## 2 Methodology for Measuring the Rate of Corrosion Reinforcement in Cores Drilled from The Structure  
### Measuring Corrosion Rate by Electrochemical Methods  
Corrosion of the rebar occurs on the contact surface area with concrete. Using a very simplified description, there is cross-section decrement in the rebar circumference over time measured as corrosion rate \(CR\) defined as the decrement of the rebar cross-section \(\Delta l\) over \(t\). Because the cross-section decrement \(\Delta l\) is equal to mass decrement \(\Delta m\) on the contact surface area \(A\) with metal solution having density \(\rho\), corrosion rate \(CR\) can be expressed in two following ways  
\[CR=\frac{\Delta l}{t}=\frac{\Delta m}{\Delta t\;A\rho} \tag{1}\]However, it is very difficult to measure the decrement \(\Delta l\) of the rebar cross-section in concrete (and impossible for mass decrement \(\Delta m\)) because such measurements require the rebars uncovered from the concrete cover, and the measurable values \(\Delta l\) are obtained only for a long-term and advanced corrosion of reinforcement. Hence, other methods are necessary, particularly the methods applicable for very small corrosion rates. Such methods, based on the electrochemical nature of the corrosion process of steel, in which the anodic reaction of iron dissolution (2) is accompanied by the reaction of oxygen depolarization (3), have been used in electrochemical laboratories for nearly 100 years and are commonly known as polarization methods. \[\mathrm{Fe}\rightarrow\mathrm{Fe}^{2+}+2\mathrm{e}^{-}, \tag{2}\]  
\[\mathrm{O}_{2}+2\mathrm{H}_{2}\mathrm{O}+4\mathrm{e}^{-}\to 4\mathrm{OH}^{-}. \tag{3}\]  
The basis is the determination of density of corrosion current \(i_{corr}\), which clearly defines corrosion rate because following [PERSON]'s law intensity of corrosion current \(I_{corr}\) is proportional to the decrement of metal mass \(\Delta m\)  
\[\Delta m=\frac{M}{z\,F}I_{corr}\,t=\frac{M}{z\,F}i_{corr}\,\,A\,\,t. \tag{4}\]  
In this equation \(M\) is molar mass for iron, \(t\)--reaction time, \(A\)--active surface of the electrode, \(z\)--charge number of reaction (2), and \(F\)--[PERSON]'s constant. Determination of the mass decrement \(\Delta m\) by electrochemical methods is used to find corrosion rate \(CR\) in the function of density of corrosion current \(i_{corr}\) from the following equation  
\[CR\,\,\Delta t\,\,A\,\,\rho=\frac{M}{z\,F}\,\,i_{corr}\,\,A\,\,t\,\,\,\, \rightarrow\,\,\,CR=\frac{M}{zF\rho}\,\,i_{corr} \tag{5}\]  
The reaction (2) in the steady state is characterized by potential \(E_{Fe^{2+}/F\varepsilon}\) calculated from the Nernst equation and the exchange current \(i_{0,Fe/F\varepsilon^{2+}}\). Similarly, the reaction (3) in the steady state is characterized by potential \(E_{O_{2}/OH^{-}}\) and the exchange current \(i_{0,O_{2}/OH^{-}}\). Because both reactions take place on the same electrode, potential of corroding steel takes the intermediate value of the mixed potential specified by the corrosion potential \(E_{corr}\) corresponding to the common value of current density \(i_{corr}\). A difference between the potential \(E_{corr}\) and the potential \(E_{Fe^{2+}/F\varepsilon}\) is specified as the overpotential of the anodic reaction \(\eta_{a}\). The difference between \(E_{corr}\) and \(E_{O_{2}/OH^{-}}\) is correspondingly specified as the overpotential of the cathodic reaction \(\eta_{c}\). Taking the activation nature of the corrosion process which means that corrosion rate depends on the unrestricted process of charge transfer, and is not controlled by the transport of reagents or by the medium resistance, then the relationship between density of anodic current \(i_{a}\) and the overpotential of the anodic reaction is given by the Equation (6), and similarly the relationship between density of cathodic current \(i_{c}\) and the overpotential of the cathodic reaction is expressed by the Equation (7)--cf. [34].