HIGH-TEMPERATURE OXIDATION BEHAVIOR OF A WROUGHT NI-CR-W-MN-SI-LA ALLOY

An investigation was carried out to study the kinetics and products of oxidation of a wrought Ni-Cr-W-Mn-Si-La alloy at temperatures in the range of 950 to 1150 degrees C. Oxidation kinetics were evaluated from measurements of weight change, metal loss, and internal penetration. Analytical electron microscopy, scanning electron microscopy, electron probe microanalysis, and X-ray diffraction were used to characterize the scale microstructure. Initially, La was observed to segregate with in a surface layer of about 5 mu m thick, which promoted Selective oxi dation of Cr and Mn. Oxidation kinetics were found to follow a parabol ic-rate law with an activation energy of about 232 kJ/mol. During stea dy-state oxidation, the scale consisted of an inner adherent layer of alpha-Cr2O3 modified by the presence of La and Si, and shielded by an outer layer of MnCr2O4. Most of the La was segregated to grain boundar ies of the alpha-Cr2O3 scale, however, Si was homogeneously distribute d. It was concluded that the characteristic oxidation resistance of th e alloy was related to the synergistic effects of Ni and Cr and to the effective minor additions of La, Si, and Mn; however, the useful life of the scale was limited by rupture and surface depletion in Cr, lead ing to accelerated internal oxidation.