Polarization Resistance Ref

The polarization-resistance, or Stem-Geary (Ref 12), method allows faster corrosion-rate measurements. The theoretical justification for this method is based on the expression for the external current given by Eq 6.5, which, on division by the specimen area to convert to current density, has the form  

This equation has the form of the solid curve in Fig. 6.12 when plotted near Ecorr (usually within 25 mV of Ecorr). 

At E = Ecorr, the exponential terms are unity, and upon rearrangement, Eq 6.24 reduces to  

Assuming that the third and higher terms in the series are negligible (i.e., x is small), Eq 6.23 takes the form  

This equation shows that E versus iex is linear (i.e., the term on the right side of the equation is a constant) provided the quantities (E - Ecorr)/pox M and (E - Ecorr)/predX are small (i.e., provided the x values in the series expansion are small). Since atypical value for the Tafel constants is 100 mV, the condition is generally considered to be met when (E - Ecorr) is less than about 10 mV. 

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