Wet gas corrosion rates of C1018 and X65 steel have been measured at the top and bottom of a high pressure, 10 cm diameter horizontal pipeline under stratified flow conditions with different chloride (Cl-) concentrations. Experiments were performed for 200 hours at 90°C, CO2 partial pressure of 3.8 bar using a superficial gas velocity (Vsg) of 10 m/s and a superficial liquid velocity (Vsl) of 0.1 m/s. Three measurement techniques; ER, LPR, and WL were used simultaneously in the experiments. Localized corrosion occurred at the bottom of the pipe around the iron saturation point. The top of line was well protected by a thin corrosion product film and no localized corrosion was detected. C1018 and X65 have different sensitivities to pitting with a variation in Cl- concentration. Thus the pitting density concept is proposed to describe localized corrosion behavior. Surface analysis on iron carbonate films, by SEM and XRD, revealed different film thicknesses and crystal structures from the top to the bottom of the pipe. Cross-sectional analysis indicates that the thin corrosion product film, usually less than 10 microns, attached to the metal surface, is responsible for the low corrosion rate on top of the line, while the thick and porous film formed on the bottom, generally detached from the metal surface, was responsible for the initiation of localized corrosion.
Before adding acetic acid (HAc) to the flow loop, a systematic glass cell test program was initiated using potentiodynamic sweeps to investigate the electrochemical properties of carbon dioxide corrosion in the presence of HAc. A standard rotating cylinder three-electrode setup was employed. Tests were performed to study the effect of HAc in solutions de-oxygenated using both nitrogen and carbon dioxide. The range of HAc concentrations was then expanded to study the effect of the concentration of HAc on the cathodic and anodic reactions in carbon dioxide solutions. The presence of HAc was found to not affect the corrosion rate of carbon steel at low temperature since the main cathodic reaction (hydrogen ion reduction) was found to be under charge transfer control. The limiting current was strongly affected by the HAc concentrations, but the charge transfer mechanism was not changed. The anodic reaction was retarded with increasing HAc concentrations but the mechanism stayed the same. The effect of pH, temperature and rotational velocity was also studied.
Natural gas does not emerge from the reservoir pure and is always accompanied by various amounts of water, carbon dioxide, hydrogen sulfide, and organic acids. These substances combine to give rise to a very aggressive environment where the survival of mild steel is not guaranteed. The multiphase mixture of gaseous and liquid hydrocarbons, water, CO2 and H2S moves through the gas pipelines in a variety of complicated multi-phase flow patterns such as annular, slug and stratified flow, depending on the terrain topography and the individual phase flow rates. It is well known that flow can accelerate corrosion of mild steel by increasing the mass transfer rates of corrosive species and/or by damaging the protective films formed on the steel surface1.
In wet gas corrosion research, there are only a handful of studies that relate to field experience, some focusing on corrosion management2 and control3, others reporting actual case histories4. In most studies the focus was on top-of-line corrosion4-6 where high uniform corrosion and sometimes localized attack were associated with rapid condensation of water by external cooling. There are no studies reported in the open literature, which investigate the nature and magnitude of the attack in wet gas transport