An accurate and reliable corrosion rate prediction is an essential parameter in design, material selection, and mitigation strategies for oil and gas transmission pipelines. While underestimating the corrosion rates may result in health, safety, environmental concerns and financial loss due to maintenance costs and production interruption, overestimation of this parameter may result in significant unnecessary costs for pipeline construction and maintenance. Such concerns have been a strong driving force for developing a better understanding of the corrosion phenomena and advancements in the corrosion rate predictive tools.

The corrosion rate predictive models developed to date can be classified depending on the mathematical description of the fundamental thermodynamics and kinetic processes, underlying the corrosion phenomena. That includes empirical models, semi-empirical models, elementary mechanistic models, and comprehensive mechanistic models. The advantages and deficiencies of each group of models was discussed in detail in an earlier review [1]. The present study is focused on improving the comprehensive mechanistic models.

Considering the importance of the chemical composition of the solution, the water speciation calculations were updated with more recent literature data. The non-ideal behavior of the gas phase (CO₂) in terms of fugacity coefficient and the liquid phase in terms of activity coefficients were incorporated into the model. It was shown that assuming an ideal behavior would result in significant error in chemical speciation at pCO₂>10 bar. A more simple charge transfer rate calculations were proposed to replace the Tafel equation commonly used in the literature. It is shown that not only these expressions are mathematically more simple, but they are also more theoretically consistent with the conditions typical in corroding systems. The effect of organic acids (acetic acid in particular) is included in the CO₂ corrosion model considering the recent mechanistic findings [2].

The existing comprehensive mathematical models usually assume an infinitely diluted solution in calculations. This matter is also disused in terms of the effect of non-ideal solutions in mass transfer and charge transfer rates. The present model was developed based on moderately diluted solution theory to partially accommodate for the non-ideal behavior but maintain a reasonable computation time. Furthermore, the precipitation and growth of an iron carbonate corrosion product layer is included in the model to account for the effect of this protective layer on the mass transfer and charge transfer rates.

References

[1] A. Kahyarian, M. Singer, S. Nesic, Modeling of Uniform CO2 Corrosion of Mild Steel in Gas Transportation Systems: a Review, J. Nat. Gas Sci. Eng. 29 (2016) 530–549. doi:10.1016/j.jngse.2015.12.052.

[2] A. Kahyarian, B. Brown, S. Nesic, Mechanism of Cathodic Reactions in Acetic Acid Corrosion of Iron and Mild Steel, Corrosion. 72 (2016) 1539–1546. doi:http://dx.doi.org/10.5006/2177.