The corrosion of copper by atmospheric sulphurous gases

The sulfurization of copper by atmospheric gases is widely recognized, but the importance of the potential causative agents of sulfurization and the mechanisms involved have remained unresolved. In this work, polycrystalline copper has been exposed to the atmospheric gases hydrogen sulfide (H 2S), carbonyl sulfide (OCS), carbon disulfide (CS 2), and sulfur dioxide (SO 2) in humidified air under carefully controlled laboratory conditions. At room temperature, the rates of sulfurization by H 2S and OCS are comparable, and are some two orders of magnitude greater than those by CS 2 and SO 2. Given the atmospheric concentrations of these gases, it is clear that OCS is the principal cause of atmospheric sulfurization of copper except near sources of the gases where high concentrations may render H 2S (and possibly SO 2) important. At constant absolute humidity, the sulfurization rate of copper by OCS is found to be inversely proportional to temperature over the range 21–80°C, a property attributed to reduced quantities of surface water at high temperatures and the subsequent decrease in the rate of hydrolytic transformation of OCS into a reactive form. In a final series of experiments, the initial sulfurization of copper by 2.2 ± 0.2 ppm H 2S in humidified air at 22°C has been studied in detail. The first stages of sulfurization involve rapid attack by H 2S at surface defect sites. As these corrosive mounds spread and merge, diffusion of copper to the surface is impeded and the fraction of H 2S molecules striking the surface that become incorporated into the corrosion film drops sharply from ∼ 5 × 10 −5 (at t = 5 s) to ∼ 8 × 10 −7 (at t = 72 h).