Recently, in CORROSION-87, one paper was presented which, assuming the electrical model of the 'transmission line' to represent the nonuniform distribution of the current in a concrete beam, demonstrated the calculation of the instantaneous corrosion rate by unit of length of the rebar through mathematical equations. In the present contribution those equations are used in order to determine whether the equations enable distinction between the corroded zones of a rebar and those which remain passive. A concrete beam was fabricated, half of it by mixing chlorides with the cement (and therefore inducing a corrosion in the rebars) and the other half without admixtures (passivated zones of the rebars). The apparent polarization resistance was measured along the beam using a small counter electrode. Through the equations previously proposed, the corrosion rate was calculated. The results obtained confirm that the boundary between corroded and passivated zones can be clearly identified using these formulae. Finally, some comments are given on the relative activity of galvanic macrocells compared with the corrosion microcells.