Dense Al2O3 coating performance and its corrosion properties in molten zinc for sink roll applications

Surface treatment for sink rolls in continuous galvanizing lines is an important issue due to the unfavorable environment of the molten zinc. This study investigated the corrosion behavior of high-velocity oxy liquid fuel (HVOLF) sprayed WC-12Co, Hybrid-LVOF sprayed Al2O3-TiO2, and Al2O3 coatings in molten zinc bath. All coated samples were tested in a zinc bath for 30 days and cross-sectioned to examine the microstructure and corrosion attack. The results were checked and compared. The results showed that the molten zinc reacts with coating material causing major destruction of HVOLF sprayed WC-12Co. Whereas Hybrid-LVOF sprayed dense Al2O3-TiO2 and Al2O3 coatings performed very well for longer exposure times of 30 days in a molten zinc bath environment. The structural integrity and no penetration of Zn into the coating were observed without any thickness reduction in both coatings. However, Zn accumulation was observed on the Al2O3-TiO2 coating surface and Zn reacted with TiO2 and form Zn2TiO4. Whereas no Zn reaction was taking place with Al2O3 coating. Further no stress has been generated in the Al2O3 coating. A very small amount of gamma-Al2O3 is present which upon exposure converts to stable alpha-Al2O3. MECPL Jodhpur has developed a special process to solve this problem, and it could greatly improve the service life of a sink roll.