## 2 Methodology for Measuring the Rate of Corrosion Reinforcement in Cores Drilled from The Structure
### Specification for Development of the Three-Electrode System for Concrete Cores  
The red lines in Figure 1 show the natural way of the formation of the resultant potential due to current flow through the electrode, which was induced by the difference in potentials (Evans diagrams). At the resultant current equal to zero, components of current density cannot be measured in a direct way, e.g., using the ammeter. Only the values \(E_{corr}\) can be measured with voltage meter of high internal resistance as a difference in potentials on poles of the open cell. Such a measurement is taken using the additional reference electrode of known and constant potential. External polarization current should be applied to the working electrode and third auxiliary electrode to determine the searched value \(i_{corr}\). New induced overpotentials of the reactions (2) and (3) maintain the relationships with relevant densities of current in accordance with the expressions (6) and (7) and are used to record the curve illustrated in blue in Figure 1.  
The system of three electrodes in the laboratory conditions, that is, the working electrode 1 (metal), the reference electrode 2 and the auxiliary electrode 3 connected to the potentiostat 4 is placed in the electrochemical cell 5 presented in Figure 2a. The construction and shape of the electrodes significantly reduce measuring errors which occur during polarization measurements, mainly by minimizing the distance between the working electrode 1 and the reference electrode 2 usually by using Luggin capillary 6. Moreover, the employed working electrode 1 is small and the auxiliary electrode 3 is much bigger. Also, the constant temperature is kept due to water jacket 7 connected to the thermostat. Figure 1: The mechanism of development of corrosion potential on iron surface: (**a**) \(E\)-\(i\) diagram on logarithmic scale; and (**b**) \(E\)-\(i\) diagram on arithmetic scale—description and notes in the text. Figure 2: The three-electrode system used in the polarization tests: (**a**) the electrochemical cell used in the laboratory; (**b**) the system formed on the drilled core with a rebar fragment; (**c**) core drilled from the structure; (**d**) drilling an opening in the rebar to connect the conductor; (**e**) connecting the conductor to the rebar using a plug; (**f**) drilling an opening in concrete for the reference electrode; (**g**) the reference electrode before its placing in the opening; (**h**) application of conductive coating which is used as the auxiliary electrode; and (**i**) the ready specimen after toluene evaporation and before putting it into the climate chamber, 1—working electrode in the electrochemical cell, 2—reference electrode in the electrochemical cell, 3—auxiliary electrode in the electrochemical cell, 4—potentiostat, 5—electrochemical cell, 6—Luggin capillary, 7—water jacket, 8—concrete core drilled from the structure, 9—a fragment of rebar used as the working electrode, 10—connection plug of the working electrode, 11—connection wire of the working electrode, 12—opening in concrete for the reference electrode, 13—reference electrode in concrete core, 14—connection wire of the reference electrode, 15—epoxy footing compound to protect the rebar face against corrosion, 16—conductive coating used as the auxiliary electrode, 17—connector of the auxiliary electrode, 18—connection wire of the auxiliary electrode. Creating the measuring system, similar to the electrochemical cell, for polarization tests for reinforced concrete structure is an extremely difficult task. Drilling from the structure cores 8 with a fragment of the rebar 9, which are used as the working electrode (Figure 2b,c) is considered as the solution for adapting the tests to the laboratory conditions. The new solution proposed in this paper consists in introducing permanently two other electrodes of the three-electrode system into the drilled core.