Effect of diffusion layer structure on the determination of corrosion rates from DC transient measurements

The boundary conditions usually used for solving the differential equations in the theory of electrochemical transient measurements are not applicable to measurements with corroding metals. In an electrode kinetic investigation the measurement starts from equilibrium, where no net current is flowing and no net chemical change is taking place before the application of the measuring signal. The situation is completely different for a corroding electrode because a corroding metal is not in equilibrium. Under these conditions, while there is no net current flowing through the system, there is a net chemical change taking place: the corrosion of the metal. The relaxation curves calculated with the modified boundary conditions can be considerably different from those calculated the conventional way; therefore, the corrosion rates calculated from polarization data corrected for mass-transport effects using dc transient techniques may be in error. The authors discuss a series of model calculations carried out to determine this error. It is concluded that classical dc transient techniques can be used to correct polarization data taken with corroding metal electrodes for mass-transport effects, as long as the system is not completely under mass-transport control and the negative polarization from the corrosion potential does not exceed 30mV.