Stern-Geary relation

The corrosion current density iC0II is related to the polarisation resistance Rv by the Stern-Geary coefficient B ... 

The dimension ofRp is ohm-cm2, iC0II is pAcm-2 andUis in V. The Stern-Geary coefficient is related to the anodic, /3a, and cathodic, /3C, Tafel slopes (Figure 7.56) as shown in Eq. (7.36) ... 

The polarization resistance method is based on the observation that the polarization curve close to the corrosion potential is linear (Fig. 8-8), the slope AE/AI (AE = step in potential, AI = resulting current) being defined as polarization resistance, Rp. The technique is called linear polarization measurement (LPR). The Rp value is related to the corrosion current by the formula (Stern-Geary, 1957) ... 

This is a well established technique applied to determine the corrosion rate of steel in concrete. A full theoretical description of this technique is given in Chapter 3. The specimen is polarized to within 25 mV of the corrosion potential (Ecorr) as the dependence of current with potential in the vicinity of corrosion potential is linear. The open circuit potential (E oii) of the reinforcement is measured against a reference electrode. Starting from this potential, the steel reinforcement is polarized to 25 mV from Ecorr at a sweep rate of 5-lOmV/min and a plot of A vs At is constructed. The polarization resistance relates the slope of the polarization curve in the vicinity of corrosion potential to the corrosion current. This technique is based on the Stern Geary equation described in Chapter 3, and given below ... 

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... 

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