Factors Influencing Localized Corrosion of Mild Steel in Marginally Sour Environments

摘要

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 H_2S, bulk pH, temperature, the partial pressure of CO_2, 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 30°C and 60°C, pH_2S 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, FeS_(Mackinawite) was under-saturated in the bulk solution in marginally sour environments, the formation of an FeS_(Mackinawite) layer was observed by scanning electron microscope (SEM) and Element Energy Dispersive Spectroscopy (EDS), embedded in the Fe_3C network of the steel microstructure. The hypothesis is that the surface saturation of FeS_(Mackinawite) was much higher than in the bulk due to mass transfer limitations within the Fe_3C 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.