A Predictive Model Accounting for the Mechanical Behavior of Galvanized Alloy Layers on the Mild Steel

Zinc coating is formed by the heterogeneous assembly of the Γ, Γ1, δ, ζ and η phases whose mechanical properties greatly differ from each other. Thermal strains resulting from large differences between thermal expansion coefficients are partially relaxed by the formation of a crack network. In order to model this phenomenon, initial hardness, thermal expansion coefficient αi and toughness (KIC) of the phases were determined. Hardness testing experiments performed on the galvanizes samples with and without annealing revealed that during the cooling down of the samples in the coating process, there was some residual stress in the coating due to the difference between the thermal expansion coefficients of the phases. In this regard, maximum hardness, 340 HV, was obtained for δ phase and its toughness was measured to be about 2MPa√m, thereby revealing that δ phase was completely brittle compared to other phases. Modeling the behavior of the phases present in the coating demonstrated that during the cooling stage, at first, some micro-cracks were formed in δ phase and grew in two stages: I) perpendicular to the δ/α interface and II) parallel to the δ/α interface. The results revealed that when the thickness of δ phase was more than 5μm, there was a good agreement between the experimental results and the proposed model. Also, due to the properties of the coating layers, the resulted stresses could not delaminate the coating.