PNNL

Abstract

Current and future power systems require chromia-forming alloys compatible with high-temperature CO2. However, critical questions concerning the mechanisms of oxidation and carburization remain unanswered. Herein we shed light onto these processes by studying the very initial stages of oxidation of Fe22Cr, Fe22Ni22Cr, and Ni22Cr model alloys. Ambient pressure X-ray photoelectron spectroscopy enabled in situ analysis of the oxidizing surface under 1 mbar of flowing CO2 at temperatures up to 530 °C, while post-exposure analyses revealed the structure and composition of the oxidized surface at the near-atomic scale. We found that gas purity played a critical role in the kinetics of the reaction, where high purity CO2 promoted the deposition of carbon on the surface of the growing oxide layer. This provided a locally decreased oxygen activity, which facilitated the selective oxidation of Cr. In contrast, no carbon deposition occurred in low-purity CO2 and Fe oxidation ensued, thus highlighting the critical role of impurities in defining the oxidation pathway of the alloy. The oxide formed on Fe22Cr was both thicker and more permeable to carbon compared to Fe22Ni22Cr and Ni22Cr, while the chemical state of carbon was consistent with atomic diffusion through the oxide.