The atmospheric corrosion of zinc in the presence of NaCl: The influence of carbon dioxide and temperature

The atmospheric corrosion of zinc has been studied at 4, 22, and 38 deg C. The samples were exposed to synthetic air with careful control of CO sub 2 concentration, relative humidity, and flow conditions. The relative humidity was 95%, and the concentrations of CO sub 2 were < 1 and 350 ppm, respectively. Sodium chloride was added before the exposures (0, 14, and 70 mu g/cm exp 2 ). Mass gain and metal loss results are reported. As expected, NaCl is corrosive toward zinc, giving rise to heavy pitting. In the absence of CO sub 2 , the rate of the NaCl-induced corrosion was found to increase strongly with temperature. However, in the presence of CO sub 2 , the corrosion rate of zinc is independent of temperature. In the absence of CO sub 2 , zincite, ZnO, is the dominant corrosion product, while zinc hydroxy carbonates and simonkolleite, Zn sub 5 (OH) sub 8 Cl sub 2 bullet H sub 2 O, dominate in the presence of CO sub 2 . A mechanism is presented that explains the observations.