Density changes, cathodic barrier

A further barrier to corrosion reactions is provided by electrical resistance. When the anodic and cathodic reactions at the metal surface take place with locally different current densities, resistance in the current circuits can cause a measurable drop in potential (resistance polarization). This resistance polarization is a linear function of the current. Resistance polarization frequently arises through the formation of passive films. The resulting relationship between the change in potential and the current usually no longer follows Ohm s law, but instead is subject to a logarithmic relationship. 

The exchange current density (given by the intersection between the anodic and cathodic Tafel lines) decreases as the thickness of the passive film changes from 1.7 to 2.7 nm. The behavior can be explained by the reduced probability of electron tunneling through the oxide barrier with increasing film thickness. 

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