Factors Influencing Localized Corrosion of Mild Steel in Marginally Sour Environments

ABSTRACT

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 30°C and 60°C, 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, 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 Fe₃C 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 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.

INTRODUCTION

Localized corrosion is a common failure mode in sour environments¹ found in oil and gas pipelines containing CO₂, H₂S, and brine. For instance, the Caroline oil spill at Sundre, Alberta, Canada on Jan 6, 1994, was caused by internal wet H₂S and CO₂ pitting corrosion at the bottom of the pipeline, where an extremely high pitting corrosion rate, approximately 30 mm/y, was observed.² Researchers in the Institute for Corrosion and Multiphase Technology (ICMT) at Ohio University have reproduced localized corrosion in CO₂ environments with low concentrations of H₂S^3,4,5,6,7 commonly found in the oil and gas industry. Although these studies identified certain sets of experimental conditions that seem to lead to the occurrence of localized corrosion, the window of operating conditions leading to the occurrence of localized corrosion is not properly defined yet.