The Effect of CO_2 and H_2S on the Passivation of Chromium and Stainless Steels in Aqueous Solutions at Elevated Temperature and under High Pressure

摘要

The effect of temperature, CO_2 and H_2S pressure on passivation of chromium and on Fe-Cr and Fe-Cr-Ni-alloys was investigated in H_2O-Na_2SO_4 and H_2O-NaCl solutions by means of linear sweep voltammetry, electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The investigations have been performed in the temperature range 20 - 150°C under the pressure of 20 bars. The growth of the temperature from 20°C to 150°C increases the (Fe~(2+)+Fe~(3+))/Cr~(3+) and O~(2-)/OH~- ratio in the passive film formed on Fe-Cr alloys in aqueous solutions of 0.1M Na_2SO_4. It seems that the presence of dissolved CO_2 also leads to dehydration of the passive film. The stability of passive layer formed on the surface of Fe-Cr and Fe-Cr-Ni alloys decreases in the solutions saturated with CO_2 and CO_2 + H_2S mixture. The effect is more distinct for austenitic Fe-Cr-Ni alloys than for ferritic Fe-Cr ones. The presence of nickel deteriorates the formation of the passive film in solutions saturated with H_2S and CO_2 + H_2S mixture because of the high susceptibility of nickel to sulfide formation.