# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 3 Results
### Corrosion Resistance of Black Coatings  
The corrosion resistance of PEO coatings, formed on two distinct titanium alloys, G2 and G5, through two different treatment durations (30 and 60 min), was evaluated using both PDP and EIS tests. It is important to note that PEO coatings are robust ceramic insulating coatings, which limited the utility of PDP primarily to qualitative assessments among the coatings and the original, untreated titanium alloys. Due to the inapplicability of the Tafel law [52], quantitative analysis was not possible. Figure 7 indicates the polarization \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **O\%** & **P\%** & **Ti\%** & **Fe\%** & **Mo\%** \\ \hline G2-30 & 76.3 & 17.0 & 5.5 & 0.6 & 0.6 \\ G2-60 & 78.2 & 16.6 & 4.0 & 0.9 & 0.2 \\ G5-30 & 79.0 & 17.9 & 2.0 & 0.9 & 0.2 \\ G5-60 & 80.1 & 16.3 & 2.2 & 1.3 & 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: XPS atomic percentage recorded on the different samples. Figure 6: XPS peaks of PEO coatings formed at 30 and 60 min: XPS core levels of Ti 2p, Mo 3d, Fe 2p, P 2p, and O 1s are shown. (Black is used for the sample G2-30, gray for the sample G2-60, dark blue for the sample G5-30 and light blue for the sample G5-60). curves, showing improved corrosion resistance for samples subjected to 60 min of PEO treatment on both titanium alloys, G2 and G5. The noticeable shift towards lower current densities, spanning up to one order of magnitude, demonstrates the improved corrosion resistance of the PEO-treated samples compared to the untreated titanium alloys. This transition occurs in the following order: substrate coated for 60 min > substrate coated for 30 min > uncoated substrate. Furthermore, shifting the corrosion potential in the PEO-treated samples toward the anodic position indicates improved corrosion resistance. Such a shift towards a more positive corrosion potential typically implies greater thermodynamic stability for the tested specimens [14]. EIS measurements were carried out to better understand the corrosion resistance of the PEO coatings. EIS measurements were performed under the same conditions as PDP at room temperature in 3.5% NaCl. Results from the EIS test are presented in the form of Bode plots, Figure 8, and Nyquist plots, Figure 9. As seen from both plots for G2 titanium alloy (indicated with the letter a), sample G2-60 has the highest impedance value through all applied frequencies. A higher value of the impedance indicates a better corrosion resistance of the system. From the Bode Modulus plot and the Nyquist plot for G5 titanium alloy (indicated with the letter b), sample G5-30 shows a higher impedance value at higher frequencies, while sample G5-60 shows a higher impedance value at lower frequencies. Generally speaking, the corrosion resistance of titanium alloys is improved by PEO coatings. Bode plot representation of phase angle (Zphz) as a function of frequency can be used to visualize the relative contributions of capacitive and resistive elements, with ideal capacitance at Zphz = \(-90^{\circ}\) and resistive processes indicated by Zphz = 0 [53]. Capacitive behavior is observed mainly at medium and low frequencies, while resistive behavior is evident at high frequencies. All four samples coated with PEO have a two-time constant compared to a one-time constant in pure titanium alloys. The first-time constant appears in the high-frequency region with a minimum of about \(10^{4}\) Hz, while the second appears in the low-frequency region with a minimum of about \(10^{0}\) Hz. It proves the presence of two layers in PEO coatings--an external porous layer and an internal barrier layer--in line with our previous research on PEO coatings [54; 55] and the work of [PERSON] et al. [56]. A one-time constant in the as-received samples indicates the presence of only an oxide layer. Figure 7: Potentiodynamic polarization (PDP) curves of the PEO-coated samples and as-received titanium alloys: (**a**) G2 and (**b**) G5 at the 3.5% NaCl.