Nitrogen Dioxide Effect on Atmospheric Corrosion of Zinc Exposed in Climate Chamber

摘要

Sulphur dioxide plays a crucial role in the atmospheric corrosion of zinc. The environmental protection and pollutants emission laws enacted about two decades ago in some industrialised countries led to a notable reduction in the sulphur dioxide concentration levels. However, levels of other atmospheric pollutants such as the nitrogen oxides (NO_x) remained constant or even increased slightly, whereby the relative significance of these air pollutants in the atmospheric corrosion has increased. For that reason, some researches have been studying in the last years the role of the NO_x in the atmospheric corrosion of metals. The studies about NO_x effect in atmospheric corrosion of zinc are few and show contradictory results. Most of them make use of accelerated tests with unrealistic exposure conditions. In this work, the role of NO_2 in the atmospheric corrosion of zinc was analysed from a detailed characterisation of corrosion products. Laboratory tests with exposure parameters close to the conditions observed in real atmospheres were performed. It was simulated the dry deposition of NO_2, SO_2 and SO_2 + NO_2, at 25°C and 35°C, and relative humidity of 90%. The exposure time was 15 days. Characterisation of corrosion products was made by SEM/EDX, grazing-angle XRD and XPS. A moderate accelerant effect of dry deposited NO_2 in the presence of SO_2 was found at 25°C. At 35°C, under similar exposure conditions, a slight accelerant effect was observed. Without presence of SO_2, no significant NO_2 effects were found. It is suggested that humidity enhance this effect. In addition, NO_2 have a indirect role, as a catalyst for SO_2 reduction to sulphate, taking into account the greater proportion of sulphate ions detected in the case in which an accelerant effect was found, and the lack of nitrogen compounds in corrosion products.