Effect of curing temperature on coating structure and corrosion resistance of ammonium zirconium carbonate on galvanized steel surface

The effect of curing temperature on the structure and corrosion resistance of an ammonium zirconium carbonate (AZC) coating on galvanized steel was investigated as an alternative to chromate. The corrosion resistance of the AZC coating was excellent when cured at 80 °C, but it decreased with an increase in the curing temperature; it was significantly inferior at curing temperatures over 160 °C. Two endothermic events were observed at 115 °C and 155 °C. The event at 115 °C resulted from the elimination of the water molecule coordinated to the zirconium atom, and the event at 155 °C resulted from the dehydration condensation of the hydroxide group involved in the bonds between zirconium, i.e., conversion of an ol-bridge to an oxo-bridge. Furthermore, it was observed that the cathode current of the coating plate cured at 160 °C or 200 °C, whose corrosion resistance was inferior to that at 80 °C or 120 °C in the salt spray test, increased. It is supposed that the dehydration occurring at 155 °C promoted shrinkage of the coating volume, which in turn caused the development of fissures on the coating surface. The fissures on the coating surface reduced the corrosion resistance significantly due to the increase in cathode current, which signifies the reduction of dissolved oxygen at the coating/metal interface.