CARBON-DIOXIDE CORROSION OF CARBON-STEEL IN 2-PHASE FLOW

Results were presented from a four-year experimental study on carbon d ioxide (CO2) corrosion of carbon (C) steel in two-phase flow. Tests we re carried out in a gas and water loop that permitted control and regu lation of relevant parameters. Flow rates of gas and water were regula ted independently to obtain a number of two-phase flow regimes, such a s bubble flow and slug flow. In more than 20 long-term experiments las ting from one to several weeks each, pH was varied from 4 to 7 while t he temperature was held at 20 degrees C, 40 degrees C, 60 degrees C, a nd 80 degrees C in different experiments. Corrosion rates were monitor ed continuously in time with a radiation detection technique. Scanning electron microscopy analysis and x-ray analysis of the specimen surfa ce and cross section were done on selected specimens after each experi ment. It was found that, in cases where the formation of protective fi lms is difficult, flow could have a ''positive'' role by eroding the i ron carbide films that otherwise would accelerate corrosion by galvani c action. When protective films did form (higher pH and higher tempera tures, Fe2+ at saturation or supersaturation), corrosion rates on the order of 0.1 mm/y were obtained. In most cases, protective films prove d very resistant to mechanical erosion even in severe flow caused by s lugging and flow disturbances. In some cases, damage at the top of the pipe was more extensive than at the bottom.