pitting (enhanced by joining to steel) normally took place on the aluminium alloy. However when the conductivity of the solution dropped (eg going from sea water to a lower chloride solution), the pitting then moved onto the pure aluminium. This was because the throwing power of the galvanic couple was insufficient to affect the aluminium alloy.

Unintended galvanic effects can also occur even where there are no detrimental constituents in the alloy.The pitting of a cast aluminium cylinder (see top right picture) was caused by localized copper deposition from copper-containing waters - this is a common problem in beer barrels funnily enough.



Aluminium usually (but not always) becomes the anode when connected to any other conductor. For example, there is the wartime story of the Spitfire factory which employed tailors to mark out the aluminium sheet panels prior to cutting to shape. They used graphite pencils to do this and, after the war, surviving planes suffered severe corrosion along these lines. Of course this tendency for corrosion is taken advantage of in the manufacture of cathodic protection anodes where small amounts of indium (and before that mercury) are added to encourage corrosion. Also, aluminium is added to some high temperature alloys to form an alumina scale which is stable at temperatures (i.e. > 900°C) where chromium oxide scales evaporate. So all in all a fascinating metal; there is masses of it in the Earth's crust, it is easily recyclable and it has not so far been discovered to be too toxic.

Two plugs to finish with. There was a questionnaire sent out in last months CM about the Corrosion Engineering Division work groups. We are desperately (!) trying to find out where the technical interests of you, the members, lie. These can be filled in very quickly and we would really like a decent number to get in to Corrosion House. So thanks in advance for sending one in. The second plug is for EuroCorr 2008 which this year is in Edinburgh (7th-11th September) and incorporates UK Corrosion/49th CSS. If you are an industrialist the exhibition will be a great place to display your company's products, if you are on the technical/ academic side you are bound to find many posters/papers of interest. And best of all Edinburgh is just a great city to be in. So let's have a good turnout from UK for this meeting! As usual any comments send to douglas@harrbridge.freeserve.co.uk
(1) R Akid and DJ Mills (2001) A Comparison between Conventional Macroscopic an dNovel Microscopic Scanning Electrochemical Methods to Evaiuate Galvanic Corrosion. Corr. Sci.Vol 43, (7), p 1203-1216