**2.1 Analysis of effects of a voltage stimulus applied to corroded metals**

One of the principal applications of *EIS* is in the study of electrolyte/electrode interfaces which is widely used in the evaluation of corrosion mechanism in metals at different environments conditions, but it has also been very useful in the performance of coatings [50–55] and in the failure detection of materials by stress corrosion cracking, similarly according to recent publications *EIS* also appears to be applied in ceramics materials [56–58]. In this sense, most of literature indicates that when applying a periodic signal of potential with amplitude from 5 to 10 mV in a given frequency domain, it is possible to detect the transitory current to obtain a change in the phase angle between *I*-*V* and the |*Z*| data, which progress over time in order to predict metal corrosion phenomena or a possible electrochemical reactions at the metal interface. It should be noted that using a known electrical circuit it is possible to characterize the impedance spectra for each system under study as it shown before. The device that allows applying a programmed potential and detected the current is a potentiostat. Therefore, in this study a galvanostatpotentiostat PARSTAT-4000 was used to evaluate the effect of the voltage applied to the two-electrode interface. In which a periodic constant signal at 1 kHz of frequency was applied over a voltage range of 1 to 1000 mV as a function of frequency domain (1 MHz to 1 mHz). For this study it was considered the following systems; i) An ideal system like circuit #1, which is designed by *RC* components, a pure capacitor of 1 μF is connected in parallel to a resistor of 3 kΩ and then connected together in series with a resistor of 200 Ω and ii) a 3 cm<sup>2</sup> of stainless steel plate were used as working electrode (*WE*) after being exposed to an aqueous solution of HCl 1 M, then the *WE* was perturbed by a sinusoidal potential at different amplitude from 1 to 1000 mV, the corresponding impedance data for each of the cases that are displayed in **Figure 10**.

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion… DOI: http://dx.doi.org/10.5772/intechopen.94470*

**Figure 10.**

*Typical impedance spectra showing the effects of the amplitude signal in; a)* EEC *model #1 (*Ro *= 276 Ω,* R1 *= 3.3 kΩ,* C1 = *1 μF) and b) a stainless steel immersed in HCl 1 M.*

The results show that when an alternate electrical pulse *V(t) of* 1 kHz fluctuates from 1 to 1000 mV through an ideal circuit like *EEC* model #1 as that shown in **Figure 10a**, a uniform current *I(t)* flows as a function of frequency domain, this signal produces a well-defined time constant in the entire frequency range. During the pulse at a time *t* the capacitor stores electrical energy causing an increase in potential difference *ZT*ðÞ¼ *<sup>t</sup> <sup>q</sup> C* and that allows the current to be phase shifted with respect to the voltage of about 60°, meanwhile the resistor *R1* connected in parallel does not allow the passage of the current, instead of it decreases gradually to zero according to the Ohm's Law, that is why the capacitor stops charging load. Finally, when the period of the capacitor's transient load ends, the potential difference in the circuit must be zero when the stored load has been exhausted, *i.e.* the circuit has been returned to its equilibrium state. Due to the characteristics of the capacitor, which is composed by a parallel polished metal plates separated with a dielectric at a distance of *d*, and due to the transient events of charging rate and discharging rate during the continuous passage of the potential at different intensities of the signal amplitude does not cause changes in the interface of the plates, so the impedance data in bode representation are overlaid showing the same behavior for all data. That is, the load capacity or capacitance of 1 μF remains constant as the amplitude of the sinusoidal signal increases from 1 to 1000 mV as is shown in **Figure 11**.

The same behavior is observed for stainless steel SS316 plate immersed in HCl 1M(**Figure 10b**), the metal interface exposed to the acid solution allows the electron transfer rate at the equilibrium potential (Ecorr) after applying lower amplitudes of the stimulus signal (between 1 to 20 mV), the impedance diagrams for this conditions do not show changes caused by the current flows into the system. In this sense the metal interface working similar as the ideal capacitor allowing ions loading charging such as Cl� and OH� with capacitances ranging between 40 to 80 μF/cm<sup>2</sup> , which is indicated by a well-defined one time constant due to the presence of a protective oxide layer (passive condition) and can be easily represented by the *EEC* model #1. Notable effects can be caused by applying high current, as is clearly seen in the distortion of the shape of EIS diagrams during increasing the amplitude of the stimulus signal from 50 to 1000 mV, the impedance value |*Z*| gradually down several orders of magnitude and severe changes in phase angle less than 20° are observed, this mean that two time constants are obvious seen and are related to the corroded interface, *i.e.* dissolution of the chrome protective

**Figure 11.**

*AC amplitude signal dependence on capacitance value for an ideal* EEC *circuit model #1 (C1 = 1 μF) and the stainless steel SS316 plate during its immersion in HCl 1 M.*

film and manifestation of the pitting corrosion process that occurs after 200 mV, for this case an increase in the interface charge of electrons is expected with capacitances over 434.40 mF/cm<sup>2</sup> , like that as shown in **Figure 11**. It can conclude that it is possible to carry out experimental tests with amplitude signals ranging from 1 to 20 mV at the steady-state of corrosion potential without surface damage by the current applied, which is in according to the literature that reports an amplitude signal of 5 to 10 mv.
