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% O-2 to 8 MPa, and heated up to temperature (50 degrees C, 100 degrees C, 150 degrees C, or 245 degrees 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 degrees C had the highest corrosion rate, followed by the specimens exposed at 50 degrees 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.