Molecular-scale investigation of the oxidation behavior of chromia-forming alloys in high-temperature CO2

Richard P. Oleksak,Tao Liu,Rafik Addou,J. Trey Diulus,Bharat Gwalani,John P. Baltrus,Arun Devaraj,Gregory S. Herman,Ömer N. Doğan

doi:10.1038/s41529-021-00194-1

Abstract

Current and future power systems require chromia-forming alloys compatible with high-temperature CO2. Important 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 and Fe22Ni22Cr 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 postexposure 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 and 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 early oxidation pathway of the alloy. The Cr-rich oxide formed on Fe22Cr in high purity CO2 was both thicker and more permeable to carbon compared to that formed on Fe22Ni22Cr, where carbon transport appeared to occur by atomic diffusion through the oxide. Alternatively, the Fe-rich oxide formed in low purity CO2 suggested carbon transport by molecular CO2.

Highlights

Current and future power systems require structural alloys that resist corrosion in CO2 at high temperatures[1]
This type of detailed peak fitting was beyond the scope of our current study, where measurements were taken at ambient pressures and with emphasis on timely acquisition to capture dynamic processes
This produced a surface with extremely low roughness (

Summary

Introduction

Current and future power systems require structural alloys that resist corrosion in CO2 at high temperatures (nominally >500 °C)[1]. Future power plants based on the oxy-combustion of fossil fuels—a process which enables efficient CO2 capture—would require structural alloys capable of surviving contact with flue gases containing high concentrations of CO23. Future power systems that utilize an indirectly heated supercritical CO2 Brayton cycle require alloys compatible with high-temperature CO24. High-temperature CO2 is significantly oxidizing to all metals present in a structural alloy. Oxidation resistance is required, which is often achieved by including high levels of chromium in the alloy. Fe–Cr alloys (e.g., ferritic and martensitic steels) and Fe–Ni–Cr alloys (e.g., austenitic steels) are among the most frequently employed materials due to a favorable combination of high-temperature strength and cost[2]

Methods

Results

Conclusion