Passivating layer equivalent circuit

Corrosion of the positive grid [Eq. (28)1 occurs equivalent to about 1 mA/lOOAh at open-circuit voltage and intact passivation layer. It depends on electrode potential, and is at minimum about 40-80mV above the PbS04/Pb02 equilibrium potential. The corrosion rate depends furthermore to some extent on alloy composition and is increased with high anti-monial alloys,... 

The impedance response with frequency can be closely simulated by the equivalent circuit shown in Figure 27a, where Re, Ra, Cdi, Rad, and Cad represent the resistance or capacitance for the electrolyte solution, charge-transfer, double layer, and adsorbed layer, respectively. An interesting correlation was found between the passivating ability of various anions and the resistances of the two impedance components R and Rad, which are high for LiPFe-and LiBF4-based electrolytes and low for LiTf- or Lilm-based electrolytes. Using the rationale proposed by the authors, the former component (Ret) is... 

The electrode/solution interface, in the simpler case (no passive layer on the electrode surface) can be modeled using the following equivalent circuit (Figure 1.18), where Rel stands for the electrolyte resistance, CDL for the double layer capacitance, and ZF the faradic impedance. 

Figure 4.5.8. Equivalent circuit of particle surface impedance taking into account a passivating layer.

Figure 4.5.9. Equivalent circuit of a battery insertion electrode. Here R is the distributed resistance of the transmission line representing electronic and ionic resistance of the layer of active material the charge transfer resistance and passivation layer resistance of the particle interface and the double layer capacitance C . is the impedance of diffusion and charge storage processes inside the particles.

As discussed above, the corrosion current density (Icorr), the critical current density (Icri), and the passive current density (Ipass) were obtained from potentiodynamic polarization. The capacitance (Qf) and the resistance (Rf) of oxide layer were obtained from electrochemical impedance spectroscopy (EIS) equivalent circuits. And the weight-loss rate (Wbss) was obtained from weight-loss immersion test. All these data were taken from experiments at ambient temperature (25°C) in 0.5 M H2SO4. 

EIS results indicate that the passive films of AkCoCrFeNi alloys become increasingly thicker and more dispersive with an increasing x. Therefore, Ipass increases with x. As x value increases to 1.00, the inductance effect appears in the equivalent circuit for severe dissolution of A1 and Ni-rich phase. As for the effect of chloride on the anti-corrosion property, chloride eases the passive layer to form metastable ion complexes and further dissolve into H2SO4. With an increasing chloride concentration and A1 content, the metastable ion complexes easily form, allowing Epu to shift to a more active region. Additionally, the microstructure of both C-0 and C-0.25 is single FCC phase, while those of C-0.50 and C-1.00 are duplex FCC-BCC and complex BCC-ordered BCC phase, respectively. 

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. 

Kinetics in the film can also be affected by the formation of pores and pits that according to the De Levie theory will modify the EIS response. Formation of active-passive corrosion pits on 304 stainless steel exposed to a sodium-chloride corrosive solution can be represented by the electrical equivalent circuit composed of two parallel branches [48,49]. The first branch represents the surface area without active pits (passive layer), where only inactive pits are present, and the second branch corresponds to the impedance of the active pits. The impedance inside the inactive pits can be represented by a transmission line, according to De Levie (Eq. 7-66). For relatively high frequencies the impedance of such a circuit can be represented by the passive layer film capacitance Cpj in parallel with the impedance in the passive pores or inactive pits Zpppp = Z,p ( tion 7-5) ... 

Immittance — In alternating current (AC) measurements, the term immittance denotes the electric -> impedance and/or the electric admittance of any network of passive and active elements such as the resistors, capacitors, inductors, constant phase elements, transistors, etc. In electrochemical impedance spectroscopy, which utilizes equivalent electrical circuits to simulate the frequency dependence of a given elec-trodic process or electrical double-layer charging, the immittance analysis is applied. 

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