THE SYNERGISTIC EFFECT OF HYDROGEN-SULFIDE AND NITROGEN-DIOXIDE ON THE ATMOSPHERIC CORROSION OF ZINC

A laboratory study of the effect of sub-ppm levels of H2S and NO2 on t he atmospheric corrosion of zinc in humid air is reported. Each sample was exposed individually to a synthetic atmosphere with careful contr ol of pollutant concentrations,relative humidity, and flow conditions. Corrosion products were analyzed by grazing angle x-ray diffraction a nd x-ray photoelectron spectroscopy. Ion chromatography was employed t o identify water soluble anions. The interaction of the pollutants wit h zinc metal was studied using trace gas analysis in real time. A stro ng H2S/NO2 synergism was observed. The main solid product was zinc ble nde (ZnS), accompanied by small amounts of sulfate. The optically smoo th zinc sulfide film was shown to grow by solid-state diffusion of ion s, the rate controlling step being the deprotonation of H2S adsorbed o n the ZnS surface. NO2 acts as a cathodic depolarizer, forming HNO2(g) , resulting in a strongly increased rate of zinc sulfidation.