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 Ag 2 S 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.