A comprehensive parametric study was performed using a small-scale laboratory setup with the aim of investigating the occurrence of localized corrosion in marginally sour environments. The parameters of interest were the partial pressure of H2S, bulk pH, temperature, the partial pressure of CO2, salt concentration and steel substrate carbon content. This series of experiments defined a window of operating conditions leading to the occurrence of localized corrosion: temperatures between 30C and 60C, pH2S from 0.02 mbar to 0.15 mbar, bulk pH below 6, and sodium chloride concentrations between 1 and 10 wt%. Even if, according to thermodynamic calculations, FeSMackinawite was under-saturated in the bulk solution in marginally sour environments, the formation of an FeSMackinawite layer was observed by scanning electron microscope (SEM) and Element Energy Dispersive Spectroscopy (EDS), embedded in the Fe3C network of the steel microstructure. The hypothesis is that the surface saturation of FeSMackinawite was much higher than in the bulk due to mass transfer limitations within the Fe3C network leading to a higher pH. This hypothesis was confirmed when similar experiments did not yield any localized corrosion when performed on 99% pure iron samples, whose microstructure does not contain any cementite.
Keywords: Localized Corrosion, Pitting, Sour Corrosion, FeS, H2S, CO2 corrosion, Fe3C, Mild Steel,
API X65 Steel, Pure Iron.