Electrochemical Behavior of Glass Melts: Application to Corrosion Processes

Molten silicate glasses are complex media in which ionic species can be easily dissolved. That typically concerns the metallic cations or oxoanions. The as-described liquids have therefore the properties of electroactive solutions in which the ions displacements allow the electrical conduction. As a consequence, the use of electrochemical techniques adapted from those classically used in aqueous media allows the characterization of specific physico-chemical properties of the system (i.e. the molten silicate matrix containing the dissolved ionic species). In particular, thermodynamic characteristics, like the redox and the basicity of the molten glass, can be measured as well as kinetic properties, like the intrinsic ion mobility or the metallic alloys corrosion rates. Some of these results, from specific studies performed by IJL-UMR 7198, are summarized here. For example, an electrochemical concentration cell has been developed to determine the activity of the strong base Na2O in silicate melts at high temperature, and the results were compared to theoretical models of basicity. A scale of formal potentials characterizing the redox couples in corrosion process was determined, allowing the prediction of the chemical reactivity with regard to thermodynamic criteria. Kinetic data evidenced the conditions leading to the passivation of the surface of alloys: it was deduced that the most efficient protection was provided by chromia. Lastly, the conditions of thermodynamic stability of Cr2O3 have been studied by considering the influence of temperature, basicity, and oxygen partial pressure in the melts.