The transition from internal to external oxidation and the formation of interruption bands in silver-indium alloys

Abstract

Wagner's theory for the transition from internal to external oxidation in alloys is evaluated for the Ag-In system and experimentally tested at 550°C and at oxygen pressures varying from 1 atm to 10-4 mm Hg. The theory assumes that the transition is caused by the blocking of the diffusion processes upon the formation of a critical volume per cent of the internal oxide in the matrix. Reasonable agreement between the theory and the experimental data leads to the conclusion that the formation of 30 vol. % In2O3 in the silver matrix is a valid criterion for the transition from internal to external oxidation for Ag-In alloys. From measurements of the thickness of internal oxidation zones, the product of the mole fraction of oxygen in silver at Po2 = 1 atm and its diffusivity, was found to be 2.5 × 10-10 cm2/seeat 550°C, corresponding to a diffusion coefficient of 2.5 × 10-6cm2/sec. An approximate theoretical description of the formation of interruption bands formed when a sudden pressure drop occurs during the internal oxidation of an alloy is presented and evaluated for the Ag-In system. Oxidation experiments at 550°C for Ag-In alloys in which the oxygen pressure was dropped from Po2 = 0.21 atm (air) to Po2′ = 10-1, 10-2, 10-3 or 10-4 mm Hg are described. The widths of the experimentally observed interruption bands are in agreement with the predicted values. It is concluded that a double reversal of the oxidation front, as suggested by Meijering, does occur, and that this process is adequately described by the approximate treatment presented here. © 1961.