Corrosion rate estimation under sweet corrosion conditions: use of the Tafel-Piontelli model

Predictive modelling has proven to be a fundamental tool for the management and long-term sustainability of the Oil&Gas infrastructures, deeply affected by corrosion problems. In particular, the two main forms of corrosion of carbon steel pipelines in this industry are sweet and sour corrosion, caused by carbonic acid and hydrogen sulfide, respectively. Carbonic acid is by far the predominant problem, despite having a relatively high pH. In this context, the Tafel-Piontelli model is introduced as a mechanistic tool for the prediction of sweet corrosion and other forms of acidic corrosion in which hydrogen evolution serves as the primary cathodic process. The model exploits the kinetic principles of corrosion and investigates the complex water chemistry interactions for the estimation of the corrosion rate. The kinetic parameters of the anodic and cathodic reactions, namely the Tafel slopes and the exchange current densities, are theoretically estimated and compared with the experimental values obtained via potentiodynamic polarization tests performed in weak acids in a wide range of pH and temperature sets. The effectiveness of the model in evaluating corrosion rates is examined first by comparing it with the experimental corrosion rates obtained from mass loss experiments. The results confirm the ability of the model to predict the corrosive behavior of different acidic solutions and the exponential dependance of the corrosion rate on pH. Finally, the case of sweet corrosion of carbon steel is tackled. A comprehensive benchmark and validation analysis is performed via comparison with other well-established models of the Oil&Gas industry: the De Waard-Lotz model (1993), the De Waard-Lotz-Dugstad model (1995), and the Cassandra model (1998). Temperatures ranging from 20 °C to 100 °C and carbon dioxide partial pressures spanning from 1 to 10 ppm are tested. The predictions of the Tafel-Piontelli model are in good agreement with the other models and similarly follow an exponential trend with respect to the pH. Moreover, the Tafel-Piontelli model is able to account for the passivating effects of the formation of a protective scale at high temperatures. The promising results confirm the good applicability of the Tafel-Piontelli model to the prediction of CO2 corrosion.