Modeling Scale Formation In CO2 Corrosion Of Carbon Steel And Comparison With Time-Resolved In-Situ Synchrotron X-Ray Diffraction Data

The CO₂ corrosion of carbon steel is one of the most prevalent internal damage mechanisms for oil & gas production tubing, flowlines and pipelines. The corrosion rate typically increases with temperature up to a certain point, and then decreases at higher temperatures due to the formation of protective scales of corrosion product on the steel surface. Companies use various empirical and mechanistic models to predict the corrosion rate for expected service conditions and determine factors such as the required inhibitor availability, maximum allowable fluid velocity and inspection interval. However, the various models differ considerably in how they implement the protective effect of the scale, to the extent that practically significant differences in predicted corrosion rates are often obtained. In the present work, we have developed a method to perform computer simulations of general CO₂ corrosion, using a finite element formulation to handle the moving surfaces that result from concurrent steel dissolution and carbonate scale precipitation. We have also used in-situ synchrotron x-ray diffraction to experimentally investigate the structure and composition of the surface scale, and the rate of formation of each phase. We accelerated the corrosion process through the use of a small applied anodic current and we focused on the changes that occur with increasing temperature across the range from 40 to 90 °C, in which a peak in corrosion rate was observed for the selected environmental conditions. Results from these experiments will be presented and compared against model simulations.