**2.2 Kinetic oxidation reaction of steels tested in their received condition at different aqueous solutions**

**Figure 12** shows the typical impedance behavior of a steel with specification of AISI 8620 (0.20 wt.%C, 0.90 wt.%Mn, 0.35 wt.%Si, 0.60 wt.%Cr, 0.70 wt.%Ni, 0.25 wt.%Mo) in its received condition after exposed to different aqueous solutions such as distilled water, NaCl 0.5 M, HCl 1 M, H2SO4 1 M. The supplied voltage signal has an amplitude of 10 mV that fluctuating around the corrosion potential (654 mV) in the frequency range of 1 MHz to 1 mHz, the response obtained is represented in Bode diagrams in which the impedance module and the phase angle serve as functions of the Log frequency, these diagrams indicate the sensitivity of the *EIS* technique to evaluate the presence of growth of a natural oxide on the steel surface, this is observed for the case of corrosion test in distilled water. Two welldefined time constants are observed in the evaluated frequency domain, one time constant at higher frequencies is related to the presence of an oxide layer, however, the intensity of the phase angle signal of 85° gives information about the oxide thickness and its adherence, however micro-cracks, closed porosity or growth defects are always present in many kinds of oxide layers that serve as conducting pathways of ions coming from the aqueous electrolyte, allowing electron charge

transfer. This causes the phase-shifted continuously to zero degrees at frequencies between 80.7 kHz to 61.5 Hz, suggesting that the system behaves like a resistive

*Bode plots of impedance response of corroding 8620 plate at different aqueous solutions; distilled water, NaCl*

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion…*

oscillates with the same phase as the potential does. In this frequency range an adsorptive process is carried out in which ions passing through the oxide layer defects, this mechanism is shown by the inductive response of the **Figure 13**. However, at lower frequencies over 8.59 Hz, an increase in the phase angle to 40° (56.6 mHz) is observed as if it were a capacitor in which the steel interface is charged by OH molecules, it is worth mentioning that this response is not related to the corrosion process, but this is a typical response to a passive system with a

*Comparison of experimental and fitted EIS data for 8620 steel after exposure to distilled water.*

Ω -cm2 . On the other hand, when the pH of the aqueous solution decreases to an acidi-

impedance diagrams has been change, for example, for NaCl solution, a slightly

, Cl, OH, SO4

, *i.e.* the current signal

, H<sup>+</sup>

, the shape of the

component with a low flow of current near 10.3 μA/cm<sup>2</sup>

magnitude of impedance about 103

**Figure 13.**

**19**

**Figure 12.**

*at 0.5 N, HCl at 1 M and H2SO4 at 1 M.*

*DOI: http://dx.doi.org/10.5772/intechopen.94470*

fied stage by the presence of ions such as Na<sup>+</sup>

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

**Figure 12.**

*Bode plots of impedance response of corroding 8620 plate at different aqueous solutions; distilled water, NaCl at 0.5 N, HCl at 1 M and H2SO4 at 1 M.*

#### **Figure 13.** *Comparison of experimental and fitted EIS data for 8620 steel after exposure to distilled water.*

transfer. This causes the phase-shifted continuously to zero degrees at frequencies between 80.7 kHz to 61.5 Hz, suggesting that the system behaves like a resistive component with a low flow of current near 10.3 μA/cm<sup>2</sup> , *i.e.* the current signal oscillates with the same phase as the potential does. In this frequency range an adsorptive process is carried out in which ions passing through the oxide layer defects, this mechanism is shown by the inductive response of the **Figure 13**. However, at lower frequencies over 8.59 Hz, an increase in the phase angle to 40° (56.6 mHz) is observed as if it were a capacitor in which the steel interface is charged by OH molecules, it is worth mentioning that this response is not related to the corrosion process, but this is a typical response to a passive system with a magnitude of impedance about 103 Ω -cm2 .

On the other hand, when the pH of the aqueous solution decreases to an acidified stage by the presence of ions such as Na<sup>+</sup> , Cl, OH, SO4 , H<sup>+</sup> , the shape of the impedance diagrams has been change, for example, for NaCl solution, a slightly

acidified substance breaks-out almost the integrity of the natural oxide layer that covers the metal matrix and the response related to ion charge transfer to the metal interface is observed at lower frequencies. In addition to, an increase in current is also observed of about 34.479 μA/cm<sup>2</sup> and a |*Z*| of 10<sup>2</sup> Ω -cm2 . Whereas, the same steel exposed to a more corrosive electrolyte such as HCl or H2SO4 at 1 M, the EIS response shows a single time constant that corresponding to the reaction's oxidation and reduction on the steel interface. That means, transient electrical charge events occur on the electrochemical double layer with ions, and is characterized by an increase of the current from 43.58 and 198.25 μA-cm<sup>2</sup> , respectively and the decrease of one order of magnitude of the impedance module 10<sup>1</sup> Ω -cm2 , that is, less resistivity. The results in **Table 3** indicates the simulation of impedance parameters with an appropriate electrical circuit that have been describe before, these data suggest that a higher current passing and large electrical charging at the interface of the steel increases the susceptible to attack by corrosion, that is, the internal energy of the aqueous solution has the ability to degrade freely the steel by pitting corrosion.

Same behavior was observed for impedance-monitored corrosion tests for a 316 stainless steel plate (18.24 wt.%Cr, 8.07 wt.%Ni, 1.76 wt.%Mn, 0.5 wt.%Si, 0.27 wt. %Mo as principal alloying elements) after exposure to different aqueous solutions such as distilled water, 0.5 N NaCl, 0.5 N KCL, 1 M HCl or 0.5 M H2SO4, **Table 4** shows the dissolution reaction. The impedance spectra that is shown in **Figure 14** indicates one of the advantages of the *EIS* technique to evaluate the performance of metal interface in full immersed to aggressiveness conditions of different electrolytes. In this sense the natural film of chromium oxide that protects stainless steel against corrosion is remarkable in distilled water by the presence of one time constant at higher frequencies with an impedance value near to 1 MΩ-cm2 .


Meanwhile, the presence of Cl ions (NaCl or KCl salt) alters the coating interface, which is electrically charged by ions causing the passivity state of stainless steel broken-down due to the dissolution of the oxide film, it is assumed that the steel is susceptible to corrosion by pitting. This is also seen through the presence of a time constant in the frequency domain studied. Similarly, the experimental tests in stronger acid media (HCl or H2SO4) indicate that stainless steel is seriously corroded in these conditions as a decrease in the impedance value below 1 mΩ-cm<sup>2</sup>

*Impedance response of corroding stainless steel (SS316) during exposure to (1) distilled water, (2) NaCl 0.5 N,*

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion…*

**2.3 Steel corroded at non-stationary solution (rotating disk electrode, RDE**

Other application of the *EIS* technique is like that shown in **Figure 15**, which is the evaluation of the effect on hydrodynamic conditions on the corrosion process in steels. This particular study has an interest to show the behavior of a pipeline steel (API-5 L-X70) that is used for transportation of hydrocarbon fluid. This steel was immersed in HCl 1 M solution at a different rotation speed of the working electrode (*WE*) from 0 to 1500 rpm, *i.e.* from static conditions 0 rpm, laminar flow 1 to 200 rpm and to turbulent flow 300 to 1500 rpm. **Figure 15** shows the *EIS* response in the representation of Bode and Nyquist for the steel interface during its exposure

At the steady-state conditions, without rotation, the impedance response is related to electrons flow from the aqueous media to the metal interface allowing the formation of an interfacial layer over the metal surface, called an electrical double layer or a thin oxide film, which is indicated by the distortion of the semicircle

**condition)**

**21**

**Figure 14.**

to a corrosive media at different flow rates.

*(3) KCl 0.5 N, (4) HCl 1 M, and (5) H2SO4 1 M.*

*DOI: http://dx.doi.org/10.5772/intechopen.94470*

.

**Table 3.**

*EIS parameters of simulated data to equivalent electrical circuit (*EEC*) for the steel 8620 during its exposure to different electrolytes.*


#### **Table 4.**

*Capacitance of electrical double layer for the stainless steel SS316 during its exposure to different electrolytes.*

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

**Figure 14.**

*Impedance response of corroding stainless steel (SS316) during exposure to (1) distilled water, (2) NaCl 0.5 N, (3) KCl 0.5 N, (4) HCl 1 M, and (5) H2SO4 1 M.*

Meanwhile, the presence of Cl ions (NaCl or KCl salt) alters the coating interface, which is electrically charged by ions causing the passivity state of stainless steel broken-down due to the dissolution of the oxide film, it is assumed that the steel is susceptible to corrosion by pitting. This is also seen through the presence of a time constant in the frequency domain studied. Similarly, the experimental tests in stronger acid media (HCl or H2SO4) indicate that stainless steel is seriously corroded in these conditions as a decrease in the impedance value below 1 mΩ-cm<sup>2</sup> .
