Corrosion rate of a low alloy tempered martensite downhole tubular steel was studied at different temperatures and CO2 partial pressures using direct weight loss measurement. Experiments were carried out in a high pressure high temperature autoclave. CO2 partial pressures of 40 (276), 80 (552), 160 (1103), and 320 (2206) psi (kPa) were examined at temperatures 25, 40, 50, and 75 °C. A linear trend between corrosion rate and CO2 partial pressure was observed at different temperatures and increasing CO2 partial pressure increased the slope of the lines. In addition, at constant CO2 partial pressure, increasing temperature increased the corrosion rate. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the corrosion products formed on the surface. At 50 °C and below, only iron and iron carbide were detected using XRD analysis, but, at 75 °C, in addition to iron carbide, iron carbonate was also detected. Formation of scattered iron carbonate crystals was studied using SEM. Finally, having the corrosion rate at different temperatures and CO2 partial pressures, the activation energy of the corrosion process was calculated using the de Waard Equation.

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