The temperature-dependence of the SO2-induced atmospheric corrosion of zinc; a laboratory study

The SO₂-induced atmospheric corrosion of zinc was studied at 4, 14, 22 and 30 °C and 95% RH. Each sample was exposed individually to synthetic atmospheres with careful control of SO₂ concentration (107 and 500 ppb), relative humidity and flow conditions. The initial reaction between SO₂ and zinc was studied in a time-resolved manner. Two-week exposures were performed to measure the corrosion rate and study the formation of corrosion products. Corrosion products were analysed by X-ray powder diffraction and ion chromatography. The corrosion rate was inversely dependent on temperature, the maximum rate being found at the lowest temperature. SO₂ deposition showed a similar trend with the highest deposition rate at 4 °C. At low temperature a thick film of ZnSO₄(aq) formed on the metal surface, whereas zinc hydroxysulphate (ZnSO₄ · 3Zn(OH)₂ · 4H₂O(s)) was the main corrosion product at 22 and 30 °C. The inverse temperature-dependence of the corrosion rate of zinc is proposed to be connected to the formation of sparingly soluble zinc hydroxy sulphate which slows down the deposition of SO₂ on the surface.