on the surface of the metal, i.e. at the metal and electrolyte interface. The figure shows hydrogen reduction on a zinc surface and serves as a simplified but accurate depiction of activation polarization. Concentration activation (see figure) occurs primarily when the reducible reagent exists in low concentrations. In this type, the hydrogens do not have to combine on the zinc surface.

It may seem trivial to differentiate between these two types of polarization. But it is important for engineers to understand what is controlling the reduction reaction. Depending on whether the reaction is controlled by the reaction sequence at the metal-electrolyte surface as in activation control or the reaction is controlled by the diffusion of the electrolyte as in concentration control, environmental factors will have drastically dissimilar effects. For example, if you increase the velocity or agitation of the electrolyte by narrowing the water passage (thus, increasing the rate of flow), there will be no effect on the rate of corrosion if the reaction is controlled by activation polarization. However, if the reactions were predominated by concentration activation, then the corrosion rate would increase.