In the literature about polarization curves and magnesium alloys, there are studies that report the values for Ba and Bc and others that only reports the values for Bc. In this material it is common to observe that the anodic lines are curved, with no linear region.
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S
A bended Tafel curve is very often due to 1) solution resistance that has not been compensated for, or 2), background current are not correctly subtracted, or 3) diffusion limitation sets in early, or 4), the Tafel slope is calculated in a region when no real data has been acquired. The latter typically happens when one is correcting for the diffusion limited current(idl). The relative error for such a correction is more than >50% at idl/2, hence no real kinetic information can be extracted from current above idl/2.
On an oxide covered surface anodic ploarization curves would exhibit a curved behaviour
curved polarization is due to start of passivation and numerous oxidation states
Corrosion current (icorr) is inversely related to Rt through the Stern-Geary equation:
where the proportionality constant B can be usually calculated from both the anodic and cathodic Tafel slopes. However, due to the uncertainty in the βa values from the polarization curves in our studies with magnesium alloys, we decided to use the constant B obtained empirically from the correlation between Rt measurements and weight loss data.
do not assume any Tafel slopes
now the question is to find out anodic Tafel slope Try one method known as accelerated Tafel plot method
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