Equivalent electrical circuit, corrosion

Figure 9.2 Equivalent electrical circuit of the interfacial impedance at the corrosion potential, where ic represents the charging current and I c represents the cathodic current.

Give examples of corrosion processes that are not adequately modeled by the simplest equivalent electrical circuit of Fig. 6.18. 

Electrochemical impedance sp>ectroscopy (EIS) provides detailed information on the corrosion reaction by determining the equivalent electrical circuit of a corroding system. 

Fig. 6. Equivalent electric circuit for corrosion of bulk Al a) corrosion system A1 - 0.5M NaCl solution, b) an equivalent circuit scheme, c) Nyquist frequency characteristics...

Equivalent electrical circuits obtained by minimizing the mean square error were further used for the analysis of experimentally identified frequency characteristics and a description of corrosive processes in the systems under investigation. A simple electric circuit consisting of three elements of type R and C with a single time constant was adopted as system model for Al. Figs. 6 and 7 show the circuits modeling respectively the... 

Fig. 6 presents the system of A1 corrosion in 0.5 M NaCl solution, its frequency impedance characteristic in the form of Nyquist plot and the equivalent electrical circuit. Individual parts of the electric circuit reflect the electrochemical and electrical characteristics of the corrosion systems. In this arrangement, the spectral characteristic of the impedance in the Nyquist plot has the shape of a semicircle, whose intersection with the real axis in the high-frequency range determines the electrolyte solution resistance Rs. Conversely, the intersection of the real axis in the low-frequency range corresponds to the sum of Rs + Rci/ where Ret indicates the charge transfer resistance of the boundary metal/electrolyte, and characterizes the rate of corrosion. On the other hand, Cdi component of the circuit represents capacity of the double layer at the interface metal/electrolyte. 

Similarly to the previous case the element Rs of the equivalent electric circuit for this corrosion system represents the resistance of the 0.5M solution of NaCl electrolyte used as the corrosive environment. Elements in parallel in the equivalent electric circuit characterize the protective properties of AI2O3 layer deposited on the bulk Al. The element Cb specifies the AI2Q3 layer capacitance, which depends on the thickness of this layer and on the dielectric properties of the material. The resistor Rb in such a system represents the resistance of the protective layer, and depends on properties of the material forming the layer, and varying with the thickness of the layer and its material composition. The... 

The equivalent electrical circuit approach adopted maps the processes occurring in the corrosion systems and enables the determination of parameters relevant to these processes. The parameter values of individual elements of equivalent electrical circuit representing investigated corrosion systems are summarized in Table 2. 

Table 2. Parameters of equivalent electrical circuit for corrosion processes of bulk A1 and AI2O3 layer in 0.5M NaCl solution.

The frequency characteristics of corrosion systems of bulk A1 and AI2Q3 layer in 0.5M NaCl solution in the form of Nyquist and Bode plots obtained by the measurements and calculations based on adopted equivalent electrical circuits are shown in Figs. 8 and 9, respectively. 

Effective modeling of complex electrochemical processes of corrosion in the systems based on iron required the use of a more complex equivalent electrical circuit, i.e., circuit containing CPE - constant phase elements. Constant phase element (CPE) is characterized by a constant angle of phase shift. Impedance of the CPE is described by the following expression Zcpe = l/Yo(jffl)", where Yo and n are parameters related to the phase angle. The more heterogeneous the corrosion processes occurring on the metal surface the smaller value of the parameter n. 

Best matching of all designated impedance spectra for experimentally studied systems of corrosion of iron and S235JR steel in the solution of 0.5 M NaCl was obtained by using an equivalent electric circuit with two time constants, whose structure is shown in Fig. 13. 

Each element of this circuit appropriately models the specific process or phenomenon occurring in the system investigated. In the circuit shown in Fig. 13 the resistance element Rs represents corrosive environment, i.e., 0.5 M NaCl solution. The resistance representing the charge transfer through the interface associated with the process of oxidation of iron, i.e., the corrosion element, is described by R, and the electrical double layer at the interface iron -0.5M NaCl solution is characterized by a constant phase element CPEdi. The use of two constant-phase elements in an equivalent electric circuit improves the quality of model fit to... 

Further assessment of the characteristics of the impedance at the system boundaries for nickel and its alloy in 0.5M NaCl solution was obtained by approximation of experimental data using equivalent electrical circuits. The equivalent electrical circuits most suitable to represent the measured impedance characteristics of studied systems of nickel with different structures and its alloy in corrosive environment of 0.5M NaCl solution are shown in Figs. 19, 20 and 21. The corresponding resulting impedances are described in the expressions (4) -5- (6). For the analysis of the corrosion of microcrystalline structure nickel with the equivalent electrical circuit a simple layout shown in Fig. 19 was used. 

Fig. 19. The equivalent electrical circuit for corrosion of microcrystalline structure nickel in 0.5 M NaCl solution.

Experimentally determined impedance spectra of nanocrystalline nickel corrosion (fig. 20) are well mapped by equivalent electrical circuit with two time constants described by equation (5)... 

Fig. 20. Equivalent electrical circuit of corrosion of the nanocrystalline structure nickel in 0.5M NaCl solution...

This circuit, besides elements such as Rs, Ret, CPEai which are needed in the equivalent electrical circuit to describe the corrosion of microcrystalline nickel contains two additional elements CPEi - modeling capacity of the passive layer on the material surface, and Ri - describing the resistance of the passive layer. 

To describe the corrosion processes occurring in the system NiP- 0.5M NaCl solution the equivalent electrical circuit shown in Fig. 21 was designed with the resulting impedance expressed by (6). 

The p>arameters of the equivalent electrical circuits of corrosive systems of nickel materials tested in this study are summarized in Table 7. 

Table 7. The parameters of equivalent electrical circuits for corrosive systems of nickel -0.5M NaCl solution.

FIGURE 8.3 Equivalent electric circuits to describe a defective coating. Cj, is the double layer capacitance, R, is the charge transfer resistance of the corrosion process, Qj is the constant phase element, Cjjj is the diffuse layer capacitance, and is the diffnse layer resistance. 

The impedance spectroscopy of steel corrosion in concentrated HC1, with and without inhibitors, exhibit relatively straightforward electrochemical phenomenology and can be represented by simple equivalent circuits involving primarily passive electrical elements. Analysis of these circuits for steel corroding in HC1 per se reveals that the heterogeneity of the surface is established rapidly and can be simulated with a simple electrical circuit model. 

Corrosivity tests frequently involve heating materials in a horizontal tubular furnace and determining the corrosivity by dissolving the gases in water and determining pH. or acids equivalent (e.g.. HCI), but more recent tests involve a.ssessing the effect of corrosive gases on electrical circuits. 

Electrochemical methods are well adapted for characterizing the corrosion behavior of coated metals in solution. Because of the high resistance of organic coatings, ac methods are generally more suited than dc polarization methods. In electrochemical impedance spectroscopy (EIC) one measures the response of the coated electrode to a small amplitude ac perturbation as a function of frequency (Chapter 5). The interpretation of the measured frequency response, in principle, requires a physical model. However, for coated metals useful information is more easily obtained by representing the metal-coating-electrolyte interface by an electrical circuit (equivalent circuit). 

Figure 3 Electrical equivalent circuit model commonly used to represent an electrochemical interface undergoing corrosion. Rp is the polarization resistance, Cd] is the double layer capacitance, Rct is the charge transfer resistance in the absence of mass transport and reaction intermediates, RD is the diffusional resistance, and Rs is the solution resistance, (a) Rp = Rct when there are no mass transport limitations and electrochemical reactions involve no absorbed intermediates and nearly instantaneous charge transfer control prevails, (b) Rp = Rd + Rct in the case of mass transport limitations.

Fig. 11.1 Electrical equivalent circuits for an organic coating (a) ideal coating, (b) real coating without corrosion, (c) adhering coating with the onset of corrosion in pinholes, (d) coating with disbonding, (e) coating with disbonding and small...

Figure 7.4 Electrical equivalent circuit of corrosion cell.

The interpretation of impedance data in terms of electrical equivalent circuits, as discussed by the examples given before, only makes sense if the elements of the circuit are unequivocally related to the physical properties of the corrosion system. Howev-... 

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