CO2 Corrosion of Carbon Steel in Two-Phase Flow.
Srdjan Nesic and Liv Lunde

In this paper results from the experiments on CO2 corrosion of carbon steel in two-phase flow are presented. An extensive experimental program has been carried out within the PROFF project initiated in 1989 by the Royal Norwegian Council for Scientific and Industrial Research (NTNF) and cosponsored by a number of companies. Tests were carried out in a gas/water loop, which enabled 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 pH 4 to pH 7 while the temperature was held at 20, 40, 60 and 80 C in different experiments. Corrosion rate was monitored continuously in time with a radiation detection technique. SEM analysis and X-ay analysis of the specimen surface and cross-section was done after each experiment on selected specimens.
It was found that in cases when it is difficult to form protective films flow can have a “positive” role by eroding the iron carbide film which otherwise accelerate corrosion by a galvanic action. In cases when protective films do form (higher pH and higher temperatures, Fe++ at saturation or super-saturation) low corrosion rates were obtained – order of 0.1 mm/y. Protective films have in most cases proven to be very resistive to mechanical erosion even in the cases of severe flow conditions caused by slugging and flow disturbances. In some cases damage at the top of the pipe was higher than the bottom.

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