Corrosion is the chemical or electrochemical reaction between a material, usually metal, and its environment that produces a deterioration of materials and its properties. Due to the phenomenon of passivity, it is not uncommon for the current to change by six orders of magnitude during a corrosion experiment. An electrochemical reaction under kinetic control obeys the Tafel equation. For the range of corrosion phenomena that can be studied with electrochemistry, the ability to measure very low corrosion rates, and the speed with which these measurements can be conducted, an electrochemical corrosion measurement system has become a standard item in the modern corrosion laboratory. For a simple activation controlled metal dissolution (anodic) process with the rate of the reverse metal ion reduction, reaction is considered to be small and the current density is given by Tafel’s Law. It is assumed that the surface concentration of the cathodic reactant did not vary with potential. This is approximately true if the cathodic current density is much less than the limiting mass transfer controlled current density. For a cathodic reaction, neglecting the reverse oxidation reaction, the limiting diffusion current density is given by Fick’s 1st Law with a surface concentration of zero. Read more..