Selective Oxidation of Silver-Magnesium Alloys in Water Vapor

Abstract

A simple method is described for measuring low rates of selective oxidation of Ag–Mg alloys at elevated temperatures when water vapor is used as oxidizing agent. It is found that the MgO layer builds up on the surface of the alloy according to a parabolic law over a range of the water vapor pressure from 7×10⁻³ to 6×10⁻² mm Hg, and the oxide growth begins to follow a linear law rather than a parabolic law at a pressure of 1 mm Hg. In the region where the parabolic law holds, the growth of the oxide layer seems to be controlled by the diffusion of Mg through the layer. 20.7 kcal/mol is obtained as the activation energy of diffusion. Applyjng the transition state theory of gas-metal reaction, as developed by Gulbransen, the diffusion of Mg through the oxide layer is given by D (cm²/sec)=2.17×10⁻⁹exp (−20,700/RT).