Mechanism of Internal Oxidation of Alloy 617 in Controlled Impurity Helium Environments at High Temperatures.

摘要

Alloy 617 has been identified as the primary candidate alloy for the intermediate heat exchangers of the High temperature gas-cooled reactors featuring very high core outlet temperatures in the range of 750 - 850 °C. In this temperature regime, the impurities in the helium coolant result in carburization, surface and internal oxidation of Alloy 617. Among these degradation modes, selective internal aluminum oxidation along the grain boundaries of the alloy is likely to be the life-limiting corrosion process as it can significantly lower the creep ductility and fatigue resistance of the alloy. Alloy 617 was exposed in a Cr-Cr2O3 Rhines pack and He-CO-CO2 gas with the objective of characterizing the surface and internal oxidation behavior and determining the governing oxygen transport mechanisms of rapid internal aluminum oxidation of Alloy 617 in the temperature range of 750 - 850 °C. Surface Cr oxidation is facilitated by grain boundary diffusion of Cr cations, whereas the internal aluminum oxidation kinetics are predominantly governed by the accelerated oxygen transport along the incoherent Al2O3 - metal interfaces in the internal oxidation zone of Alloy 617. The oxygen diffusion coefficients along these interfaces are several orders of magnitude greater than those reported in pure nickel and binary nickel based alloys. Another contribution to the rapid internal oxidation is the gas permeability of Cr2O3, which results in enhanced oxygen concentration at the alloy - Cr2O3 interfaces. Despite formation of a surface Cr2O3 film, the atomic fraction of oxygen established at the alloy - Cr2O3 interfaces in He-CO-CO2 environments is a factor of 4 greater than that did at the alloy surface by the dissociation oxygen partial pressure of Cr2O3 in the Rhines pack exposure at 850 °C. The oxygen solubility and diffusion coefficient values determined in this dissertation 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.%.