Corrosion charge transfer overvoltage

The charge transfer from the surface of the semiconductor must be fast enough to prevent corrosion and reduce energy losses due to overvoltage. 

Overvoltage depends on the current density. When a cathodic current is appHed to the electrode, the electrons accumulate in the metal as a result of slow charge transfer. This phenomenon causes cathodic polarization, t], to be negative. Conversely, when electrons are removed from the metal like anodic polarization (corrosion), the polarization is positive. 

Most metal sulfides and certain metal oxides are electronic conductors and are capable of establishing corrosion and galvaific couples in aqueous solutions. Similarly, metals react by well-established kinetic patterns involving corrosion couples displaying charge transfer or diffusion overvoltages or combinations of these depending on the metal. 

The Butler-Volmer equation written in the form (44) is frequently used in corrosion. It describes the relationship between current density and potential of an electrode reaction under charge transfer control in terms of three easily measurable quantities P, and P, . For large anodic overvoltages (tl/p 1) Eq. (44)... 

Because the rate of the cathodic reaction is proportional to the surface concentration of the reagent, the reaction rate will be limited by a drop in the surface concentration. For a sufficiently fast charge transfer (small activation overvoltage), the surface concentration will fall to zero, and the corrosion process will be totally controlled by mass transport. For purely diffusion-controlled mass transport, the flux of a species O to a surface from the bulk is described with Fick s first law [Eq. (D.77)] ... 

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