Synergistic Effect of NaCl and SO2 on the Initial Atmospheric Corrosion of Zinc Under Wet–Dry Cyclic Conditions

Abstract

The synergistic effect of NaCl and SO2 on the atmospheric corrosion of zinc has been studied at equivalent total but different ratios of molar deposition rate under wet–dry cyclic conditions. The results show that the corrosion rates, corrosion morphologies and the composition of the corrosion products are strongly influenced by the molar deposition rate ratio of NaCl and SO2 (NaCl/SO2). The corrosivity of NaCl and SO2 toward zinc increases in order of SO2 < 1:3 < 3:1 < NaCl < 1:1. The corrosion morphology is patchy corrosion for Group 1:1, while it is pocking corrosion for the other four experimental groups. The corrosion product containing sulfur and chlorine detected on zinc is Gordaite (NaZn4SO4(OH)6Cl·6H2O), which has quite porous structure and was supposed to cause the patchy corrosion tendency of zinc in Group 1:1. In addition, soluble zinc corrosion products, which can inhibit the atmospheric corrosion process of zinc, were found on zinc samples in Groups 1:3 and SO2 and connected to the lower corrosion rates of zinc in these two groups.