Diffusion of sodium ions driven by charge compensation as the rate-limiting step of internal redox reactions

The kinetics of iron oxidation in sodium borosilicate melts have been determined as a function of time from 710 to 1570 K for four compositions of constant SiO2 content and B2O3 contents ranging from 0 to 22 mol%. The iron redox ratios have been measured by iron K-edge XANES spectroscopy as a function of time at constant temperature. The redox kinetics become slower with increasing B2O3 content. Near the glass transition range, they are controlled by the diffusion of network modifying cations toward the surface of the melt whereas they are limited at superliquidus temperatures by the diffusion of free Na+ ions to charge compensate Fe3 + in tetrahedral coordination, along with the diffusion of oxygen.