AISI 316L stainless steel is extensively used in the oil and gas industry, partly due to the alloy’s low
cost and good availability. The applicability of 316L in wet CO2 or sour environments is due to the
protective passive oxide film formed on the surface. The corrosion risk for 316L in sour service
environments containing chloride is due to localized forms of corrosion. The role of H2S has been
attributed to its ability to depassivate materials.
Results from electrochemical impedance spectroscopy (EIS) and DC polarization measurements in Cl-/
CO2/H2S environments are reported and assessed against models proposed for initiation of passive film
breakdown in the presence of aggressive ions. The observed extent of the reactions involving H2S
implies a significant transport of electrons through the passive film, thus reducing its internal field. This
may in turn lead to film thinning and eventually film breakdown (depassivation). Density-functional
theory (DFT) calculations on the adsorption of chloride at a defect-free Cr2O3 surface, on the other
hand, indicate that this adsorption will have little influence on the initial breaking of the film (cf. filmbreaking
mechanism).