Electrochemistry and Corrosion Science

The concentration of a specie j is lower at the electrode surface than in the bulk C _b <C _o . This is the opposite of activation polarization in which C _b >C _o The kinetics of concentration polarization is a rate-controlling electrochemical process since the electrode is cathodically polarized. The mass transfer may be by diffusion, migration, convection or a combination of these modes. Thus, the Nernst-Plank equation can give reasonable results. However, if diffusion is the solely mechanism, Fick’s first law states that the diffusion molar flux depends on the concentration gradient at a steady-state since the concentration rate is dC/dt=0. On the other hand, Fick’s second law of diffusion requires that dC/dt≠0, but it strongly depends on the concentration gradient ∂² C/∂x ² and time. The solution of Fick’s second law depends on the type of diffusion problem and related boundary conditions, but the presented solution given in Appendix A is based on the error function of the Bell-Shaped Function y=exp(−_x ²), and it predicts that the concentration of a specie j the concentration gradient, and current density decay with time t ^−1/2 at the electrode surface. Despite that the current density is a time-dependent parameter, it is influenced by the flux of specie j and it is restrictive to the limiting current density (i _L ) as its maximum value. Therefore, the overpotential needed for concentration polarization depends on the current density ratio i _c /i _L as described by the Nernst equation as η _c= f(i _c /i _L ).

The mixed-potential theory includes both anodic and cathodic polarization, in which diffusion of species is related to the current flowing in the electrolyte. The principles of mixed-potential allows the characterization of electrochemical corrosion systems by developing an Evan’s diagram if the Tafel slopes and exchange current densities and at least one oxidizer agent are known. The kinetic parameters that are determined from an Evan’s diagram are the corrosion potential and the corrosion current densities. Potentiodynamic polarization curves also know as Stern diagrams are obtainable very easily and exhibit different features that are useful in characterizing the electrochemical behavior of electrodes.

In addition, a departure from electrochemical equilibrium leads to a polarization scheme due to a developed overpotential η>0 for anodic polarization and η<0 for cathodic polarization. If current flows and η≠0, irreversible effects occur on the electrodes surfaces during polarization, which in turn, causes changes in local potentials until a steady-state is reached.

Characterizing galvanic corrosion in a particular environment simulating the service atmosphere must be done with caution. As previously mentioned, the galvanic corrosion potential is sensity to changes in temperature, ionic concentraion and cathode-toanode surface area ratio.

Anodic protection (AP) is a potential-control electrochemical technique suitable for preventing corrosion of a metal in aggressive environments, such as sulfuric acid (H₂SO₄. In this technique, the metal to be protected must exhibit passivity at relatively low current density so that the passive current density (i _p ) is at least one order of magnitude lower than the corrosion current density (i _corr ) care must be exercise in selecting a material that shows a wide enough passive potential range. Thus, the protective potential is E_pa>E_x<E_p.

Furthermore, AP is normally used when coatings and cathodic protection methods do not provide adequate protection against corrosion.