CORROSION OF AUSTENITIC STAINLESS STEELS IN SUPERCRITICAL AQUEOUS SOLUTIONS

摘要

Severe corrosion damage occurs on metals and alloys in contact with high subcritical and supercritical aqueous systems (SCAS), and few materials can withstand those harsh working environments. Two corrosion mechanisms, "chemical oxidation" (CO) and "electrochemical oxidation " (EO), have been postulated to describe the corrosion processes on titanium and on Type 304 SS in SCAS, depending upon the density and dielectric constant of the medium. Electrochemical Emission Spectroscopy (EES) is used to differentiate the mechanisms, by postulating that only EO gives rise to spontaneous fluctuations in current and potential. EES is used as a measure of the instantaneous electrochemical corrosion rate in SCAS, based on the assumption that a proportional relationship exists between the standard deviation of the electrochemical current noise density and the corrosion current (rate). Experiments on Type 304 SS and titanium in 0.01m HCl, 0.01m H_2SO4,, 0.01m NaOH, and in water 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 increase and/or the pressure (density) falls. Furthermore, the form of corrosion of both Type 304 SS and titanium at supercritical temperatures is acid attack, with the primary corroding species being H*. Oxygen is found not to be a strongly corroding species in supercritical aqueous environment, but is expected to become so at very high fugacities.