Deformation induced degradation of hot-dip aluminized steel

In this work the fracture and corrosion behaviour of hot-dip aluminized steels is investigated in controlled dipping experiments which allowed to separately study the effects of Si in the Al bath (1-10 wt%) and the intermetallic phase thickness (5-30 µm). The addition of Si had no direct influence on the performance of the coating system for similar thickness values of the IMP seam, which in turn showed to be the dominant factor independent from the amount of Si. Thin intermetallic phase seams (< about 10 µm) exhibited more (about 5-10 per 100 µm interfacial length) but smaller cracks with a fishnet pattern on the outer Al-Si coating, which remained intact and interconnected until a tensile deformation of 15-20%. Thicker intermetallic phase seams resulted in less (about 2 per 100 µm interfacial length) but broader cracks perpendicular to the tensile direction, giving rise to a lamellar pattern on the Al-Si coating, which cracks and uncovers the steel already at strains below 10%, and readily flakes off leaving the steel substrate to accelerated corrosion in chloride environments. Our results indicate that the reduction of the intermetallic phase seam thickness remains the main target to improve the performance of hot-dip aluminized coated steel by combining appropriate Si additions with minimized dipping temperatures and times.