The electrochemical corrosion measurements at temperatures above 300 °C present a specific challenge due to limitations on material stability in sub- and super-critical hydrothermal environments, the very low concentrations of ionic species in the typical boiler water and the sluggish corrosion process. The actual carbon steel corrosion rates, after formation of protective oxide layer, are as low as few microns per year. The traditional mass loss corrosion studies would require a substantial experimental time to measure such a low rate process. We developed an approach using a number of electrochemical techniques that are able to provide in-situ information on the instantaneous corrosion rates at any stage of the process. In this paper, the experimental corrosion data obtained by electrochemical impedance spectroscopy, DC polarization, and cyclic voltammetry at temperatures up to 350 °C are reported to illustrate the corrosion behavior of the carbon steel in de-oxygenated all-volatile treatment (AVT) boiler solutions with different chloride contents (up to 3000 ppb).