Nyquist plots coating

Figure 7. Nyquist plots - chlorine containing vinyl acrylic coating alternatively immersed in 3 NaCl.

Figure 10. Nyquist plots for bitumen coated mild steel. Day 0 and Day 50.

Figure 12. Nyquist plots for polyurethane (1 coat) unpig ...

Fig. 11.5. Typical Nyquist plots of Zim vs. Zre for a freshly prepared coat at ( ) -200 mV, ( ) -100 mV, ( ) OmV. The frequencies of the highest and lowest points in the plots are marked, along with the dc potential at which these data were collected.

Fig. 11.6. Typical Nyquist plot for the cycled coat (as in Fig. 11.4(c)) at a dc potential of...

Basic Analysis The easiest way of using impedance data is to make visual examination of the shape of the spectrum and of its evolution with time. In a coated system of high protection, the Nyquist plot... 

In summary, the source voltage is a cosine function, v= Vcos cot). The current lags the voltage by an angle 0 between 0° and 90° represented by i = Icos o)t — 0. A plot of Z vs. Z" represents a Nyquist plot, which can be used to evaluate the corrosion properties of metals and coatings. 

Fig. 5.30 Nyquist plots obtained for various coatings in 0.5 M Na2SO4+0.5 M H3BO3 solution at pH 7.0 solution [Popov, Unpublished data].

Figure 5.35 shows the impedance response of a bare and a cobalt-coated metal-hydride electrode. Two semicircles are seen in the Nyquist plot initially for the bare alloy and Co-coated alloy. After 50 charge/discharge cycles of the electrode for the bare alloy, the two semicircles merge and a significant increase in the alloy resistance is observed. The increase in resistance for the Co-coated alloy is comparatively less even after 120 cycles [76]. AUoy oxidation is responsible for the increase of the parti cle-to-particle resistance observed in the bare alloy in Fig. 5.35. 

Fig. 5.35 Nyquist plot of the impedance response of bare and cobalt-coated metal hydride alloy at different charge-discharge cycles [76].

Very recently Chen and coworkers [56] demonstrated the use of ElS for label-free electrochemical detection of DNA sequences relevant to anthrax lethal factor on gallium nitride (GaN) nanowires. The GaN nanowires were grown on a silicon substrate coated with Au catalyst using Ga as the source material and NH3 as the reactant gas in a tubular furnace via air pressure chemical vapor deposition. ElS measurements of the "as grown" GaN nanowires, observed in the Nyquist plot in Fig. 14.12A, exhibited a semicircle and a straight vertical line, indicative of finite impedance at the GaN/electrol3Ae... 

The use of electrochemical impedance spectroscopy (EIS) for the assessment of coatings has been under development for many years now [83]. The first period application of the technique was restricted to the study of the corrosion occurring with poorly protective coatings and coating impedances were evaluated by shape of the Nyquist plots. However, relatively few reports have been published describing results obtained from protective organic coating systems as used commercially [84]. 

The EIS spectra for coated metal-electrolyte systems are characterised by two time constants, two semicircles in Nyquist plots, two negative slopes in Bode magnitude plots and two negative inflections in Bode phase plots [114, 115]. Figs. 1.6, 1.7, 1.8, 1.9 show coated metal solution interface (two time constant system) and C i is double-layer capacitance. 

The effect of scribed holidays on coated WS panels has also been studied by EIS. Bode and Nyquist plots are given in Figs. 3.23 and 3.24. Although the order of Rp value has decreased to some extent due to creation of these holidays, but is still sufficiently higher (27.23 x 10 ) to produce high corrosion resistant surface which is better than Rp (29.87 x 10" ) of ZP 4- MS (Table 3.8). Phase angle (-80°) of WS shows capacitive behaviour. CPE and CPE 4- D model fit were used for ZP 4- MS and ZP 4- WS, respectively. 

Figure 10.15 Morphology of freshly prepared copper coating (a) and of that exposed for 30 min in the solution containing 0.01 M Cu(ll), 0.04 M gluconate, 0.3 M KjSO at pH 5 (b). Respective Nyquist plots obtained at the open-circuit potentials are shown in the insets.

Figure 19-22. (a) Impedance evolution for aluminium coated with non-particulate polymer coating. Two time constants revealed the contribution of the substrate to the total impedance of the system. Thus, points out to the presence of defects ofpores, (b) Impedance corresponding to a sample coated with a particulate coating. Only one time constant is depicted either in Bode and Nyquist plot, revealing that the contribution of the substrate is negligible. 

Figure 17 Two Nyquist plots of measurement on coating A (a) after 1 h and (b) after...

Nyquist plot of coating B after I hour immersion... 

Since for a purely resistor the imaginary components of the resistance is zero, the Nyquist plot for a resistor is a single point on the real axis with a value R. A purely capacitive coating can be represented by a series of a capacitor and a resistor. The Nyquist plot for a series RC circuit is a vertical line where the intercept of the line with the real axis gives an estimate of the resistance value. The imaginary component of the impedance (contributed by the capacitor) dominates the response of the circuit. One limitation of this Nyquist plot is that the value of the capacitance cannot be determined from the plot. It can be determined by a curve fit or from an examination of the data points. Also, the plot does not indicate which frequency was used to take each data point. 

Figure 12.34 Nyquist plot for a metal with porous insulating coating...

The Electrochemical impedance spectroscopy (EIS) results for the Mg alloy without and with surface Al coated from the 53 m/o and the 60 m/o ionic liquid, respectively, are depicted in Fig. 14.10. For bare Mg alloy, the polarization resistance was about 470 Qcm. A substantial increase in the polarization resistance, as evidenced by an enlarged diameter of the semicircle of the Nyquist plot, can be obtained for Mg alloy if it is electroplated with Al. For those with surface Al electrodeposited at -0.2 V from the 53 m/o and the 60 m/o ionic liquid, the polarization resistance in 3.5 wt% NaCl solution are 3000 and 5200 Qcm, respectively. The results were consistent with those revealed in the polarization curves demonstrated in Fig. 14.8. The improved polarization resistance of AZ91D Mg alloy with Al coating from ionic liquid is clearly demonstrated. However, the passivity or the polarization resistance of the Al-coated Mg alloy depends on the deposition conditions. The Al film formed in more acidic AICI3-EMIC and at a lower deposition rate renders a better passivation behavior. 

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