Atsushi Takei1 and Kazuyoshi Nii1
1National Research Institute for Metals, Tokyo
The effect of Si content on the oxidation behavior of 80Ni-20Cr alloys has been studied in the repeated oxidation in air stream at the temperature of 1100°C. The addition of 1% or 5%Si to the alloys lowered the weight gain in oxidation, whereas the amount of spalling of oxide scale was increased with the addition of Si. The structures of oxide layers observed by microphotography, X-ray diffraction and electron probe microanalysis (EPMA) were different with the Si content of alloys. The oxide layer of the alloy with 1%Si consists of multilayers, that is Ni oxide, Cr2O3 and SiO2 as the external oxide layer. The oxide layer remained on the alloy with 5%Si, however, consists of a single oxid layer of Cr2O3 containing small amounts of Si and Ni. Inspite of the fact that the amount of Si in this alloy is larger than that of the alloy with 1%Si, the SiO2 oxide layer was not observed at the interface of alloy-oxide. It was found by EPMA that the concentration of Si in the oxidized 5%Si alloy substrate was increased in the vicinity of the surface, although Si in the 1%Si alloy was depleted.
From the above results the internal oxidation of Si is assumed in the near surface region of 5%Si alloy. The internal oxidation of 5%Si alloy was confirmed by an increase in hardness in the near surface region.
The difference in oxidation behavior between the 1%Si and 5%Si alloys can be understood under the assumption that the oxide layer formed on 5%Si alloy contained much larger amounts of Ni and Si than that on 1%Si alloy, and that this oxide layer tends to crack more easily, thus being less protective for the penetration of oxygen.
Please do not copy without permission.