Evolution of oxide scale on a Ni-Mo-Cr alloy at 900 °C

The cyclic oxidation behavior of a Ni-Mo-Cr alloy was studied in air at 900 °C for exposure periods of up to 1000 h. The morphology, microstructure and composition of the oxide scale was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Oxidation kinetics was determined by weight gain measurements. The results show that steady state oxidation was achieved within 1 h of exposure. During transient oxidation, the alloy grain boundaries intersecting the alloy surface became depleted in Ni and enriched in Mo and Cr. The scale initially formed at the surface was NiO which grew outwardly. However, a protective Cr₂O₃ layer developed, rapidly retarding the rate of oxidation. Formation of NiMoO₄ was also observed. The presence of Mo in the alloy facilitated the formation of a Cr₂O₃ layer at an early stage of oxidation. The alloy exhibited considerable oxide spalling during prolonged exposure.