External Α-Al₂O₃ scale on Ni-base alloy 602 CA. – Part I: Formation and long-term stability

A multi-step oxidation treatment was developed for the NiCrAl-base alloy 602 CA to promote the growth of protective α-Al₂O₃ scales during exposure at 1100–1200 °C in air. Above 1000 °C, the alloy forms external chromia scales accompanied by internal alumina precipitation. On the other hand, external alumina grows on alloy 602 CA surface at 800–900 °C. The alloy specimens were pre-oxidized at 800 °C to form the external α-Al₂O₃ scale and further exposed at 1100 and 1200 °C. The resulting oxide scales were characterized with SEM/EDX, EBSD, and TEM. The pre-grown alumina scale demonstrated good adherence, long-term stability and healing capability.