The most common cause of galvanised coating delamination is ‘heat peeling’. This occurs when the steel, usually heavy section thickness, is cooled slowly or is not cooled sufficiently in the quenching process, causing residual heat in the steel mass to reheat the coating.The thermal stresses generated by this differential heating or cooling will create high shear forces at the steel/coating interface, resulting in localised delamination of the coating. This will take the form of blisters, or in the worst case, flaking of the coating from the surface.Mechanical delamination will generally occur on edges or areas where the galvanised coating is subjected to high localised pressure or impact.On rough steel surfaces, galvanised coating delamination is generally less likely to occur because of the better mechanical keying of the coating to the steel surface. Very smooth surfaces on ERW pipe, RHS or other cold rolled sections are more likely to delaminate on reactive steel and the presence of residual alloying elements like phosphorous increase the risk if coating delamination.
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