Galvanically coupled process for the conversion of Ag2O to AgI

The conversion of Ag2O, grown-on Ag substrates, to AgI has in been investigated, using open-circuit potential, linear polarization, and potentiostatic control measurements. Three distinct reaction stages were clearly identified from the time-dependent behaviour of the open-circuit potential, EOC, and the corrosion currents measured by linear polarization. In the early stages, when the silver surface was covered with a coherent Ag2O film with low porosity, EOC∼(Ee)Ag2O/Ag and only chemical conversion of Ag2O to AgI was observed. As Ag2O was consumed and the area of Ag exposed to I− solution increased, galvanic coupling of Ag2O reduction to Ag and the oxidation of the Ag substrate to AgI also occurred. Once all the Ag2O has been reduced, AgI formation stopped and EOC fell to (Ee)AgI/Ag. Chemical conversion was shown to produce fine particulate AgI whereas that formed anodically via galvanic coupling was in the form of large crystals. Since few larger crystals were observed, conversion via galvanic coupling appeared to be a minor process.