Abstract
The oxidation kinetics of Ni-20Cr alloys with additions of 0.5% Ce or less and 1% Si were studied in air at temperatures between 1000 and 1260°C for 500 minutes using thermogravimetric method. After completion of kinetic runs, the amount of surface oxide spalled on cooling from the oxidation temperature was measured. The addition of more than 0.1% Ce or 1% Si resulted in decrease in the rate of oxidation. In each case the surface oxides formed were very susceptible to the spalling. The best improvement in the oxidation resistance was attained by the addition of both 0.5% Ce and 1% Si owing to the marked decrease in the rate of oxidation and the good adherence of surface oxide. In order to clarify the effect of Ce addition and to know the limit of the practical performance, cyclic and continuous oxidation tests for long period were carried out on Ni-20Cr-1Si alloys with 0.5% Ce and without Ce in air at 1, 200°C. The surface morphology, microstructure and composition of oxide scales were examined by means of scanning electron microscopy, X-ray diffraction and electron probe microanalysis. The spalling of surface oxide formed on the alloy without Ce initially occurred at the grain boundaries, and subsequently extended to the interior of grains with increasing time of oxidation and number of cycles. No spalling of surface oxide was observed in the alloy with 0.5% Ce even under the cyclic condition. The addition of 0.5% Ce to the Ni-20Cr-1Si alloy promoted the formation of protective Cr2O3 layer as an outer scale, and the internal oxidation of Cr and Si beneath the continuous Cr2O3 layer. The marked improvement of scale adherence was attributed to the keying of the protective surface oxide to the alloy substrate due to the internal oxide and the highly convoluted oxide/allay interface.
