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434 Freeze/thaw effects on RHSs and unexpected hydrogen generation
Report ID: 434
Published: Newsletter No 36 - October 2014
Report Overview
A view of the freezing effects on RHSs and how this could be related to Liquid Metal Assisted Cracking. Also the reporter is aware of two instances where corrosion of sealed RHS has resulted in the section being pressurised by evolved hydrogen.
Report Content
A correspondent is interested in reports on freezing effects on galvanised hollow sections (Newsletter 33 Report 314 More on freezing and galvanised hollow sections). Without having done calculations, he is surprised that ice would actually split an intact hollow section at the corners. He is not sure ice could generate enough stress, and would expect to see bulging of the faces first. It is significant, he thinks, that the sections were galvanised. He suspects that what has occurred is Liquid Metal Assisted Cracking - the strain hardened corners of cold formed RHS are known to be vulnerable to this - and the ice has simply broken the zinc that filled the cracks and bent the then unrestrained sides of the RHS outwards. Sealing the section to prevent water ingress may not be a good remedy. Apart from the problems discussed in the linked CROSS report, he is aware of two instances where corrosion of sealed RHS has resulted in the section being pressurised by evolved hydrogen. This has resulted in burns to people drilling or cutting the section. One incident occurred on a handrail standard; a check revealed significant hydrogen in the other standards. This was documented in an internal company memo. The other incident occurred in the USA when an agricultural plough (or plow!) was being modified; a jet of flame occurred where a hole was being drilled. To add weight to the plough, the hollow section had been filled with steel and aluminium punchings (swarf). Following the accident, simulations showed that the pressure of the hydrogen may have been as much as 12 bar. Unfortunately the reporter has been unable to find the original source of this report but it does seem that this risk is not widely known.
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In plumbing pipework it is normally the thaw phase which creates bursting since a quantity of water has maximum volume at ~4oC (not as ice). A complication is that RHS walls will be thinner at the corners but as the material is strain hardened it is also stronger and the extra strength is presumed to compensate for the loss of section. If calculations were done, they would need to be based on strain not stress since the volumetric expansion causing stress would have to be enough to stretch the steel beyond its fracture point. Hydrogen embrittlement can occur but is normally associated with higher strength steels. In addition tothe RHS sections previously reported to CROSS (253 Freezing splits RHS galvanised column) there have been cases of bridge parapet posts bursting and because of this it is normal put a drain hole in to the post if the might be doubt as to whether complete sealing is possible. There have also been examples of RHS sign posts and gantry legs splitting through freeze/thaw action, or just filling with water. Supposedly sealed units are often found to be leaky and any defect can lead to differential pressure issues and consequent water ingress.
So far as the debate on unexpected hydrogen generation is concerned, any other experiences will be welcome. Previous CROSS reports on Liquid Metal Assisted Cracking and hydrogen generation can be found on the Structural-Safety data base.