In experiments simulating direct supercritical CO₂ power cycle conditions, a change in the corrosion behavior of 316 and 347H stainless steels has been observed during the startup and shut down periods. In these exposure tests, the steels experienced environments which are CO₂-saturated water systems at elevated temperature and pressure. Due to the electrochemical nature of corrosion processes occurring in these systems, corrosion behavior of ferritic-martensitic P91 steel and austenitic 347H stainless steel was studied in CO₂-saturated H₂O using potentiodynamic and potentiostatic experiments. The solution was prepared by bubbling CO₂ through H₂O at constant temperature. The solution preparation and the experiments were carried out at 25 and 50°C.

Potentiodynamic experiments, including cyclic voltammetry, were performed to determine the corrosion behavior of selected materials. Corrosion rates were determined from linear polarization resistance experiments. Optical and scanning electron microscopy of the corroded surfaces were performed after the potentiostatic experiments.