The Effects of Rare Earths and Reactive Elements on the Oxidation Resistance of Ni–20Cr Alloy

Abstract

The isothermal oxidation behavior of Ni–20Cr alloys with 0.7 mass% of various reactive elements (Y, La, Al, Zr, Ti and Si) was investigated in air at 1273 and 1373 K.
It was found that the alloying elements except for Ti reduced the oxidation rate of the Ni–20Cr alloy at 1273 and 1373 K . There was a tendency that the parabolic rate constants obtained during the first 36 ks oxidation at both temperatures for the alloys with the alloying elements except for Ti were closely related to the affinity for oxygen of the alloying elements. The larger the standard free energy for the oxide formation of the alloying element, the smaller was the parabolic rate constant for the oxidation of the alloy with the alloying element. The oxidation behavior after 36 ks oxidation at 1373 K was rather complex compared to that at 1273 K . The highest oxidation resistance was obtained by a Ni–20Cr–0.7Y alloy at a lower temperature (1273 K) and during the early stage of oxidation at a higher temperature (1373 K). Whereas, a Ni–20Cr–0.7Al alloy showed the highest oxidation resistance after 72 ks at 1373 K with the formation of the internal oxide layer α-Al₂O₃. The addition of 0.7%Ti considerably reduced the oxidation resistance of the Ni–20Cr alloy.
The spallation of the scale was also reduced by the addition of the reactive alloying elements, especially Al and Si, which formed their internal oxides beneath the scales.
The difference between the improving effects of the alloying elements observed in the Ni–20Cr and Fe–20Cr alloys were discussed from the difference in the diffusivities of oxygen and the alloying elements in both alloys.