Degradation modes of austenitic and ferritic-martensitic stainless steels in He-CO-CO₂ and liquid sodium environments of equivalent oxygen and carbon chemical potentials

The objective of this work is to explore possible thermodynamic correlations between the degradation modes of austenitic and ferritic-martensitic alloys observed in high temperature He-CO-CO₂ environments with oxygen and carbon chemical potentials equivalent to that in a liquid sodium environment containing 2-5 molppm oxygen and 0.02-0.2 molppm carbon at temperatures 500-700 °C. Two He-CO-CO₂ environments (Pco/Pco₂ = 1320, Pco = 1980 molppm, and Pco/Pco₂ = 9, Pco = 13.5 molppm) were selected to test alloys NF616 and 316L at 700 and 850 °C. Upon exposure to He environments at 850 °C, 316L samples exhibited thick surface Cr2O₃ scales and substantial internal oxidation; however at 700 °C no significant internal oxidation was observed. NF616 samples exhibited relatively thinner surface Cr2O₃ scales compared to 316L samples at both temperatures. NF616 samples exposed to liquid sodium at 700 °C and He-Pco/Pco₂ = 9 at 850 °C showed decarburization. No surface oxide formation was observed on the sample exposed to the Na environment. Results obtained from He exposure experiments provide insight into what may occur during long exposure times in a sodium environment.