*4.1.2. Linear polarization resistance*

LPR is a real-time technique regulated by the ASTM G59 standard method and is based on the potential variation around the open circuit potential (OCP; typical variations approximately ±10 mV [76] to ±20 mV are used) [83,84]. The current required to maintain a specific displace‐ ment of the resting potential is directly related to the corrosion process on the electrode surface. This technique is particularly useful in aqueous systems and is applicable to obtain the polarization resistance (Rp) [75]. It is also possible to calculate the current and the corrosion rate if the values of Rp and the Tafel slopes are known [32,83]. In addition, it is possible to evaluate the porosity of the coating by comparing the Rp values of the coating and of the substrate [82].

#### *4.1.3. Electrochemical impedance spectroscopy*

EIS is a technique that provides detailed information on the electrical characteristics of the electrode/solution interface. EIS is based on the application of a small sinusoidal signal of potential (or current) to the working electrode according to a particular desired frequency range. As a response, it obtained the impedance (Z), which can be related to the opposition to the current flux in the system [32]. Important information about the charge transfer kinetics, structure, and properties of the interface electrode/electrolyte can also be achieved. The frequency range usually used for disturbing the system varies from 100 kHz to 10 MHz, using the amplitude signal in the range of 5 to 50 mV rms, depending on the studied system [32,85].

The results obtained from the EIS measurements can be used to construct diagrams repre‐ senting the behavior of the electrode in a particular electrochemical process. One of these diagrams is the Nyquist diagram, in which the real (Z) and imaginary (Z) impedance data are represented in a complex plane. The real impedance (Z) incorporates the ohmic resistance (i.e., the pure resistance that is independent of the frequency). The imaginary impedance (Z) incorporates the capacity and/or inductive reactance (i.e., the resistance that is dependent on the frequency applied to the system) [32].

The Bode and phase diagrams are also used to represent the EIS results. These diagrams show the variations of the impedance modulus (|Z|) or the phase angle with the frequency, respectively. An advantage of the Bode diagram is the possibility of observing the impedance variation at high frequencies, which is generally omitted in the Nyquist diagram representa‐ tion [32].

EIS can be considered as a nondestructive technique [32,85,86]. The application of this technique in electrochemical systems considers the combination of physical and chemical interfacial processes with the components of an equivalent electrical circuit as an analogy of the electrochemical phenomena. Therefore, it is possible to obtain the electrolyte resistance (Re), the charge transfer resistance (Rct), the polarization resistance (Rp), and the double layer capacitance (Cdl) as variables that allow the assessment of coating/substrate characteristics [32,86]. In addition, it is also possible to evaluate the corrosion rate and the porosity of the coating by comparing the Rp values of the coating and of the substrate in the same electrolytic medium [32,82].
