Stern-Geary approximation

Oldham and Mansfeld" approached the problem of linearity in a different way and their derivation avoids the approximation used by Stern and Geary. They conclude that although linearity is frequently achieved this is due to three possible causes (a) ohmic control due to the IR drop rather than control according to linear polarisation, (b) the similarity of the values of b, and be and (c) a predisposition by the experimenter to assume that the AE — Ai curves near must be linear. In a later paper Oldham and Mansfeld" showed that linearity of the AE — Ai curve is not essential and... 

Many investigators have experimentally observed that im was approximately linearly related to applied potential within a few millivolts of polarization from Econ (3). Stern and Geary simplified the kinetic expression to provide an approximation to the charge transfer controlled reaction kinetics given by Eq. (1) for the case of small overpotentials with respect to corr (4-6). Equation (1) can be mathematically linearized by taking its series expansion (e.g., ex = 1 + x + x2/2 + jc3/ 3 + x4/4 ...) and by neglecting higher terms when AE/p <0.1. This simplified relationship has the form... 

Based on low-field approximation, a simple procedure for the evaluation of corrosion currents and corrosion rates was developed in 1938 by Wagner and Traud [22]. Stern and Geary [23] and Stern [24,25] developed an experimental procedure for measuring the corrosion rates known as the linear polarization technique. This technique wiU be discussed in detail in Chapter 5. 

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