The corrosion of silver by atmospheric sulfurous gases

Polycrystalline silver has been exposed to the atmospheric gases H 2S, OCS, CS 2 and SO 2 in humidified air under carefully controlled laboratory conditions. OCS is shown to be an active corrodant while CS 2 is quite inactive. At room temperature, the rates of sulfidation by H 2S and OCS are comparable, and are more than an order of magnitude greater than those of CS 2 and SO 2. It appears that OCS is the principal cause of atmospheric sulfidation of silver except near sources of H 2S where high concentrations may render the latter gas important. At constant absolute humidity, the sulfidation rate of silver by both H 2S and OCS decreases from 20 to 40°C and then increases to 40 to 80°C. This behavior is interpreted as indicating a decrease in water-enhanced sulfidation as the relative humidity is reduced, followed by dominance of sulfidation by a water-independent process with strong positive temperature dependence. The initial sulfidation of silver by 3.9 ± 0.2 ppm H 2S in humidified air at 22°C has been studied in detail. The data are consistent with an initial stage of sulfidation involving rapid attack by H 2S at surface defect sites. As these corrosive products spread and merge, diffusion of silver to the surface will be impeded. In agreement with this picture, the results show that the fraction of H 2S molecules striking the surface that become incorporated into the sulfide film drops sharply from ∼3 × 10 −6 (at t = 5 min) to ∼1 × 10 −8 (at t = 500 h).