INFLUENCE OF COOLING RATE ON MICROSTRUCTURE AND CORROSION BEHAVIOR OF COATINGS ZN-7AL HOT DIP ON STEEL

Abstract

In many commercial processes of hot-dip galvanized, aluminum is added to the bath for various purposes, including inhibiting the formation of Zn-Fe layer to increase corrosion resistance. This work aims to determine the influence of the cooling rate (water at room temperature, still air and inside the furnace) on the microstructure and corrosion behavior of coatings Zn-7Al deposited by hot dip on a steel AISI 1015. Methodology includes visual inspection, thickness measurement and adhesion tests in according to the standards ASTM A123 and ASTM B571. Microstructural characterization was performed by optical microscopy, scanning electron microscopy and EDS spectrometry. Potentiodynamic polarization tests were made in NaCl 3.5% solution. The results revealed that the water-cooled coatings do not comply with the quality requirements for standards. The microstructure of the coating in air-cooled consist Zn-rich dendrites and microconstituent interdendritic eutectic Al-Zn-Fe; water quenched coatings show eutectic segregated at the interface coating/steel, and cooled in the furnace, show a monotectoide microconstituent of dendritic morphology. For corrosion testing were excluded water quenched coatings for not having acceptable quality and commercial zinc coating was used to compare. Coatings cooled in the furnace presented active-passive behavior, while the cold air and commercial zinc are in dissolution. The difference in corrosion rate between the coatings cooled in air and furnace can be attributed phases greater amount of aluminum present in the furnace cooled. It is concluded that the cooling rate during solidification of the Zn-7Al coatings significantly influences the quality, microstructure and corrosion behavior.