Abstract
The isothermal oxidation behavior of Ni–20Cr alloys with various amounts of TiO2 was investigated in air at 1273, 1373 and 1473 K . The oxidation rate was minimum in the alloys with 0.7 mass%TiO2 at 1373 and 1473 K and with 0.3%TiO2 at 1273 K . The oxidation rate markedly increased with increasing TiO2 content when the TiO2 content exceeded the one corresponding to the minimum oxidation rate, and then the TiO2 addition showed a deleterious effect on the oxidation resistance of Ni–20Cr alloy. Spalling behavior of the scale was also significantly dependent on the TiO2 content of the alloy. The composition of Cr2O3 scales formed on the Ni–20Cr–xTiO2 alloys during the early stage of oxidation was determined by IMA, which revealed that the scales contained NiO and TiO2. In order to clarify the relation between the TiO2 content of the scale and the oxidation behavior of Ni–20Cr–xTiO2. alloys, the electrical resistivity of the sintered Cr2O3–xTiO2 was measured in the temperature range from 873 to 1473 K in air. The electrical resistivity of the Cr2O3–xTiO2 was strongly dependent on the TiO2 content. The resistivity had a maximum at 0.1–0.2 mass%TiO2 and then gradually decreased beyond this TiO2 content. IMA results of the scale were used to correlate the oxidation behavior of the alloys with the electrical resistivity of the sintered Cr2O3–xTiO2. Consequently it was found that the TiO2 content of the scale formed on the alloy with the minimum oxidation resistance corresponded with the TiO2 content of the sintered Cr2O3–xTiO2 with the maximum electrical resistivity. These results suggested that the oxidation behavior of the alloy changed due to the alteration in the defect structure of the Cr2O3 scale by the introduction of Ti.
