Polarisation resistance Tafel constants

Stern and Weisert " by taking arbitrary values of the Tafel constants showed that corrosion rates determined by the polarisation resistance techniques are in good agreement with corrosion rates obtained by mass loss methods. 

The controversy that arises owing to the uncertainty of the exact values of and b and their variation with environmental conditions, partial control of the anodic reaction by transport, etc. may be avoided by substituting an empirical constant for (b + b /b b ) in equation 19.1, which is evaluated by the conventional mass-loss method. This approach has been used by Makrides who monitors the polarisation resistance continuously, and then uses a single mass-loss determination at the end of the test to obtain the constant. Once the constant has been determined it can be used throughout the tests, providing that there is no significant change in the nature of the solution that would lead to markedly different values of the Tafel constants. 

Where, / a and are anodic and cathodic Tafel slopes (mV/decade), is corrosion current density (A/cm ) and Rp is polarisation resistance (ohm-cm ). This involves a potential scan 250 mV of /tcorr at a scan rate of 0.1-1.0 mV/s. The technique is used to determine the equilibrium corrosion current, potential, Tafel constants and corrosion rates. The corrosion rate (CR) is determined from the Faraday s law ... 

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