Coating capacitance

Thus it appears that by incorporating parameters such as pore resistance and coating capacitance to the existing theoretical impedance model dealing with metal dissolution one would obtain valuable overall information (14,27). Complemented by results from regular immersion and salt spray tests it should be possible to find satisfactory solutions to corrosion problems of coated metals (9 ). 

Measurement leads used to connect to the electrochemical cell contribute measured capacitance. Lead capacitance between the potentiostat and the electrometer is normally accounted for in the potentiostat calibration, but stray capacitance associated with the leads and connections to the cell may not. Moreover, this capacitance may change with time the leads age, and the connections become oxidized. Uncompensated capacitance may also exist within the measurement circuitry of the potentiostat itself. These uncompensated capacitances impose a practical lower limit on the coating capacitance that can be measured. 

In these expressions, R°po = pt (ohm-cm2), and R°v (ohm-cm2) and Ch (pF/cm2) are characteristic (area-specific) values that do not change with time t is coating thickness, and p is coating resistivity, which must also be time-independent. Furthermore, the coating capacitance, Cc, is given by... 

PANI Dodecyl benzene sulphonic acid (DBSA) Steel Casting EIS in 3.5 NaCl PANI (DBSA) coating shows a coating resistance and a coating capacitance stable up to 77 days. [73]... 

It is often found that the double-layer capacitance or a coating capacitance does not behave like an ideal capacitor, experimentally manifested in the complex plane plot by a depressed semicircle whose center lies below the real axis. This behavior is usually attributed to some distribution (or dispersion) in some physical property of the system (e.g., the porous surface of the metal or the varying thickness or composition of a coating) and is modeled by the use of a constant phase element (CPE) [30]. 

To evaluate a coating, along with / , 7 p, Cdi and W (Warburg impedance) two additional circuit elements, namely coating capacitance (C ) and resistance of coating pores (Tfpo) come into account. The presences of Cai or can be idealised by a constant slope in versus frequencies plot and peaks in 9 versus frequencies plots. 

The coating time constants are smaller and manifest at higher frequency regions of the impedance spectra, whereas time constants corresponding to metal corrosion appear at lower frequency regions. An impedance plot obtained can be correlated with one or more equivalent pore resistance Rpo), coating capacitance (Cc) and polarisation resistance (Rp). 

FIG. 12—Electrical equivalent circuit to simulate a coated steel panel with a defect [146,147,149. Rg is the coating defect resistance, R is a charge transfer resistance (similar to R) at the metal Interface where water has penetrated. Is the coating capacitance. 

C, = the coating capacitance after some exposure time, and Co = the initial coating capacitance at time zero when the exposure begins. 

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