Corrosion Rate Measurement of a Downhole Tubular Steel at Different CO₂ Partial Pressures and Temperatures and Calculation of the Activation Energy of the Corrosion Process

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.