## 3 Reinforcement Corrosion Monitoring Techniques
### Potentiodynamic Method
#### 3.1.4. Galvanic Current  
The study of corrosion of concrete contaminated with chlorides has led to the development of an embedded sensor based on the measurement of galvanic current (\(i_{gal}\)) in macrocells [67]. This type of sensor is based on concrete with construction steels and nobler metals, which are usually stainless steel. The \(i_{gal}\) generated when depassivation of steel reinforcement occurs, due to contamination of the concrete with aggressive agents, is measured. The construction steel sections act as anodes and the stainless steel sections act as a cathode. As the penetration of aggressive agents in the concrete progresses, the corrosion of the first anode begins to generate a \(i_{gal}\). By simultaneously monitoring successive sections of construction steel embedded at different depths, it is possible to detect the moment at which the depassivation of steel reinforcement occurs for each depth. Figure 9 shows a schematic representation of a sensor composed by several rebars (construction steel) embedded in concrete at different depths (anodes) and a stainless steel rod that will act as cathode. This system shows if the reinforcement will corrode and when corrosion will start. Therefore, corrosion protection measures can be planned and carried out before the spalls and cracks are visible. Nevertheless, this corrosion monitoring system does not allow determination of the absolute chloride contents. It only determines the depth of the critical chloride content. This is an advantage as the interpretation of absolute chloride contents is quite challenging. The critical chloride content for reinforcement corrosion depends on concrete composition [19; 20; 213; 214; 215], rebar surface finishing [12; 25; 216; 217], environmental conditions [20; 218; 219], and others [11; 15; 23; 213; 214; 215]. Figure 8: Randles EEC modified with a CPE to simulate the behavior of the steel embedded in concrete (concrete-steel interface). Where: \(R_{e}\)—ohmic resistance of the concrete; CPE—constant phase element that replaces the double layer capacitance at the steel rebar surface; and the corresponding charge transfer resistance—\(R_{t}\); W—Warburg impedance [161; 165; 211]. According to [PERSON] [221], the influence of temperature in \(i_{gal}\) in macrocells might be quantified as:  
\[I=\frac{I_{0}}{e^{a(1/\tau-1/n_{i})}} \tag{8}\]  
where: \(I\) is the current intensity at temperature \(T\) (in A); \(I_{0}\) is the current at temperature \(T_{0}\) (in A); \(T\) and \(T_{0}\) are absolute temperatures (in K); \(a\) is a constant (in K) and depends on the degree of saturation of concrete. In 2009, [PERSON] et al. [82; 182] reported a study on the development of \(i_{gal}\) sensors (Figure 10) in saturated Ca(OH)\({}_{2}\) aqueous solutions. Several conditions simulating the most important parameters that can affect the corrosion of concrete in RCS were studied, such as carbonation, ingress of chloride ions and O\({}_{2}\). In all cases, the influence of time and temperature (25 to 55 \({}^{\circ}\)C) were considered. Figure 11 shows the \(i_{gal}\) values of the steel-working electrode vs. immersion time under the different experimental conditions studied. The results showed that for the steel in the passive state no variation of \(i_{gal}\) was found (Figure 11a). As the pH was lowered (from 12.5 (Figure 11a) to 9.5 (Figure 11b-d), the process was influenced by the temperature. Higher \(i_{gal}\) values were obtained for higher temperature values either in the presence of Cl\({}^{-}\) or the absence of O\({}_{2}\) (Figure 11b-d). Figure 11 also shows that due to the formation of oxides on the working electrode surface, a decrease of \(i_{gal}\) with time was recorded (Figure 11c,d). A relationship between the galvanic currents and the corrosion rates of reinforcing steel, under a great variety of controlled laboratorial conditions, was also established. The authors have shown that \(i_{corr}\approx 10\)\({}^{9/2}\)\(i_{gal}\)\({}^{6/5}\)[82; 182].