The corrosion resistances of several alloys were investigated to optimize performance and cost in seawater and supercritical CO2 environments. Many alloys are prone to corrosion in seawater and/or supercritical CO2 containing impurity environments. Exposure and electrochemical experiments were conducted in both environments to evaluate alloys’ corrosion resistance. In seawater corrosion tests, salt fog spray test was conducted on 12 alloys to down select alloys with good corrosion resistance; alloy hardness in annealed and hardened conditions were considered as well. Cyclic polarization and electrochemical impedance spectroscopy were conducted to evaluate down selected alloys’ corrosion resistance in different heat treatment conditions. Alloy with high Cr and Ni content generally have higher corrosion resistance, and heat treatment of alloys have negatively affected alloys’ corrosion resistance. In supercritical CO2 environments with O2 and H2O impurities, high pressure and temperature autoclave exposure tests were conducted to evaluate the corrosion resistance of 347H stainless steel and P91 martensitic-ferritic steel. Electrochemical tests at ambient pressure at 25°C and 50°C were performed with H2O saturated with CO2 to calculate corrosion rates of the steels. X-ray diffraction and scanning electron microscopy were used to characterize corrosion products before and after corrosion testing. This study revealed that 347H has higher corrosion resistance than P91 due to higher Cr content and severe corrosion occurred during the condensation of H2O during the shutdown period of autoclave tests.