CROSS Safety Report
Liquid metal assisted cracking (LMAC)
This report is over 2 years old
A reporter raises concerns about the risk of liquid metal assisted cracking during the hot dip galvanising process.
Key Learning Outcomes
For civil and structural design engineers:
Be aware that liquid metal assisted cracking can occur if due attention is not paid to the design, fabrication, galvanising and inspection processes of susceptible assemblies
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A reporter writes in connection with a problem on hot dip galvanising (HDG) which he says is routinely specified in favour of paint systems and thermally sprayed metals for corrosion protection. This is a well established and effective technique provided that recommended procedures are adopted.
According to the reporter possible problems (if the recommendations in BCSA/Galvanisers Association publication 40/05 are not followed) may include:
Strain age embrittlement
Restraint cracking and distortion
And recently, Liquid Metal Assisted Cracking (LMAC).
They give the example of a girder which was fabricated in normal welded steelwork using beam and angle sections where possible and plate fabrication for the remainder. The design and specification were given to the main contractor, who in turn passed it to their own sub-contractor. Minor changes were made to the design and after fabrication the structure was galvanised as a complete unit.
It was then found to have suffered severe distortion and there were numerous visible cracks (Figure 1 and 2). Some were in materials 20mm thick whilst others were in 6mm thick sections. The level of distortion was considerable and affected both primary and secondary members. All cracks seen were brittle in appearance with no indication of ductile tearing.
The reporter says that repair is not simple as it is possible that cracks in the steel may be obscured by the galvanising. It is not known if the guidance given in the BCSA/GA publication (40/05) was followed.
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BCSA document 40/05 emphasises that LMAC is a rare phenomenon. However, it can occur if due attention is not paid to the design, fabrication, galvanising and inspection processes of susceptible assemblies. It is apparent that not all designers are yet familiar with the guidance.