Abstract
Pitting corrosion of carbon steel in CO2 environments occurs mostly in high turbulence regimes. Several experimental investigations have been conducted to achieve a better understanding of the complex phenomenon of flow induced localized attack. In this work, a theoretical model is developed to predict the extent of pit propagation under the effects of high turbulence regimes. Given the flow conditions in the main stream and an initial shape of a pit along the pipe wall, the model predicts the hydrodynamics inside the cavity and its extent of propagation or repassivation. It takes into account the equilibrium conditions, the surface kinetics, the electrochemical process at the surface, and the fluid flow inside the pit. The hydrodynamics of the model have been based on flow separation and reattachment for shallow and medium size pits, whereas the skimming flow analysis has been applied to deep pits. The criterion of pit passivation or propagation is evaluated, depending on the iron carbonate supersaturation kinetics and the concentration of Fe++ at the surface of the pit walls. The former is influenced by both the iron dissociation and the mass transfer rates. The model, as developed, has been used to study the effect of velocity on the pit wall shear stress, the surface concentration of ferrous ion, the mass transfer coefficient in and out of the cavity, and, hence, the overall propagation rate of an existing pit.
