Corrosion diffusion overvoltage

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. 

Figure 9.1 Determination of corrosion current density by extrapolation of linear parts of the polarization curves, a) Both the cathodic and the anodic reaction are tmder activation control (the overvoltage curves are Tafel tines), b) The cathodic reaction is diffusion controlled and the anodic reaction activation controlled, c) The cathodic reaction is activation controlled, the anodic curve is irregular, d) The cathodic curve is irregular, the metal is passive, i.e. the corrosion current equals the passive current.

Fig. 10.1 The corrosion of an iron rivet in a copper plate. The large copper surface results in a low O2 overvoltage, allowing the corrosion to proceed at a rate controlled by O2 diffusion...

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)] ... 

Concentration polarization prevails when the corrosion rate is limited by diffusion in the solution. When overvoltage versus the logarithm of current density is plotted, the resulting curve will appear as shown in Figure 17.9a. 

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