A mechanistic model of uniform carbon dioxide (CO 2) corrosion is presented that covers the following: electrochemical reactions at the steel surface, diffusion of species between the metal surface and the bulk including diffusion through porous surface films, migration due to establishment of potential gradients, and homogenous chemical reactions including precipitation of surface films. The model can predict the corrosion rate as well as the concentration and flux profiles for all species involved. Comparisons with laboratory experiments have revealed the strengths of the model such as its ability to assist in understanding complex processes taking place during corrosion in the presence of surface films.

A theoretical carbon dioxide (CO 2) corrosion model was used to conduct numerical experiments, which allowed total insight into the underlying physicochemical processes. The focus was on factors influencing protective iron carbonate film formation and the effect that these films have on the CO 2 corrosion process. It was confirmed that high bulk pH, high temperature, high partial pressure of CO 2 , high Fe 2+ concentration, and low velocity all lead to favorable conditions for protective iron carbonate film formation. The model can be used to identify threshold values of these parameters. Corrosion rate was not strongly correlated with protective film thickness. The so-called surface film "coverage" effect appeared to be more important. Corrosion rates decreased rapidly as the film density increased. It was shown that in the presence of dense films diffusion of dissolved CO 2 through the film is the main mechanism of providing the reactants to the corrosion reaction at the metal surface. It was demonstrated that "detached" films have poor protective properties even when they are very dense. Serious errors in prediction/ reasoning can be made by operating with bulk instead of surface water chemistry conditions. The former is made possible by using advanced models such as the one used in the present study.

In many offshore oil and gas projects under development, the pipeline costs are a considerable part of the investment and can become prohibitively high if the corrosivity of the fluid necessitates the use of corrosion resistant alloys instead of carbon steel. Development of more robust and reliable methods for internal corrosion control can increase the application range of carbon steel and therefore have a large economic impact. Corrosion control of carbon steel pipelines has traditionally often been managed by the use of corrosion inhibitors. The pH stabilization technique has been successfully used for corrosion control of several large wet gas condensate pipelines in the last few years. Precipitation of scale and salts in the pipeline and process equipment creates further challenges when formation water is produced. Different corrosion prediction models are used in the industry to assess the corrosivity of the transported fluid. An overview of the present models is given together with a link to fluid flow modeling.

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