Passivation Behavior and Surface Chemistry of 2507 Super Duplex Stainless Steel in Acidified Artificial Seawater Containing Thiosulfate

In this work, passivation behavior and surface chemistry of 2507 super duplex stainless steel (SDSS) in acidified artificial seawater (ASW) containing different concentrations of thiosulfate are investigated using electrochemical techniques, X-ray photoelectron spectroscopy and the Point Defect Model. The 2507 SDSS is resistant to thiosulfate-induced pitting corrosion in the acidified ASW with a {[Cl−] + [SO42−]}: [S2O32−] ratio ranging from 59 to 5900, while the film dissolution rate is enhanced, resulting in a higher passive current density and lower polarization resistance. The passive film exhibits n-type semiconductivity in the acidified ASW and is independent of the thiosulfate levels. The donor density shows little change while the point defect diffusivity increases with increasing thiosulfate concentration. No elemental sulfur and sulfur-containing compounds can be detected on the well-passivated surface of 2507 SDSS, but increasing thiosulfate concentration results in the depletion of Cr2O3, enrichment of oxidized Fe, and thinning of the passive film. The influence of thiosulfate on the film composition depends on the passive states. Thiosulfate promotes the formation of Fe(II) in the prepassive region and accelerates the transformation of Cr(III) to Cr(VI) in the transpassive region.