Corrosion Behavior of Ferritic-Martensitic Steel in H<sub>2</sub>O Containing CO<sub>2</sub> and O<sub>2</sub> at 50°C to 245°C and 8 MPa
In order to understand the corrosion mechanisms of structural materials in low-temperature components of direct supercritical CO<sub>2</sub> cycles, immersion experiments were performed in the aqueous environments expected at these conditions. A ferritic-martensitic steel [UNS K91560] was selected as the candidate material. Steel specimens were fully submerged in H<sub>2</sub>O pressurized with 99% CO<sub>2</sub> and 1% O<sub>2</sub> to 8 MPa, and heated up to temperature (either 50°C, 100°C, 150°C, or 245°C), with a test duration of 500 hours. Corrosion rates were calculated based on mass loss. SEM, XRD, XPS, 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 non-continuous non-protective 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 temperature increased.