A laboratory study of the effect of NO2 on the atmospheric corrosion of zinc

Studies on the effect of NOx on zinc corrosion are scarce and their results are variable and at times seemingly contradictory. This paper reports laboratory tests involving the dry deposition on zinc surfaces of 800 μg m−3 NO2, alone and in combination with 800 μg m−3 SO2, at temperatures of 35 and 25 °C and relative humidities of 90% and 70%. From the gravimetric results obtained and from the characterisation of the corrosion products by optical microscopy, scanning electron microscopy (SEM/EDX), grazing incidence X-ray diffraction (GIXD) and X-ray photoelectron spectroscopy (XPS), it has been verified that the corrosive action of NO2 alone is negligible compared with SO2. However, an accelerating effect has been observed when NO2 acts in conjunction with SO2 at 25 °C and 90% RH. At 35 °C and 90% RH, the accelerating effect is smaller, and at low relative humidities (70%), the synergistic effect is only slight, which suggests it may be favoured by the presence of moisture. In those cases where an accelerating effect has been observed, a greater proportion of sulphate ions has been found in the corrosion products, and nitrogen compounds have not been detected, indicating that NO2 participates indirectly as a catalyst of the oxidation of SO2 to sulphate.