Manganese sulfide (MnS) is known to providing pitting corrosion sites in stainless steels [1, 2]. The roles of MnS on pitting corrosion have been generally agreed that the electrochemical and/or chemical reactions of MnS release S species [1-3]. Although many suggestions have been discussed to understand an initiation of pitting corrosion, it is still difficult to elucidate an initiation process of pitting corrosion because of intricate steps of a pit initiation which includes passivity and complicated passivity breakdown. It is important to investigate detailed studies for understanding the effects of primal S species released from MnS on a passivity behavior of stainless steel. Despite the fact that MnS chemically dissolves and releases hydrogen sulfide ion (HS–) as primary reaction, very little attention is given to the effects of HS– on a passive behavior of stainless steel which can be additional roles to understand an initial stage of a pitting corrosion. The authors have developed a liquid-phase ion gun (LPIG) to release an infinitesimal amount (ppm-order) of HS– by reducing a silver/silver sulfide microelectrode and applied to induce the local sulfidation on a silver surface [4]. The application of LPIG enables to elucidate the passivation and/or depassivation of stainless steels induced by hydrogen sulfide ions. In this study, effects of HS–on passivity of type-316L stainless steel were examined by using LPIG for the first time. The fabrication procedure of a silver/silver sulfide LPIG microelectrode was basically identical with that reported previously [4]. A diameter of the microelectrode was 500 µm. A three-electrode electrochemical cell with a platinum counter electrode and an Ag/AgCl/sat. KCl reference electrode (SSE) was used for electrochemical measurements of a substrate electrode of the stainless steel. The surface of the stainless steel was passivated by a potentiostatic polarization at 0.4 VSSE for 100 s. When a distance between microelectrode and substrate electrode was sustained with 125 μm, the microelectrode was connected with the substrate electrode and galvanostaticaly reduced to generate HS– by using a battery-driving DC signal source in pH 8.4 boric acid-borate buffer solution, while the substrate stainless steel electrode was continuously polarized at 0.4 VSSE. Electrochemical impedance was measured at a constant frequency of 15 Hz and in a frequency range of 10−2 to 104 Hz with an amplitude voltage of 10 mV. Mott-Schottky (MS) analysis was also conducted at a frequency of 15 Hz while the applied potential was stepwise shifted from the potential of 0.4 VSSE to –0.4 VSSE. Moreover, silver wire and tungsten wire electrodes were used to estimate concentration of HS− and pH value, respectively, in the narrow space between microelectrode and substrate electrode before and after the galvanostatic polarization of LPIG microelectrode. Auger electron spectroscopy (AES) was also used to analyze the passive film on the stainless steel when it was formed with or without presence of HS–generation by LPIG. From the open circuit potential measurement of the silver and tungsten wire electrodes during the LPIG polarization for generation of HS−, concentration of HS− and pH values in the narrow space were estimated with less than 10 ppm and ca. pH 9.5, respectively. During potentiostatic polarization of the stainless steel and LPIG microelectrode, current density increased with increase in concentration of HS−. It is considered that further reactions could be induced by HS− during the passivation of the steel surface. The impedance at 15 Hz decreased during the polarization in the presence of HS− by LPIG, while that sustained after a dilution of HS− by stopping the generation of HS− and removing the microelectrode to the solution bulk. Moreover, a charge transfer resistance of passive film formed in the presence of HS− decreased with increase in HS– concentration. MS results indicated that the more defective n-type passive film is formed in the presence of HS− than that formed without presence of HS−. AES results revealed the increase of metal cations and the decrease of oxygen anion in the passive film formed in the presence of HS−. It was indicated that the passive film formed on type-316L stainless steel at 0.4 VSSE in pH 8.4 buffer solution was unstable due to the presence of HS−.
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Sep 1, 2015
Hong Luo + 4
