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

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 to a corrosive media at different flow rates.

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

**Figure 15.** *Experimental impedance diagrams of corroding pipeline steel (API-5 L-X70) during exposure to HCl 1 M at different electrode rotation speed (0 to 1500 rpm). a) Bode plots representation and b) Nyquist complex plane.*

presenting two time constants not very well-defined, besides in the diagram of bode two changes of slopes are shown for the impedance module. When applying rotation from 20 to 200 rpm an increase in the magnitude of the Zreal and Zimag is observed due to the reaction kinetics at which the interfacial layer is forming at instantaneous rate and is controlled by electron charge and mass transfer mechanism. However, at turbulent conditions (>500 rpm) it does not allow the ions adsorption at the metal interface to maintain the presence of the double electrochemical layer or oxide film allowing only transients of electron transfer as a function of time, which promote the interfacial degradation of the steel. Therefore, the impedance diagrams show that under equilibrium conditions there is a corrosion rate controlled by the presence of a natural oxide on the steel surface, but this increased by the hydrodynamic conditions at turbulent flow, which is what is seen in real cases of application. But at moderate rotation speed the mass transport toward to the metal surface is carried out, giving opportunity to adsorption of molecules that come from the aqueous solution, which is consistent with the review literature [59].

specific resistance of the concrete that could be controlled by charge transfer process; while the straight line indicates a diffusion mechanism of ions through the pores. It is observed that the semicircle amplitude for the reference sample [REF.- 0d, non-carbonated] is shorter than the carbonated samples at 7 or 120 days, this suggest that its resistance to the ions diffusion through the porous structure is much lower (a favorable condition for the ions coming from the aqueous solution driven easily into the porous structure of the concrete, resulting in the faster flow of electrons with chemical reactions and molecules adsorption processes around the vicinity of the steel interface), in addition to this, a typical signal describes a passive stage of the concrete. However, notable changes in the semicircle amplitude of the EIS spectra are observed, these changes are associated to the increase in electrical resistance (*R*) value of the concrete from 23.62 to 101.54 k<sup>Ω</sup>cm<sup>2</sup> as the carbonation progress until to 84 days of CO2(g) exposure, this resulted to the blockade of the concrete pores by a calcium carbonate products, this reduces de alkalinity condition of the concrete matrix. However, the EIS diagrams for 106 days of exposure the

*EIS spectra for the particular system of concrete with reinforcing steel exposed to different days of a CO2(g) environment, carbonation process [6]. a) Nyquist complex plane showing the carbonation progress and b)*

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

plete, but after 120 days the resistivity still remains lower than 84 days of CO2(g)

frequency domain. The changes registered by the *EIS* data for carbonated samples for 7 to 84 days are well-defined by one semicircle located at high frequencies (concrete porous resistance) with an infinite linear response at low frequencies (diffusion mechanism) only seen in the frequency domain of about >10<sup>6</sup> to 10<sup>3</sup> Hz by imposing a small amplitude of *AC* signal perturbation to the concrete/steel reinforcement system, this linear response was then modified by a second

depressed semicircle with an inductive loop at lower frequencies in the domain of 10<sup>6</sup> Hz, using the EEC model #6 represents this behavior. The characteristic behavior of a second semicircle formed at lower frequencies for 106 or 120 days indicates that a process of corrosion may occur on the steel bar surface. The *EIS* parameters effectively demonstrate that after 106 days of exposure the carbonation is almost complete and corrosion damage is clearly progress on the steel bar. Carbonation progress was monitored by a significant increase in the diameter of the

) and the *EIS* spectra show remarkable changes in the low

, the carbonation is almost com-

resistance value decreases of about 58.26 k<sup>Ω</sup>cm2

exposure (65.59 k<sup>Ω</sup>cm<sup>2</sup>

**23**

**Figure 16.**

*Nyquist response for steel corroding.*

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

#### **2.4 Corrosion monitor in concrete reinforced materials**

*EIS* technique can also be used for monitoring the evolution of the carbonation progress on concrete and the corrosion of the steel that serves as reinforcement. Carbonation results in a decrease in the pH of the cementation matrix when CO2(g) from the environment diffuses into the concrete structure, that can cause the loss of the passivity condition on the reinforcing steel surface and leads to an early failure of concrete by corrosion attack. Change in electrical resistance (Rpo) and capacitance (Cpo) of the concrete bulk is measured by a semicircle at high frequency region, which is the typical response of EIS diagram as that shown in **Figure 16**. More details are available in the research of H. Herrera in 2019 [6]. The corrosion test of this study was carried out on a fresh cross section of concrete sample after 7, 14, 21, 42, 61, 84, 106 and 120 days of artificially CO2(g) exposure periods (carbonation process). The characteristic impedance diagrams (*EIS*) of the concrete specimens after carbonation process at different ages of CO2(g) exposure during immersion in tap water are shown in **Figure 16**.

The *EIS* spectra is displayed in the Nyquist plots (Zreal vs. Zimaginary), these results show a single capacitive well-defined semicircle at higher frequencies followed by a straight line for 7 to 84 days of carbonation, which indicates the *Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion… DOI: http://dx.doi.org/10.5772/intechopen.94470*

#### **Figure 16.**

*EIS spectra for the particular system of concrete with reinforcing steel exposed to different days of a CO2(g) environment, carbonation process [6]. a) Nyquist complex plane showing the carbonation progress and b) Nyquist response for steel corroding.*

specific resistance of the concrete that could be controlled by charge transfer process; while the straight line indicates a diffusion mechanism of ions through the pores. It is observed that the semicircle amplitude for the reference sample [REF.- 0d, non-carbonated] is shorter than the carbonated samples at 7 or 120 days, this suggest that its resistance to the ions diffusion through the porous structure is much lower (a favorable condition for the ions coming from the aqueous solution driven easily into the porous structure of the concrete, resulting in the faster flow of electrons with chemical reactions and molecules adsorption processes around the vicinity of the steel interface), in addition to this, a typical signal describes a passive stage of the concrete. However, notable changes in the semicircle amplitude of the EIS spectra are observed, these changes are associated to the increase in electrical resistance (*R*) value of the concrete from 23.62 to 101.54 k<sup>Ω</sup>cm<sup>2</sup> as the carbonation progress until to 84 days of CO2(g) exposure, this resulted to the blockade of the concrete pores by a calcium carbonate products, this reduces de alkalinity condition of the concrete matrix. However, the EIS diagrams for 106 days of exposure the resistance value decreases of about 58.26 k<sup>Ω</sup>cm2 , the carbonation is almost complete, but after 120 days the resistivity still remains lower than 84 days of CO2(g) exposure (65.59 k<sup>Ω</sup>cm<sup>2</sup> ) and the *EIS* spectra show remarkable changes in the low frequency domain. The changes registered by the *EIS* data for carbonated samples for 7 to 84 days are well-defined by one semicircle located at high frequencies (concrete porous resistance) with an infinite linear response at low frequencies (diffusion mechanism) only seen in the frequency domain of about >10<sup>6</sup> to 10<sup>3</sup> Hz by imposing a small amplitude of *AC* signal perturbation to the concrete/steel reinforcement system, this linear response was then modified by a second depressed semicircle with an inductive loop at lower frequencies in the domain of 10<sup>6</sup> Hz, using the EEC model #6 represents this behavior. The characteristic behavior of a second semicircle formed at lower frequencies for 106 or 120 days indicates that a process of corrosion may occur on the steel bar surface. The *EIS* parameters effectively demonstrate that after 106 days of exposure the carbonation is almost complete and corrosion damage is clearly progress on the steel bar. Carbonation progress was monitored by a significant increase in the diameter of the

semicircle, thus demonstrating the increase in resistivity of ions transmission due to blockade of pores by precipitation of CaCO3 compounds. Finally, the *EIS* technique results a practical tool for evaluating the carbonation progress on reinforced concrete structures without causing structural damage, and its sensitivity to predict the activation of the reinforcing steel to be corroded.
