New insights into near-surface grain refinement in Ni-containing alloys during high-temperature corrosion at 600 °C

Ni-alloys often exhibit near-surface microstructural changes due to surface deformation or reaction with the environment. The phenomenon of grain refinement has been of great interest in terms of understanding material behaviour and applicability. This prompts careful tailoring of alloy composition and processing parameters to achieve suitable microstructure. In this study, detailed experimental evidence of near-surface grain refinement of Alloy 27Cr33Ni3Mo during high-temperature corrosion at 600 °C is presented. Corrosion tests were carried out in a KCl-rich laboratory environment, mimicking boiler environment, for 24, 168, 500, 1000, 2000 and 8000 h. Additional tests were performed in evacuated quartz tubes to investigate the role of environment. XRD and advanced electron microscopy (SEM/EDX and EBSD) and conceptual modelling were used to characterize corrosion products and fine-grain microstructure. The alloy experiences breakaway oxidation, forming chromium-rich oxide scale due to selective oxidation. Below the scale, alloy grain refinement occurs through gradual disintegration driven by chemical potential of chromium oxidation. The growth kinetics of the fine-grain region undertakes a parabolic behaviour in the earlier stages and shifts to cubic behaviour in the later stages. The findings in this study highlight the potentials to design alloys that could exhibit beneficial microstructural changes during application.