Effect of Hydrogen Sulfide Ions on the Passive Behavior of Type 316L Stainless Steel

The effect of hydrogen sulfide ions (HS−) on the passivity of type 316L stainless steel was investigated in pH 8.4 boric acid-borate buffer solution. Galvanostatic polarization of a silver microelectrode covered with Ag2S generated both OH− and HS− above the stainless steel surface. During potentiostatic polarization of the stainless steel, the passivity-maintaining current density increased with increase in the concentration of HS− in the vicinity of the surface. The impedance of the stainless steel at a constant frequency decreased during polarization in the presence of HS−, while it was sustained after dilution of HS−. Electrochemical impedance spectroscopy (EIS), Mott-Schottky (M-S) analysis and scanning electrochemical microscopy (SECM) showed that a defective and n-type semiconductive passive film was formed in the solution containing HS−. Auger electron spectroscopy (AES) revealed that metal cations and oxygen vacancies in the passive film on the stainless steel increased when it was formed in a HS−-containing solution. The series of changes in passive film properties is thought to be due to adsorption of HS- on the film surface during the polarization.