## 1 Introduction  
Concrete prepared with a suitable water/cement (\(w\)/\(c\)) ratio, well consolidated and properly cured offers excellent protection against corrosion to the reinforcing steel in concrete. When the steel is embedded in the concrete it is protected by a physical barrier and by a passivation film formed on the surface of the steel from the contact with aggressive species such as chloride ions (Cl\({}^{-}\)), carbon dioxide (CO\({}_{2}\)), sulfur dioxide (SO\({}_{2}\)), nitric oxides (NO\({}_{x}\)), or moisture. The passivation film formed on the reinforcing steel surface is due to the high alkalinity of the concrete (pH \(>\) 13.5). These high pH values are due to the presence of calcium hydroxide (Ca(OH)\({}_{2}\)) and potassium hydroxide (KOH). Furthermore, its high electrical resistivity restricts the rate of corrosion [1]. However, during the hydration and curing processes strain and tensions may be generated and promote the formation of cracks that may reduce the efficiency of the concrete barrier. Generally, the degradation of concrete properties results from the combination of several internal and external factors. It is a complex process, widely determined by the physico-chemical properties of the concrete (internal factors) and by the environment that the concrete is exposed to (external factors). Furthermore, the physical barrier provided by the concrete is not perfect due to the porous structure of the concrete. The existence of imperfections during concreting and curing allow the ingress of aggressive species at the steel/concrete interface causing the rupture of the passivation film. The most common causes are the incorporation of Cl\({}^{-}\) and the reaction of atmospheric CO\({}_{2}\) with the constituents of concrete and/or a combination of the two. The volume of the corrosion products is much higher than steel, approximately 4 to 6 times [2]. Therefore, the corrosion evolution in the reinforced concrete structures (RCS) produces expansion forces that, if they exceed the tensile strength of the concrete, initially lead to cracking or spalling followed of the detachment of the concrete [2]. The evolution of corrosion also leads to loss of adhesion between concrete and building steel, loss of ductility of steel and reduction of the section of steel rebar which may have implications on the stability of RCS [2]. Several methods for corrosion monitoring of RCS have been proposed in the last few decades. These systems may be used either in new, existing or repaired structures. These may be destructive, use different degrees of complexity and cost, and provide information on the progression and kinetics of the corrosion phenomena. The destructive methods are limited to sampling. Therefore, sampling may not be representative of the whole structure, which is extremely important in RCS with large heterogeneities both in terms of materials used and in terms of the exposure environment. Furthermore, in situations where the evolution of the corrosion over time is assessed it may not be advisable to use destructive methods. For example, for repair procedures it would not be suitable to remove a large quantity of samples required for assessment. Within this context, non-destructive methods have been widely developed, which are intended to provide quick information about the entire structure. These methods should be able to detect the corrosion state of the steel inside the concrete, the main causes of corrosion, as well as the evolution of the corrosion phenomena over time. The development of non-destructive electrochemical methods for the assessment of corrosion in RCS began in the 1970s with the measurement of corrosion potential. The measurement of corrosion rate, resistivity, galvanic current, oxygen limiting current and chloride content was followed in the early 1990s, and particularly in the last two decades.