Corrosion resistance from Tafel plots

Concerning the Z modulus curves, at high frequencies, chromated and ppHMDSO coatings display the same characteristics. At low frequencies, slightly better corrosion resistance is observed for the chromated film (300 kO) than the polymer one (100 kO). On the other hand, the resistance of the untreated substrate was lower by one order of magnitude. Therefore the results obtained from Tafel plots and impedance spectra are consistent. 

The data in Figure 6.23 show that the films are stable up to -1.2 V. Polarization resistance of the films (calculated from the tafel plots) was found to be enhanced by increasing the oxidized form of the particles in the film. The corrosion current obtained was smaller compared to the uncoated Fe. 

Linear polarization measurements are executed rapidly. The currents in linear polarization measurements are measured in the potential range between 10 and 20 mV from the equilibrium potential. The E-I dependence in this potential range follows a linear relationship. The slope of the plot, dE/ di, represents the polarization resistance. The corrosion current is calculated using the Stem-Geary equation for known values of the anodic and cathodic Tafel slopes. The ratio of the overpotential to the current represents the resistance in Ohm s law and is often termed the charge transfer resistance or the polarization resistance, Rp. 

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