# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 3 Results
### Corrosion Resistance of Black Coatings
To compare samples with only PEO coatings, the results, including PEO coatings on both titanium alloys, G2 and G5, are shown in the form of a Bode Modulus plot and Nyquist plot in Figure 10. In general, the corrosion resistance of PEO-coated samples on G2 titanium alloy is better than that of samples on G5 alloy, while the best corrosion resistance is found in sample G2-60. The larger arcs on the Nyquist graph and higher impedance values support this conclusion. Figure 8: Results of the EIS tests in the form of Bode Modulus plots (impedance vs. frequency (Zmod vs. Freq)) and Bode Phase plots (phase vs. frequency (Zphz vs. Freq)) for the (**a**) PEO-coated samples on the G2 titanium alloy and the G2 titanium alloy without the coating; (**b**) PEO-coated samples on the G5 titanium alloy and the G5 titanium alloy without the coating. Figure 9: Results of the EIS tests in the form of Nyquist plots (impedance vs. frequency (\(-\)Zimag vs. Zreal)) for the: (**a**) PEO-coated samples on the G2 titanium alloy and the G2 titanium alloy without the coating; (**b**) PEO-coated samples on the G5 titanium alloy and the G5 titanium alloy without the coating. EIS data were fitted, applying two distinct equivalent electrical circuits for quantitative assessments of the PEO-coated samples. One circuit was designed for untreated samples (Figure 11a), while the other was developed for PEO-treated samples (Figure 11b). Since the untreated sample contained just a natural oxide layer, it was examined using a simple Randles \(R_{\mathrm{s}}(R_{\mathrm{o}}CPE_{\mathrm{o}})\) circuit configuration. On the other hand, the analysis of PEO-treated samples required a dual-circuit \(R_{\mathrm{s}}(R_{\mathrm{p}}CPE_{\mathrm{p}}(R_{\mathrm{b}}CPE_{\mathrm{b}}))\) configuration due to the presence of two distinct layers: an external porous layer and an internal barrier layer. A Constant Phase Element (_CPE_) was used in both electrical circuits, according to the typically less-than-ideal nature of capacitance measurements [57]. _CPE_ can be represented as follows:  
\[Z=(1/Y_{0})/(j\omega)^{\alpha} \tag{2}\]  
In the given expression, \(Y_{0}\) represents a constant phase element (_CPE_), \(\omega\) stands for angular frequency, and \(\alpha\) is an empirical exponent. \(\alpha\) can assume a value of 1, indicating a perfect capacitor, and 0 perfect resistor. Within the equivalent circuit, various components are delineated: \(R_{\mathrm{s}}\), which signifies the resistance of the solution; \(R_{\mathrm{o}}\), serving as a measure of the corrosion resistance of the natural oxide layer; \(R_{\mathrm{p}}\), indicative of the corrosion resistance of the porous layer; and \(R_{\mathrm{b}}\), representing the corrosion resistance of the barrier layer. A _CPE_ includes (\(CPE_{\mathrm{o}}\)) connected to the oxide layer, (\(CPE_{\mathrm{p}}\)) connected to the porous layer, and (\(CPE_{\mathrm{b}}\)) connected to the barrier layer. The fitting results of the experimental EIS data are reported in Table 5 in the case of the PEO-coated samples and Table 6 for the as-received samples. Good fitting results were obtained, as confirmed by chi-squared values varying around 0.01 and 0.001. Figure 11: Equivalent electrical circuits employed to fit the EIS data, (**a**) simple Randles circuit \(R_{\mathrm{s}}(R_{\mathrm{o}}CPE_{\mathrm{o}})\); (**b**) dual-circuit \(R_{\mathrm{s}}(R_{\mathrm{p}}CPE_{\mathrm{p}}(R_{\mathrm{b}}CPE_{\mathrm{b}}))\). Figure 10: Results of the EIS tests in the form of (**a**) Bode Modulus plot (impedance vs. frequency (Zmod vs. Freq)) and (**b**) Nyquist plot (impedance vs. frequency (\(-\)Zimag vs. Zreal)) for the PEO-coated samples on the G2 titanium alloy and the G5 titanium alloy. A two order of magnitude increase in the corrosion resistance (considering both Rp and Rb) was observed for the 60-minute treated samples on the G2 titanium alloy (G2-60) compared to the 30-minute treated sample (G2-30).