Corrosion Behavior of Mild Steel in Sour Environments at Elevated Temperatures

The objective of this work was to determine the corrosion rate of mild steel and characterize the corrosion products in sour environments at temperatures ranging from 80°C to 200°C. First, a H₂S–H₂O water chemistry model was developed based on available literature for a closed system at high temperature. Then, H₂S corrosion tests were conducted at 80°C, 120°C, 160°C, and 200°C with an exposure time of 4 d. Linear polarization resistance (LPR) and weight loss (WL) methods were used to measure the corrosion rates. X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive x-ray spectroscopy microanalysis (SEM/EDS) were used to characterize the corrosion products and surface morphology. The results show that the initial corrosion rates increased with temperature then decreased as they achieved steady state. The corrosion product was comprised of two distinct layers. The inner corrosion product was always an iron oxide layer (hypothesized to be Fe₃O₄), while mackinawite, troilite, pyrrhotite, and pyrite were identified as the main components of the outer layer at 80°C, 120°C, 160°C and 200°C, respectively. Pourbaix diagrams generated based on the analysis of water chemistry corroborated the experimental characterization of the corrosion products.