Effect of surface conditions and alloying elements on the early oxidation behaviour of two austenitic alloys in the pure steam environment

Abstract The early oxidation behaviour of 347 SS and Alloy 800H with various surface preparation methods, including grinding, mechanical polishing (M−polishing), and electropolishing (E−polishing), were investigated in 650 °C pure steam. For the ground specimens, a protective Cr-rich oxide layer was formed at the M/O interface and lowered the oxidation rate owing to the presence of a nano-crystalline subsurface layer which provided the fast Cr outward diffusion path. On the other hand, M− and E−polished specimens showed significant spallation for 347 SS, while larger weight gain for 800H. Low Cr in 347 SS resulted in the formation of Fe2O3 nodules due to easier Cr2O3 degradation for ground specimen and the absence of Cr-rich oxide at the internal oxidation zone (IOZ)/matrix interface for M− and E−polished specimens. Higher Cr in 800H contributed to duplex Cr2O3 for ground specimen and Cr-rich oxide at IOZ/matrix interface for M− and E−polished specimens. Higher Ni in 800H enhanced Cr outward diffusion and Cr activity at the M/O interface, which improved the oxidation resistance. Low Al content in Alloy 800H showed no positive effect on oxidation resistance due to the Al internal intergranular oxidation.