Determination of Corrosion Mechanisms and Estimation of Electrochemical Kinetics of Metal Corrosion in High Subcritical and Supercritical Aqueous Systems

Severe corrosion damage occurs in high subcritical and super-critical aqueous systems (SCAS), and few materials can withstand those harsh working conditions. In this study, two corrosion mechanisms, “chemical oxidation” (CO) and “electrochemical oxidation” (EO), have been postulated to describe the corrosion processes in SCAS, depending upon the density and dielectric constant of the systems. Electrochemical emission spectroscopy (EES) is used to differentiate between the two corrosion mechanisms, by postulating that only the electrochemical mechanism gives rise to spontaneous fluctuations in current and potential. EES is used to measure the instantaneous electrochemical corrosion activity in SCAS based on the assumption that a proportional relationship exists between the electrochemical current noise and the corrosion current. Experiments on Type 304 (UNS S30400) stainless steel (SS) and titanium in water, 0.01 m hydrochloric acid (HCl), 0.01 m sulfuric acid (H2SO4), and in 0.01 m sodium hydroxide (NaOH) show that the electrochemical mechanism is the dominant corrosion mechanism when the temperature is below 350°C and that it becomes of progressively lower importance as the temperature increases above the critical temperature (Tc = 374.15°C). The energy of activation for electrochemical corrosion of Type 304 SS and titanium in 0.01 m HCl is estimated and the result shows that titanium is more corrosion-resistant than Type 304 SS at high subcritical temperatures. In addition, the dynamic polarization study performed on Type 304 SS and titanium also suggests that titanium is more corrosion resistant in 0.01 m HCl at supercritical temperatures.