Growing interest in transporting supercritical carbon dioxide (scCO2) for enhanced oil recovery and carbon capture and storage has led to an increased interest in the corrosion behavior of pipeline steels in scCO2 fluids with varying levels of water contamination. Electrochemical measurements in scCO2 fluids have been limited by the very low solution conductivity, even in fluids saturated with water vapor. Electrochemical corrosion measurements using linear polarization resistance, electrochemical impedance spectroscopy, and electrochemical frequency modulation are presented here for X65 carbon steel in supercritical carbon dioxide contaminated with water vapor. The low solution conductivity was overcome using a coating of ion conducting polymer over the corrosion probe. The results showed that increasing membrane thickness resulted in reductions to both charge transfer and mass transfer rates. It is proposed here that this behavior was a result of the thickness of the aqueous film varying with the membrane thickness. Improving the process of applying the membrane could allow electrochemical corrosion measurements to be performed in low-conductivity environments with respect to varying thicknesses of the aqueous film.