Tafel-Piontelli model for acidic corrosion: theoretical developments and engineering applications on carbon steel exposed to weak acids

Carbon steel is widely employed in a variety of applications thanks to its versatility and relatively low cost. Nonetheless, industrial applications often involve aggressive environments that can lead to the fast degradation of the metal. In this context, acidic corrosion represents a diffused threat and can be caused by a variety of acids. While strong acids may seem the most dangerous ones due to their very low pH values, weak acids can often result in more aggressive corrosive behaviors, despite their relatively high pH. A predictive strategy is therefore fundamental in order to properly design and monitor the life of industrial structures, and to avoid catastrophic failures. The Tafel-Piontelli model was developed with the goal to predict the corrosion rate of active metals in different acidic environments characterized by hydrogen evolution as the dominant cathodic process. The model, mechanistic thanks to its strong electrochemical and kinetic roots, is here applied to forecast the corrosion rate of carbon steel across different weak acids (namely: formic, citric and azelaic acid) under a wide range of pH levels (spanning from 3 to 5) and temperatures (ranging from 20 to 60 °C). The validation of the model is performed via two main tests: mass loss tests are used to compare the predicted corrosion rates with real laboratory data; potentiodynamic polarization tests allow to gain realistic information regarding the four kinetic parameters that characterize the corrosion mechanism: Tafel slopes and exchange current densities of the anodic reaction (iron dissolution) and cathodic reaction (hydrogen evolution). The estimations of the model are in good agreement with the experimental results and accurately predict the exponential trend of the corrosion rate with respect to the pH, thus confirming the potential of the Tafel-Piontelli model.