When a metal electrode is immersed in a corrosive electrolyte,
anodic and cathodic reactions occur simultaneously on its surface.
Generally, the anodic reaction involves the oxidation of the metal
to metal ions, which are released into the environment, while the
cathodic reaction involves the reduction of species present in the
environment, such as molecular oxygen or hydrogen ions [1]. The
rate at which the anodic and cathodic reactions proceed can fluctuate
with time but, overall, the rate of the anodic and cathodic reactions
are balanced, at open-circuit potential, in order to preserve
electroneutrality [2]. Generally, the rate of the anodic reaction increases
with increasing electrode potential, while the rate of the
cathodic reaction increases with decreasing electrode potential.
Consequently, the average corrosion potential represents the potential
at which the average rates of anodic and cathodic reaction
are balanced. However, if for example, a rapid increase of the anodic
reaction rate proceeds, some of the charge generated by metal oxidation
can be transiently stored in the capacitance that is generated
due to charge separation at the metal-solution interface, known as
the double-layer capacitance, before being consumed by the cathodic
reaction on the electrode surface