Full Factorial Experiment of Laboratory Exposure of Ag with Ozone and Ultraviolet Radiation

The discrepancy between laboratory ASTM B117 salt spray test and field exposure of Ag makes research on new lab accelerating test technique necessary and important for atmospheric corrosion research of metals. Exposure to a lab environment containing moist air, ozone, NaCl and UV light was shown in previous research to simulate the atmospheric corrosion behavior of Ag. To better understand and to attempt to predict corrosion rate of Ag, a full factorial experiment has been carried out with three factors: ozone concentration, UV intensity and relative humidity. Ozone ranged from 250 to 2000 ppb; UV intensity at the window of the chamber ranged from 0 to 4.7 mW/cm²; relative humidity was adjusted between 0 to 90%.  Loading of NaCl was fixed at 50 µg/cm², which is much more chloride than that necessary to form the AgCl observed during lab exposures. By XRD analysis, AgCl was identified as the only corrosion product in wet environments, while Ag₂O was the only corrosion product in dry environments. Corrosion products were quantified by galvanostatic reduction of exposed samples in 0.1 M Na₂SO₄ solution. At least two samples of Ag were exposed to each environment condition. The addition of UV light did not always increase the Ag corrosion rate. In high relative humidity environments, UV in fact decreased the Ag corrosion rate.  This odd effect of UV is probably due to photodecomposition of AgCl by UV radiation.  Based on quantification results of Ag in different environments, an equation to represent corrosion rate of Ag as a function of ozone concentration, relative humidity and UV intensity was achieved.  Furthermore, corrosion rates of Ag exposed to field environments were analyzed and correlation of lab and field corrosion rates is ongoing.