Predicting the Effect of Reactive Impurities on the Corrosion Rates of Carbon Steel in CCTUS

Abstract It is well established that under CO2 transport conditions, acid dropout can occur as a separate aqueous phase precipitating out of liquid or supercritical CO2 due to reactions between impurities such as SOx, NOx, H2S, H2O, and O2. Such dropout phases primarily consist of concentrated sulfuric acid and, in some cases, nitric acid. Sulfuric acid is highly corrosive to carbon steel assets up to a certain concentration limit but, at sufficiently high concentrations, the corrosion rate decreases significantly due to the formation of a protective scale layer from solutions supersaturated with respect to corrosion products. The Mixed-Solvent Electrolyte (MSE) thermodynamic framework is well suited for describing both thermodynamic properties and speciation across the whole concentration range of such solutions from 0 wt% to pure sulfuric acid and even beyond, extending from pure H2SO4 to water-deficient H2SO4-SO3 systems (i.e., oleum). At the same time, the MSE Corrosion mechanistic model has been previously shown to reliably predict the corrosion behavior of stainless steels in wide ranges of concentrations and temperatures. This study presents a mechanistic electrochemical framework that extends the MSE Corrosion model by developing a new theory for quantitatively calculating the corrosion rates of carbon steel in supersaturated solutions in which mass transfer of metal species and dissolution of corrosion product layers play a key role in the corrosion of the metal.