Overpotential transport

The relation between transport overpotential and current density for a cathodic reaction is given by... 

Fig. 1.31 Shape of cathodic polarisation curve when transport overpotential is rate controlling, (a) Effect of velocity on ( l and corrosion rate, (b) effect of concentration on tY and corrosion rate and (c) effect of position and slope of anodic curve (after Stern...

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

Stern and Geary on the basis of a detailed analysis of the polarisation curves of the anodic and cathodic reactions involved in the corrosion of a metal, and on the assumption that both reactions were charge-transfer controlled (transport overpotential negligible) and that the /R drop involved in determining the potential was negligible, derived the expression... 

The corrosion current (it is also assumed that the area of the metal is 1 cm so that - ho ) occurs at a value within the Tafel region for the anodic and cathodic reaction, i.e. transport overpotential is negligible. 

The Nernst equation written in terms of the transport overpotential of a cathode is... 

Concentration (diffusion or transport) Overpotential change of potential of an electrode caused by concentration changes near the electrode/solution interface produced by an electrode reaction. 

The effect of transport phenomena on the overall electrode process can also be expressed in terms of the concentration or transport overpotential. The original concentration of the oxidized form cQx decreases during the cathodic process through depletion in the vicinity of the electrode to the value (c0x)x=o and, similarly, the concentration of the reduced form cRed increases to the value (cRed)A.=0. It follows from Eqs (5.4.18), (5.4.19) and (5.4.20) that... 

See also - diffusion overpotential, -> diffusion time, -> fractals in electrochemistry, -> mass transport overpotential, -> mass transport processes. 

Mass transport overpotential (or concentration polarization (overpotential)) tjc — is a departure of the - electrode potential (or cell potential), E, from the - equilibrium electrode potential (or zero-current potential), Eeq, caused by a -> faradaic current flow and the associated surface concentration, Cox(x = 0), deviation from the bulk concentration, Cox(°°)> of the reacting redox species, Ox... 

The concentration and transport overpotential can be estimated by solution of Tick s law of diffusion across a small layer of thickness 10-100 /tm at the electrode surface. The result is [2]... 

Wood, D. L. and Borup, I. R. L. 2010a. Estimation of mass-transport overpotentials during long-term PEMFC operation. Journal of the Electrochemical Society 157 B1251-B1262. 

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