A current pulse I is imposed on the reinforcement from a counter electrode placed on the concrete surface. A guard ring confines the current to an area A of the reinforcement below the central counter electrode. The applied current is usually in the range of 5 to 400 A and the typical pulse duration is 5 to 10 seconds. The reinforcement is polarized in the anodic direction compared to its free corrosion potential, Ecore.
The resulting change of the electrochemical potential of the reinforcement is recorded as a function of time using a reference electrode (Ag/AgCl). A typical potential response for reinforcement actively corroding is shown in the right figure above. When the current is applied to the system, there is an ohmic potential drop IRo as well as change in potential due to polarization of the reinforcement, IRp.
The polarization resistance of the reinforcement Rp is calculated by curve fitting to the transient portion of the potential data. By means of the Stern-Geary equation for active corrosion (Icorr = (26 A)/Rp) and Faraday’s law of electrochemical equivalence, the corrosion rate is estimated as: Corrosion Rate (m/year) = 11.6 Icorr /A where A is the confined area (in cm2) of the reinforcement below the central counter electrode. The factor 11.6 is for black steel. The value of Ro, the electrical resistance of the concrete between the counter electrode and the reinforcement, is also determined.