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CO₂ corrosion of mild steel constitutes a significant integrity threat within hydrocarbon transport systems, particularly in crude oil and natural gas pipelines. This study comparatively investigates the temperature-dependent corrosion behaviour of mild steel in CO₂-saturated and CO₂-free environments using weight loss test, surface morphology and phase characterisation. Mild steel samples were exposed to two media which are CO₂-saturated 3.5% NaCl solution and 3.5% NaCl solution without CO₂, 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 accelerated corrosion rate. However, in 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 CO₂ environment underwent uniform corrosion whereas localised corrosion was observed in the absence of CO₂. The corrosion scale 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 demonstrate that temperature and environmental conditions critically influence the corrosion mechanisms and product formation where the protective siderite scales significantly mitigate corrosion in CO₂-rich systems at higher temperatures.
