Mechanism of Internal Oxidation of Alloy 617 in He-CO-CO₂ Environments at 1123 K (850 °C)

Alloy 617 was exposed in Cr-Cr₂O₃ Rhines pack and in He-CO-CO₂ gas with the goal of determining the mechanisms that govern the rapid internal aluminum oxidation kinetics of the alloy at 1123 K (850 °C). The predominant mechanism is the accelerated oxygen transport along the incoherent Al₂O₃-metal interfaces in the internal oxidation zone of Alloy 617, which results in values of oxygen diffusion coefficients up to five orders of magnitude greater than the bulk oxygen diffusion coefficient in pure nickel at 1123 K (850 °C). Another contribution to the rapid internal oxidation is the gas permeability of Cr₂O₃, which results in enhanced oxygen dissolution in the alloy. Despite formation of a surface Cr₂O₃ film, the dissolved oxygen concentration attained in Alloy 617 in He-CO-CO₂ environments is a factor of 4 greater than that established by the dissociation oxygen partial pressure of Cr₂O₃ in the Rhines pack exposure. The dissolved oxygen concentrations and oxygen diffusion coefficient values established in this study are used to determine that internal oxidation of Alloy 617 in He-CO-CO₂ environments can be mitigated by increasing the Al concentration of the alloy at least to 5.0 at. pct.