Isothermal oxidation at high temperature of a nickel-chromium alloy in water vapor-rich air. Part 2: Metallographic characterization of the oxidized samples
The Ni-25Cr samples oxidized at high temperature with thermogravimetric measurements exploited on the mass gain kinetic field in the first part of this work, were here metallographically prepared and subjected to different investigations: X-ray diffraction and SEM observation of the outer scales, cross-section examination of the quality and thickness of the external oxides and chromium concentration in the subsurface. It appeared that, if sensible differences were found again between the present samples and samples of the same alloy but oxidized in dry air, the differences between oxidationwith this higher water vapour content and oxidation with a less humidified air studied in a previous work, were not significant. This confirms the first feelings deduced from the kinetic results. In this range of water vapour concentration, the oxidation behaviour remains almost the same. Further investigations devoted to demonstrate an effect of water concentration in air must concern higher vapour concentrations associated to higher pressures.