Mitigation of Cr$_{2}$O$_{3}$’s reduction by MnO in UHV and oxidising environments at 500°C and 700°C : an observation of initial oxidation of Fe-Cr alloy with in-situ X-ray photoelectron spectroscopy

Abstract

This work is on the understanding of the oxidation mechanism of Fe-Cr alloys, the Crofer 22 APU alloy. This alloy is designed for interconnector part of a planar Solid Oxide Fuel Cell System which subjects the alloy to gaseous environments (O2, H2, H2O and CO) at temperatures from 500 to 750 °C. The evaporation of Cr causes corrosion from the alloy during SOFC’s operation. The critical information on the mechanism of Cr evaporation lies on in-situ observation of the alloy’s initial oxidation. This work is using the in-situ X-ray Photoelectron Spectroscopy (XPS) to understand the mechanism by utilising the ability of XPS to probe (up to 5 nm) of the outer alloy's surface in which initial oxidation reaction occurs. This work involves two XPS instruments with different capabilities. The first instrument: PHI Versa Probe has enabled the measurements in UHV which representing a low oxygen partial pressure (low ${P_{O_2}}$) environment. The second instrument: ScientaOmicron has enabled the measurements in O2 which representing the high oxygen partial pressure (high ${P_{O_2}}$) environment.The first experimental part is on the observation of the behaviour of high- purity Fe and Cr in UHV for 500 °C and 700 °C. Subsequently, Fe-22Cr model alloy is also investigated in UHV at the same temperatures to rule out the behaviour that might only be observed on commercial Fe-Cr alloy Crofer 22 APU. In such environments, Fe’s surface is being reduced from oxide into metallic while Cr exists as both metal and oxide. Model sketches are suggested based on the outcomes of these in-situ observations in UHV. This experiment has provided a base on understanding the behaviour of Crofer 22 APU alloy in the same conditions and further for the measurements done at high ${P_{O_2}}$. Second experimental part embarks on the investigation of Crofer 22 APU alloy in UHV for 500 °C and 700 °C. Being a commercial alloy, while it has the same amount of Fe and Cr as in Fe-22Cr model alloy, Crofer 22 APU has some other minor elements added up to 0.005 wt.%. These minor elements particularly Mn and Ti affects the behaviour of Fe and Cr at high-temperature. While studies have acknowledged the effect of these minor elements but not all of them have been able to describe the mechanism. Due to the low ${P_{O_2}}$ environment provided by the UHV, coupled with the presence of Mn and Ti has an influence on the selective oxidation behaviour of Cr. This is an evidence of the role of Mn and Ti in regulating the formation of Cr oxidation in such environment.The third experimental part is the highlight of the work because the AP-XPS has allowed for the observation of the oxidation mechanism on Crofer 22 APU in high p_(O_2 ) environment. The oxidation mechanism behaviour differs in higher ${P_{O_2}}$ environment. This chapter showcases the first result on in-situ XPS measurements employing AP-XPS at temperatures of 500 °C and 700 °C on Crofer 22 APU. The measurements, with ${P_{O_2}}$=5 mbar for 500 °C and 700 °C, has demonstrated the significant roles of Mn in regulating the oxidation of Cr.Oxidation mechanism of alloy has been investigated in-situ with both UHV- and AP-XPS to provide a means of low and high ${P_{O_2}}$ environments at 500 °C and 700 °C. The outcome from different environments provided has been different from each other. Nevertheless, it could provide a foundation for a systematic study with in-situ XPS on the initial oxidation mechanism of high-temperature alloy in the future