Corrosion Behavior of Ferritic-Martensitic Steel in H₂O Containing CO₂ and O₂ at 50°C to 245°C and 8 MPa

To understand the corrosion mechanisms of structural materials in low-temperature components of direct supercritical CO₂ power cycles, immersion experiments were performed in aqueous environments expected at these conditions. A ferritic-martensitic steel (UNS K91560) was selected as the candidate material. Steel specimens were fully submerged in H₂O pressurized with 99% CO₂ and 1% O₂ to 8 MPa, and heated up to temperature (50°C, 100°C, 150°C, or 245°C), with a test duration of 500 h. Corrosion rates were calculated based on mass loss. Scanning electron microscope, x-ray diffraction, x-ray photoelectron spectroscopy, and Raman spectroscopy were used to characterize microstructure, phases, crystallinity, and composition of the corrosion product layer. Experimental results show that specimens exposed at 100°C had the highest corrosion rate, followed by the specimens exposed at 50°C. The specimens exposed at the highest temperature exhibited the lowest corrosion rate. An outer noncontinuous, nonprotective Fe-rich oxide layer and a well-adhered inner oxide layer containing both Fe and Cr formed on the specimen surfaces. The inner oxide layer changed from amorphous to crystalline as the temperature increased.