The three case histories presented in this paper concern defects and causes of failure associated with galvanized steel sheet material. Optical metallography and microindentation hardness testing were the principal methods employed in determining the cause of failure but were supplemented as necessary by scanning electron microscopy and energy-dispersive X-ray spectrometry.
One of the case histories presents defects associated with the actual hot-dip galvanizing process itself. In one instance, tiny hydrogen blisters were observed to have occurred at oxide scale deposits at the interface between the steel and galvanized coatings.
When galvanized sheet steel cracks in bending during forming operations, most end users tend to place the blame on the galvanized material. However, several separate investigations of formed and cracked galvanized steel parts have revealed that the cracking often initiates at the sheared edges of the part where plastic deformation has produced a local increase in hardness and a corresponding decrease in ductility. Ductility is the property of steel required to successfully withstand forming operations.
The final case history presents some observations made during failure analysis of corroded galvanized and painted panels at a utility plant. The coating system was composed of a galvanized layer, a thin primer, and a final top coat. The problem was identified as localized development of white rust. The problem was found to be caused by the presence of a second-phase precipitate in the galvanized layer that adversely affected its ductility allowing it to crack during forming. This produced localized stress in the paint layer that then cracked permitting moisture to come into direct contact with the zinc, resulting in white rust formation.