This study investigated the microstructure and high temperature oxidation properties of Fe-25Cr-20Ni-1.5Nb, HK30 alloy manufactured by metal injection molding (MIM) process. The powder used in MIM had a bi-modal size distribution of 0.11 and 9.19 μm and had a spherical shape. The initial powder consisted of γ-Fe and Cr23C6 phases. Microstructural observation of the manufactured (MIMed) HK30 alloy confirmed Cr23C6 along the grain boundary of the γ-Fe matrix, and NbC was distributed evenly on the grain boundary and in the grain. After a 24-hour high temperature oxidation test at air atmospheres of 1000, 1100 and 1200°C, the oxidation weight measured 0.72, 1.11 and 2.29 mg/cm,2 respectively. Cross-sectional observation of the oxidation specimen identified a dense Cr2O3 oxide layer at 1000°C condition, and the thickness of the oxide layer increased as the oxidation temperature increased. At 1100°C and 1200°C oxidation temperatures, Fe-rich oxide was also formed on the dense Cr2O3 oxide layer. Based on the above findings, this study identified the high-temperature oxidation mechanism of HK30 alloy manufactured by MIM.
In order to improve the efficiency of power generation system and reduce CO2 emissions power plants work at high temperature and pressure. Under such conditions modified steel 9Cr, which fulfils the requirements concerning creep resistance, is used. However, Cr2O3 formed on the steel does not protect the construction material in the atmosphere which contains CO2 and SO2. The aim of the experiment was to study the behaviour of P91 steel in CO2 atmosphere with the addition of 1% and 5 vol.% of SO2 at different temperatures (700, 800 and 900°C). It was concluded that the corrosion rate of P91 steel is increasing with a rise in temperature. Scales formed in CO2 atmosphere at 900°C contain a mixture of iron oxides in the outer layer and chromium-iron spinel in the inner layer. The FeS and Ni were found in the inner zone of scales formed in SO2 atmosphere.
In this investigation, the formation of oxide scales on different Co-Ni based superalloys of γ–γ′ type was analyzed. Co-20Ni-7Al-7W (at. %) alloy as well as its W-free modifications based on Co-Ni-Al-Mo-Nb and Co-Ni-Al-Ta systems was analyzed under conditions of high temperature oxidation at 800 and 900°C. Therefore, the alloys were isothermally oxidized at selected temperatures for 100 h in laboratory furnace. Afterwards, the oxidation products were evaluated by means of X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). The performed tests showed that W-free alloys exhibit worse oxidation resistance compared to those of Co-Ni-Al-W alloys. After oxidation at 900°C, all alloys were prone of oxide spallation. The scales characterized by oxide peeling were mostly composed of complex Co-based oxides, including CoWO4, CoTa2O6, Co2Mo3O8, CoNb2O6.