Developing and Testing Standards to Assess Atmospheric Corrosion of Silver Witness Coupons

Atmospheric exposure of Ag witness coupons yields corrosion products such as AgCl and Ag 2 S that are highly dependent on local atmospheric chemistry. The principal goal of this study was the synthesis of a comprehensive catalog of silver corrosion product standards with characterization using electrochemical and surface analytical methods. Several silver corrosion products were generated using galvanostatic oxidation with good charge efficiency and were analyzed using coulometric reduction, cathodic polarization, and surface techniques. Average reduction potentials estimated by coulometric reduction were − 0.22 ± 0.005 V MSE (Ag 2 O), − 0.34 ± 0.03 V MSE (AgCl), − 0.50 ± 0.001 V MSE (AgBr), − 0.74 V MSE (AgI), and − 1.2 5 ± 0.0 1 V MSE (Ag 2 S). Results from surface analytical techniques confirmed the presence of supporting halides, sulfides, or oxides. Surfaces were also analyzed after the film was reduced, confirming that coulometric reduction consumed the corrosion product. The standards were used to identify Ag 2 O, AgCl, and Ag 2 S on coupons exposed to two outdoor environments and one indoor environment, and the average reduction potentials from these coupons were within ± 0.02 V MSE of the standards.