The Effects of Temperature on CO2 Corrosion of Mild Steel in 3.5 wt.% NaCl: Corrosion Rate, Surface Morphology, and Phase Characterisation

Abstract

CO2 corrosion of mild steel constitutes a significant integrity threat within hydrocarbon transport systems. This work comparatively investigates the temperature-dependent corrosion behaviour of mild steel in CO₂-saturated and CO₂-free environments through weight loss test, surface morphology, and phase characterisation. Mild steel samples were exposed to two media, which are CO2-rich 3.5% NaCl solution and CO2-free 3.5% NaCl solution at 25°C, 40°C, 60°C, and 80°C for 7 days. Morphology changes, microstructure of corrosion products, cross sections, and phase characterisation were analysed using field emission scanning electron microscopy (FESEM), optical microscope (OM), and X-ray diffraction (XRD). Results from weight loss tests found that an increase in temperature resulted in an accelerated corrosion rate. However, in a CO2 environment, a decrease in corrosion rate was observed at higher temperatures, attributed to the emergence of protective layers. FESEM images revealed that mild steel in a CO2 environment underwent uniform corrosion, whereas localised corrosion was observed in the absence of CO2. The corrosion scales thickened with increasing temperature in both media; however, the development of a protective carbonate layer in the CO₂ environment inhibited further deposition, leading to a thinner final layer. XRD analysis confirmed the formation of siderite, hematite, and ferrous hydroxide in a CO2 environment, while cementite, hematite, and ferric oxyhydroxide were identified in a non-CO2 environment. These findings suggest that while temperature initially accelerates sweet corrosion, the emergence of a compact FeCO3 scale above 60°C transitions the behaviour to partial protection, guiding the developing temperature-dependent mitigation strategies and the application of nano-enabled monitoring in CO2-rich pipeline environments.