Modeling the effect of chloride content on hydrogen sulfide corrosion of pure iron by coupling of phase and polarization behavior

Localized corrosion of higher alloyed steels has frequently been shown to produce locally aggressive solutions with low pH levels in crevices and pits. It also has been established that such solutions can serve to initiate and propagate stress corrosion cracking. The present model aims at a basic understanding of the time-dependent local acidification process and, in a first approach, uses pure iron as a model metal. The model couples the anodic polarization resistances to the precipitated equilibrium masses of iron disulfide (FeS2), which in a closed loop time stepwise procedure are calculated from the solute concentrations in a diffusion boundary layer. With the cathodic process controlled by hydrogen ion reduction, the precipitation of FeS2 enforces local pH reductions at the anodic site depending on the total concentrations of HS-, Fe++, and H+ in the assumed diffusion layer. In the present work, the effects of chlorides on ion migration, sulfide solubility, anodic polarization resistances, and solid phase precipitations of FeS2, together with ferrous chloride (FeCl2), are integrated into the model. As a result, the increase of chloride contents accelerates the anodic acidification and increases the mean corrosion currents. This effect is more pronounced at higher H2S partial pressures depending on bulk pH levels. As in the previous work, the reduction of bulk pH accelerates acidification and increases the corrosion currents. As a specific result for pH levels below 7, the effect of H2S partial pressures is characterized by an initial drop of corrosion currents followed by an increase at higher H2S contents. Also, the increase of total pressures at a constant H2S volume content enhances corrosion rates due to the respective effect of increasing H2S partial pressures. The results are shown to reflect presently known experimental and sour service field corrosion experiences.