CROSS Safety Report
Freeze/thaw effects on RHSs and unexpected hydrogen generation
This report is over 2 years old
A reporter provides their views of the freezing effects on rectangular hollow sections (RHSs) and how this could be related to Liquid Metal Assisted Cracking.
They are also aware of two instances where corrosion of sealed RHS has resulted in the section being pressurised by evolved hydrogen.
Key Learning Outcomes
For asset owners and managers:
Regular inspections and maintenance can help keep a structure and its elements safe and detect any obvious safety issues
Consider including a risk assessment for internal corrosion in the inspection and maintenance regimes for external hollow section members
A check for internal corrosion should be carried out by a suitably qualified person where internal corrosion has been assessed as a significant risk
For civil and structural design engineers:
Where there is a potential risk of moisture build-up in external hollow section members, consider using a different section type that may be more appropriate for the given environment
When carrying out structural inspections, be aware that water build-up in external hollow sections is a possibility and, where appropriate, consider specific investigation
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A correspondent is interested in reports on freezing effects on galvanised hollow sections that CROSS has published. Without having done calculations, they are surprised that ice would actually split an intact hollow section at the corners. They are not sure ice could generate enough stress, and would expect to see bulging of the faces first.
Liquid Metal Assisted Cracking
It is significant, they think, that the sections were galvanised. They suspect that what has occurred is Liquid Metal Assisted Cracking - the strain hardened corners of cold formed rectangular hollow sections (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, they are 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.
The risk of hydrogen
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 aluminum 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.
Expert Panel Comments
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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 and 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 to the RHS sections previously reported to CROSS, there have been cases of bridge parapet posts bursting and because of this it is normal to 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 signposts 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. You can search for safety information on Liquid Metal Assisted Cracking and hydrogen generation on the CROSS website.
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