Microscopic mechanism of big-white-spot defect formation in hot-based galvanized strip steel: Theoretical calculation and molecular dynamics simulation

Abstract

By combining experimental, thermodynamic, dynamics, and molecular dynamics simulations, the microstructure characteristics and microscopic processes generated of big-white-spot defects on the surface of hot-based galvanizing were studied. OM, LCM, FESEM, EPMA, XRD are used to characterize the microstructure of the coating surface and cross-section from different angles, and electrochemical tests are used to test corrosion mechanism of the coating. Molecular dynamics is used to calculate the atomic diffusion coefficient. The distribution of the aluminum-rich layer in the big-white-spot coating is extremely uneven. And the aluminum-rich layer loses its hindrance to the diffusion of Fe and Zn. There are many holes inside the defect coating. The fracture of aluminum-rich layer leads to a large number of Fe2Al5 particles within the defect. The defects cause the transformation of Al5Sb3 phase from interdendritic segregation to particle distribution. Nucleation theory, diffusion model, coating growth model are used to analyze the influence of temperature on coating. The nucleation rate increases exponentially with temperature and is mainly controlled by diffusion. The simulation results indicate that diffusion coefficient of Al is greater than Fe, and unequal diffusion leads to an increase in holes. The intensification of element diffusion, increase of holes, and poor adhesion between aluminum-rich layer and substrate jointly contribute the fracture of aluminum-rich layer.