Carbon Dioxide Corrosion of Carbon Steel in Two-Phase Flow^⋆

Results were presented from a four-year experimental study on carbon dioxide (CO₂) corrosion of carbon (C) steel in two-phase flow. Tests were carried out in a gas and water loop that permitted control and regulation of relevant parameters. Flow rates of gas and water were regulated independently to obtain a number of two-phase flow regimes, such as bubble flow and slug flow. In more than 20 long-term experiments lasting from one to several weeks each, pH was varied from 4 to 7 while the temperature was held at 20°C, 40°C, 60°C, and 80°C in different experiments. Corrosion rates were monitored continuously in time with a radiation detection technique. Scanning electron microscopy analysis and x-ray analysis of the specimen surface and cross section were done on selected specimens after each experiment. It was found that, in cases where the formation of protective films is difficult, flow could have a “positive” role by eroding the iron carbide films that otherwise would accelerate corrosion by galvanic action. When protective films did form (higher pH and higher temperatures, Fe²⁺ at saturation or supersaturation), corrosion rates on the order of 0.1 mm/y were obtained. In most cases, protective films proved very resistant to mechanical erosion even in severe flow caused by slugging and flow disturbances. In some cases, damage at the top of the pipe was more extensive than at the bottom.