Formation of basic copper sulfates and chlorides during atmospheric copper corrosion

Basic copper sulfates and copper chlorides are known to form in addition to cuprite during outdoor atmospheric corrosion of copper. Among the basic copper sulfates, posnjakite is reported to form in the early stage and then transform into brochantite when during the course of exposure. In contrast, copper chlorides, such as atacamite and nantokite, are reported not to form in the early stage. Using thermodynamic data at 298 K, we have calculated the conditions under which basic copper sulfates and chlorides are deposited on the surface of exposed copper. For pos njakite, the minimum sulfate ion concentration needed for deposition was calculated to be 6.3 × 10 ^-4 M given a cupric ion concentration of 1 × 10^-6 M and a pH of 7. The posnjakite formation found on copper exposed in summer in urban areas is reasonably explained by the observation that the sulfate ion concentration in the surface electrolyte exceeded the minimum concentration. Although the sulfate ion concentration in the surface electrolyte exceeded the minimum concentration during winter, there was no evident posnjakite formation. This is explained by the low relative humidity and the short 'time of wetness'. For atacamite, the minimum chloride ion concentration needed for deposition was calculated to be 2.5 × 10^-2 M given a cupric ion concentration of 1 × 10^-4 M and a pH of 7. Although the chloride ion concentration in the surface electrolyte on copper exposed in urban areas exceeded the minimum concentration needed to deposit atacamite, peaks originating from this phase were not observed. A possible explanation for this is the slow formation rate of atacamite. Posnjakite depostition was found to be predominant, particularly in winter. This is reasonable give that posnjakite is more stable than atacamite. This also explains why there was a lack of atacamite formation in the early stage of the exposure.