Oxygen diffusion and internal oxidation of Mg in Ag/1.12at.%Mg

Internal oxidation of Mg in Ag/1.12at.%Mg was studied at 450–825 °C in various oxygen partial pressures. Measurements of O weight gain and oxidation-front velocity showed that at 450 °C the rate of O penetration into the alloy was ≈400 times slower than O diffusion in pure Ag. At 825 °C, this factor decreased to ≈60 times slower. A comparison of these results with a calculation based on a model of internal oxidation indicated that O diffusivity in pure Ag was nearly identical to O diffusivity in the alloy. The decreased O penetration rate in the alloy was attributed to O uptake by the Mg species, which are able to capture superstoichiometric amounts of O. O fixation proceeded according to predictions of the model; oxidation rate increased with partial pressure due to an increased O activity gradient. However, below ≈600 °C and at 0.08 atm partial pressure, O fixation progressed at a rate notably faster than that predicted by the model. This result was attributed to a change in Mg–O fixation stoichiometry, such that smaller amounts of O were absorbed, allowing the oxidation front to proceed more rapidly.