Technical Topics - Let It rust!

At the start of his UK Corrosion lecture in Sheffield earlier this week, David Worsley stated that there are only three things certain in life "Death, Taxes and Corrosion" (he was quoting someone else I believe!). Anyway that seems to be a good enough introduction to this issue's column which is all about deliberately allowing corrosion to proceed without any protection method applied.

Weathering steels are an example. This is use, probably for cladding ie. atmospheric exposure, of an uncoated low carbon steel containing a percent or two of copper. This in theory decays to a nice brown colour and the corrosion rate slows dramatically after a year or so. I say in theory because there can be problems such as excessive corrosion rates, (a paper in UK Corrosion on weathering steel in bridge structures in the Czech republic was mainly about this), staining and the like.

Last weekend I was lucky enough to walk past a recently built, "sculpture" in the East End of the city of London (Broadgate) comprised of a number of large (30 feet high?) unprotected steel plates. I would guess this structure has been "up" for about a year and was made from normal (ie. non copper containing) steel (cheaper?). At the moment the rusting could hardly be described as attractive; being quite like rusting that typically occurs if a steel coupon is immersed in water ie. somewhat non uniform and following rolling directions. Still, this may well be the effect the artist intended! Of course atmospheric rusting can work well and we are familiar with attractive green patina on bronze alloys (I touched on this in the bells article a couple of issues ago).

So that really covers the decorative aspects. What about deliberate rusting of functional components? I toured in Washington State (home of Seattle and Mt St Helens among other things) in the summer and was intrigued by the crash barriers in the mountain areas (which is mainly where we were). These are made from ungalvanised steel (possibly of the weathering type?) although occasionally coupled to galvanized sections or joined with coated bolts/rivets.

Unlike the Broadgate "art", the attack by now (and my estimate is that these structures are at least ten years old) is quite uniform and to my eye is not unattractive, blending the barrier in with the mainly green and brown natural environment. But what compromise of strength will occur? There must be some because however uniform the corrosion looks, there is inevitably some degree of pitting and under sudden stress these will act as crack initiating points diminishing the ability of the barrier to do its job. What made Washington State Highways Agency decide on this approach? I do not know but my guess is that replacement will be necessary significantly earlier compared with galvanized steel barriers, although the rural atmosphere will help delay it.

There are of course many other examples of "letting it rust" where rusting is quite deliberate and a corrosion allowance has tobe built in. An example is steel piling .This picture was taken approaching Vancouver (Horseshoe Bay) on the ferry from Vancouver Island. Sea water corrodes steel at a significant rate and thus one has to allow for several hundred urn a year of metal loss. So if you are going to go for this approach you have to overdesign by a margin.

Finally in terms of unprotected iron structures one cannot avoid mentioning the Delhi Pillar. This of course has been standing for a long, long time and is remarkably lacking in corrosion despite no protection system. It is generally agreed that the benign warm atmosphere plus the effect of rubbing by human backsides has played quite a significant part in keeping the structure in such pristine condition. Now back to the last TT column (the one on corrosion in concrete), I was warned when I sent it to a couple of "experts" for perusal before publication that I might get some correspondence and they were right! (see letter published in this month's CM) People who work in concrete seem more inclined to put finger to keyboard than most corrosion people.

There was also the topicality of it with the Minneapolis Bridge Collapse in early August. Certainly the level of interest suggests I will have to return to this topic in a subsequent TT and incorporate some of my correspondents' helpful points. Anyway as usual if anyone has any comments on this month's offering please get in touch.

The e-mail address is douglas.mills@northampton.ac.uk.co.uk