Direct estimation of the true polarization resistance (RT) of embedded metallic bars in concrete is not feasible in a large concrete structure, since the electric signal tends to vanish with increasing distance from the smaller counter electrode located on its surface. Measurements give apparent values of polarization resistance and erroneous corrosion rates when this parameter is introduced into the Stern-Geary equation. The possibility of estimating the true RT value in large-sized reinforced concrete slabs by means of a simple model of easy mathematical treatment is discussed. Despite the approximations made, the model proves sufficiently correct for such a purpose.