Microstructure understanding of high Cr-Ni austenitic steel corrosion in high-temperature steam

The microstructure and microchemistry of the oxide scales formed on Fe-21Cr-32Ni and Fe-17Cr-9Ni steels after exposure to deaerated high-temperature high-pressure steam at 60 0 & DEG;C for 150 0 h have been analysed and compared by several advanced characterization techniques. By comparing the oxide scales formed at different-stages of exposure, it is shown that Fe-21Cr-32Ni steel was internally oxidized at the early-stage, and then an external oxide scale was developed together with an inner chromia band under the internal oxidation zone. In comparison, Fe-17Cr-9Ni steel was internally oxidized together with an ex-ternal Fe-rich oxide scale during the entire experimental period. The thicknesses of the internal oxidation zone of Fe-21Cr-32Ni and Fe-17Cr-9Ni steels were-7 and-70 mu m, respectively. Further characterisation revealed that the internal oxidation zone contained (Cr, Fe, (Ni))3O4 and nanoscale nickel networks, to-gether with numerous nano-pores. The effects of these structures on mass transfer and reaction product formation were discussed, in connection with the alloy composition and the formation of the chromia layer.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.