Influence of Manganese and Silicon on High Temperature Oxidation of Nickel-base Heat Resistant Alloys in Simulated VHTR Helium Environment

Abstract

Roles of Mn and Si in Hastelloy alloy X were examined in terms of the high temperature oxidation behavior of several experimental heats at 1000°C in flowing helium containing very small amounts of impurities. Both elements were found to have significant influence both on the steady state oxidation rates, evaluated as Cr consumption rate, and on the oxide film spallation resistance. In such a special environment, characterized by very low oxygen potential, the following effects were observed:
1) Within the range (max. 1.32 wt%) tested addition of Mn improved the oxidation resistance nearly propotionally to the increase of Mn content. The effect was interpreted in formation of the outer MnCr₂O₄ spinel oxide layer outside the inner Cr₂O₃ oxide layer.
2) Addition of Si caused the similar effect on the steady state oxidation rate to the maximum (1.02 wt%) amount tested. The degree of adherence of the protective oxide film, however, was maximized at around 0.3 wt%, above and below which spallation of oxide film tended to occur with either increasing or decreasing Si content. These two could be explained in formation and morphology of the Si oxide phase in the oxide and metal interface.