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 degrees C and 95% RH. Each sample was exposed individually to synthetic atmospheres with careful control of SO2 concentration (107 a nd 500 ppb), relative humidity and flow conditions. The initial reacti on between SO2 and zinc was studied in a time-resolved manner. Two-wee k 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 wa s 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 degrees C. At low temperature a thick fi lm of ZnSO4(aq) formed on the metal surface, whereas zinc hydroxysulph ate (ZnSO4 3Zn(OH)(2) . 4H(2)O(s)) was the main corrosion product at 2 2 and 30 degrees C. The inverse temperature-dependence of the corrosio n rate of zinc is proposed to be connected to the formation of sparing ly soluble zinc hydroxy sulphate which slows down the deposition of SO 2 on the surface. Copyright (C) 1996 Elsevier Science Ltd