Carbon dioxide corrosion in wet gas annular flow at elevated temperature

Abstract

Wet gas corrosion rates of plain carbon steel at the top and bottom of a high-pressure, 10-cm-diameter, horizontal pipeline were measured under annular flow conditions at 0.45 MPa carbon dioxide (CO2) and 90°C. The corrosive medium consisted of deionized water and a low viscosity hydrocarbon phase at water cuts of 80%. Superficial gas velocities (Vsg) of 15 m/s and 20 m/s were applied at a superficial liquid velocity (Vsl) of 0.05 m/s for 100 h. The corrosion rates measured by electrical resistance (ER technique advanced in three stages, showing an exponential decay corresponding to the iron carbonate (FeCO3) scale forming process. The general corrosion rates measured by ER were low, which were also confirmed by the coupon weight-loss measurements of average corrosion rates. No flow-induced localized corrosion (FILC) was found for the flow and corrosion conditions applied. Surface analysis techniques such as scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) revealed that the FeCO3 film forming process was flow dependent and different for scales formed on the top and on the bottom of the pipe, resulting in different crystal sizes. Cross-sectional analysis by use of metallurgical microscope (MM) indicates that a low thickness of the corrosion product film, usually <10 μm, was responsible for the high protection of the scales formed under these conditions.