Corrosion processes and the rates of the individual electrode reactions in carbon dioxide containing solutions are well known to be affected by solution composition and hydrodynamics. There are many different mechanistic explanations for the observed, from rotating-disk studies, cathodic current-potential-rotation speed relationships, most of which obtain the observed currents by the simple addition of two or more currents due to processes (for example, chemical reactions and diffusion) that are considered to act independently. The validity of this is not clear, since many of the species contributing interact in the boundary layer.
A model is described that derives cathodic current densities and surface solution chemistries for the rotating-disk electrode over the range of conditions of practical relevance. The model allows for the slow homogeneous chemical step of carbon dioxide hydration, diffusion, convection, charge transfer processes, and the rapid acid-base equilibria. It is shown that the experimental data are adequately modeled without the need to consider a heterogeneous surface hydration of carbon dioxide, and that the electrochemical response under conditions not previously studied is predicted. Further, the increase in cathodic limiting current at pH > 6 is shown to be a consequence of the solution chemical equilibria and the direct reduction of bicarbonate is not necessary to explain the experimental data. None of the mechanisms previously proposed is valid over the entire pH range of interest.
The concentration profiles in the near surface region throw light on the rate determining steps in the cathodic process and can be used to predict the possibility of surface film formation.