A laboratory study of the initial stages of the atmospheric corrosion of zinc in the presence of NaCl; Influence of SO 2 and NO 2

Abstract

A laboratory study of the NaCl-induced atmospheric corrosion of zinc in air containing sub-ppm concentrations of sulphur dioxide and nitrogen dioxide at 70 and 95% RH is reported. Each sample was exposed individually to synthetic atmospheres with control of pollutant concentrations, relative humidity and flow conditions. NaCl (14–280 μg cm −2) was added ex situ either before or periodically during exposure. Mass gain and metal loss results are reported. The corrosion products were investigated with respect to phase composition and sulphur, chloride, nitrite, nitrate and zinc content by a combination of X-ray powder diffraction, ion-chromatography and gravimetry. This results in a detailed knowledge of the corrosion product composition, both quantitatively and qualitatively. Crystalline ZnO(s), Zn 5(OH) 8Cl 2 · H 2O(s), ZnSO 43Zn(OH) 2 · 4H 2O(s) and ZnSO 43Zn(OH) 2NaCl · xH 2 O(s) were detected. In addition soluble (normal) sulphate, nitrite and nitrate, and HNO 2(g) were found. The deposition of NO x was studied in a time-resolved manner using gas analysers. Evidence was found for NO 2 reduction on zinc. In the presence of sodium chloride, NO 2 was produced, whereas HNO 2(g) was formed on the clean zinc surface. In the presence of moderate to high concentrations of NaCl(s), sulphur dioxide slowed down the corrosion of zinc somewhat. The weak inhibitive action of sulphur dioxide was connected to the formation of sodium zinc hydroxychloride sulphate, ZnSO 43Zn(OH) 2NaCl · xH 2O, on the surface. In the presence of NaCl there was no evidence for a synergistic effect of the combination of SO 2 and NO 2.