Morphology changes in Fe-Cr porous alloys upon high-temperature oxidation quantified by X-ray tomographic microscopy

Abstract

The effect of high-temperature oxidation at 850 °C (10 h, 30 h, 100 h) and 900 °C (10 h) on porous (∼30 % porosity) ferritic stainless steel (Fe22Cr) has been investigated using synchrotron tomographic microscopy, which allowed for visualisation, separation and quantitative analysis of the metallic core, closed pores, open pores and oxide scale phase. The same regions within the samples were investigated before and after oxidation performed at different conditions. Quantitative analysis of the tomographic data provided information on changes upon oxidation of the relative volume of the different phases, the specific surface area (SSA) of the metallic core, the thickness of the oxide scale and pore size distribution. The results were discussed in the context of thermogravimetric analysis of the samples and supported by SEM imaging. It was observed that oxidation leads to an increase of the SSA of steel and the largest increase (∼50 %) was obtained for the sample processed for 100 h at 850 °C. It is demonstrated the open porosity forms a network of connected channels within the sample and it dominates in the volume. In addition, the 3D imaging revealed breakaway oxidation areas for samples, for which this phenomenon remained undetected using 2D SEM analysis.