X-Ray Photoelectron Spectroscopy Study of the Effect of Nitrogen Dioxide and Sulfur Dioxide on the Atmospheric Corrosion of Copper at Low Relative Humidity Values

A study has been made of the individual and combined roles of nitrogen dioxide (NO₂) and sulfur dioxide (SO₂) atmospheric pollutants on corrosion and patina formation on copper in atmospheres with comparatively low water contents (70% and 50% relative humidity [RH]) for exposure times of up to 28 days. X-ray photoelectron spectroscopy (XPS) analysis has revealed some important compositional changes in the outermost atomic layers of films formed on copper, depending on the nature of the atmospheric pollutant, the humidity level, and the exposure time. After 7 days of exposure to NO₂, a significant nitrogen content is detected (mainly as nitrates). In contrast, in exposure to SO₂, copper oxides are present on the patina surface even after 21 days of exposure. In exposures to the mixture of NO₂ + SO₂, a nitrate layer grows on a copper sulfate film. As with NO₂ alone, the appearance of nitrates leads to an appreciable increase in mass gain values.