Effect of heat treatment on the interlayer and corrosion behavior of Zn and Zn-0.5Al coated high carbon steel for marine application

摘要

Hot dip galvanizing has been an important technique for corrosion protection in the industry. In recent years, longer service life for hot dip galvanized steel is thus crucial to offset the rapid rises in cost of maintenance in service. This project is aimed to investigate the effects of heat treatment on the intermetallic and corrosion behaviour of Zn and Zn-0.5%Al coated high carbon steel wire rope for marine application. A total of nine set of heat treatment parameters with heating time one, three and five hours at temperatures 250, 350, 400°C respectively were conducted onto Zn and Zn-0.5%Al coated high carbon steel substrate (0.87%C). These parameters were to evaluate the effects of heat treatment time and temperature on the microstructural evolution and formation kinetics of the coating. The coated steels had undergone corrosion tests namely salt spray test and electrochemical test. The samples before and after corrosion test were analyzed with optical microscopy, scanning electron microscopy, energy dispersive X-ray and X-ray diffractometer The result shows that heat treatment affects the coating thickness and weight loss due to powdering effect. Besides, by increasing heat treatment time and temperature, it was observed that the gamma layer for Zn-coated steel was increased. However, the growth rate of gamma layer for Zn-0.5%Al galvanized wire rope was slow. It was also observed that heat treatment affects the corrosion rate of Zn and Zn-0.5%Al coated samples. The optimum heat treatment parameter for Zn coated samples was heating at temperature 350ºC for 3 hours which gives the lowest corrosion rate. Optimum heat treatment parameter for Zn-0.5%Al coated samples was heating at temperature 400°C for 5 hours.