Corrosion experiments on Au, Ag and Au-Ag alloys of various composition in 1mol/L solutions of NaHS and Na
2S have been carried out at 150°C over 120 days. Ag
2S was formed in NaHS solutions on Au
25Ag
75, Au
10Ag
90 and Ag
100 (pure silver) and in Na
2S solutions on Au
50Ag
50, Au
25Ag
75, Au
10Ag
90 and Ag
100, but its morphology differed: The Ag
2S formed in the NaHS solutions was platy, while that formed in the Na
2S solutions was mostly whisker-like. It is thought that Ag
2S morphology was controlled by HS
- and S
2- concentrations in the solutions and by the rate of sulfidation. The remarkably developed Ag
2S scale seen on Ag
100 was found to consist of an outer layer formed by the outward diffusion of Ag
+ and an inner layer formed by the inward diffusion of S
2-.
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