Atmospheric Corrosion of Silver in Outdoor Environments and Modified Accelerated Corrosion Chambers

The accelerated corrosion of pure silver (Ag) exposed in a standard salt spray test chamber under standard conditions (5 wt% sodium chloride [NaCl] solution at 95°F and 95% relative humidity), as well as in a modified salt spray chamber test that included ozone (O₃) and ultraviolet (UV) irradiation with 5 wt% NaCl, was investigated. The effects of O₃ and UV on the atmospheric corrosion of exposed samples were determined with coulometric reduction and scanning electron microscopy–energy dispersive spectroscopy. The coulometric reduction is based on the cathodic reduction of the film, which forms on the Ag surface during exposure. A quantitative value of film formation on Ag sample could be indicative of the cumulative corrodent level present at the exposure sites. Coulometric reductions on exposed Ag coupons were performed in 0.1 M Na₂SO₄. The corrosion rates of the samples exposed in the modified ASTM B117 corrosion chamber under UV and O₃ conditions were much higher than the standard B117 salt spray test. Each O₃ exposure level (100 ppb and 800 ppb) and UV irradiation level (5.44 W/m² and 46.8 W/m²) condition affected the corrosion rate of Ag in the following order: high UV and high O₃ > high UV and low O₃ > low UV and high O₃ > low UV and low O₃. The corrosion rates of the samples in the modified B117 corrosion chamber under UV and O₃ conditions were higher than the field samples.