Technical Topics No 23

As regular readers of this column will know I am involved with Corrosion Engineering Division, CED (fairly near the bottom of the pile in terms of the organisation but I like to think I punch above my weight!).  The way it works is that Nick Smart is the Chair (Nick.smart@serco.com), then there are the working group chairs (eg Brenda Peters for Coatings), then there are task force leaders (eg within coatings Ian Stewart and Jeremy Twigg), then there is myself as CED coordinator. Actually I have a dual role as I have had my arm twisted to also be the Coatings group’s secretary.  After a long confinement we are now very close to giving birth to the first document which is entitled “Guidance note on the selection, specification and use of intumescent coating systems for corrosion protection and fire protection of structural steel”.

Staying with CED, apart from some unofficial activity of the coatings work group nothing much happened at Correx. However a major CED meeting is now being planned for next Spring. It is expected that this will take place at Birchwood Park,  Warrington area on around Thursday 29th April 2010.

So please if you have not already done so DO GET INVOLVED IN CED – e.g. by signing up for one or more work groups (you can do this on the web site (www.icorr.org then go to CED section). Apart from coatings there are active work groups in Steel in Concrete, Cathodic Protection, Oilfield Chemical and Corrosion, Monitoring, Nuclear and Water Treatment.  Or you can start up your own! (with a few friends - they do not even have  to be ICorr members). Contact Nick! 

Now I mentioned in the last issue that TT No 21 had stimulated interest: both topics that  I covered namely Accelerated Low Water Corrosion and  the zinc coated wire. But it is the barbed wire that I want to discuss again here. You may remember the photo showed loss of zinc on the barbs whereas it remained in the other sections. A good term to describe this (used by Eric Martin- see below) is selective corrosion.

You may remember my explanation was extra stress in the barbs. Well some correspondents have offered other explanations. Nick Smart suggested that water stays on those sharp barbs longer than other places. Hence overall it is a more aggressive environment. But Hans Arup has really gone to town with an explanation related to a different forming process of the barbs.

I will include what he said pretty much verbatim, “It is quite possible that different qualities of “Galvanized Wire” have been used. In the galvanizing plant you can finish the process in two different ways. At the exit of the molten zinc bath the wire has a thin alloy layer next to the steel, and a much thicker layer of still molten zinc. This can be blown away by a jet of hot steam or air, leaving only the thin, brittle alloy coating.

Or you can allow the layer of still liquid zinc to solidify while the wire is led vertically several meters through a sort of cooling chimney.  I would guess that the thinner
and more uniform coating of the “blown” wire is more suitable (convenient) for the processes that the wire must undergo in forming the “barbs” (or whatever they should be called), and the more durable, slowly cooled wire can with advantage be used for the longitudinal wires”.

Thanks to Hans! Inspired by the same article Eric Martin has sent in a spectacular example of selective corrosion of steel (see photos).These were taken in Broughty Ferry in May 2008 ( a place I know - my grandmother was born and brought up in Newport-on-Tay and my mother lived as a teenager  in Dundee).

Quoting now from Eric “These are of the cycle racks, which appear uncoated, having only the original millscale. The corrosion appears to be worse at the bends and on the seaward side of the racks. (The quayside can be seen at the top right corner of photo)”.

So stress has probably contributed to greater attack  But not on all bends - so part of the explanation appears is in line with Nick’s for the barbed wire ie differing environment.

Anyway the picture is definitely a good one for a Vulture lecture!. So thanks for that contribution. Also thanks to others who have written in. One such is Rick Simpson. Although he didn’t actually proffer an alternative explanation for the barbed wire corrosion (perhaps he agreed with me!), he writes very authoritatively on zinc. As an example he gives a goodish explanation for why zinc is ok as a coating for rebar when an instinctive reaction might be that it would not be because zinc is an amphoteric metal (apparently the pH that the solution in the concrete pores gets to is generally below the critical pH). Anyway further examples of selective corrosion from my readers would be welcome!  As usual the e-mail address is: 
Douglas@harrbridge.freeserve.co.uk

P.S. Thanks are due to Eric Martin for permission to publish his two photos.