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Description
The issue of CO₂ corrosion on mild steel has been the main problem in oil and gas industry, particularly in transportation systems for crude oil and natural gas. The corrosion behaviour of mild steel in CO₂ environment at varying temperatures was studied through weight loss test, surface analysis and phase characterization of corrosion products. Mild steel samples were exposed to two media, which are CO₂ saturated 3.5% NaCl solution and 3.5% NaCl solution without CO2 at temperatures of 25°C (control), 40°C, 60°C and 80°C for 7 days. Morphology changes, microstructure of corrosion products, cross sections and phase characterization were analysed using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and optical microscope (OM). Results from weight loss tests found that the corrosion rate increased with temperature. However, in CO₂ environment, at higher temperatures, corrosion rate decreased due to the formation of protective layers. FESEM images revealed mild steel in CO₂ environment underwent uniform corrosion whereas localized corrosion was observed in the absence of CO₂. The thickness of the corrosion product layer increased with temperature in both environments, but the thickness of the layer decreased when a protective layer formed, mainly in CO₂ environment. Furthermore, XRD analysis confirmed the formation of siderite, hematite and ferrous hydroxide in CO₂ environment, while the cementite, hematite and ferric oxyhydroxide were identified in non-CO₂ environment. In conclusion, corrosion rate of mild steel increases with temperature in environment without CO₂ as the corrosion products formed do not provide protection to the sample while in CO₂ environment, corrosion rate decreases at higher temperature due to the formation of siderite layer to protect mild steel.
