# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 4 Discussion  
According to the optical analysis, all PEO-coated samples have an absorptance greater than 0.80, indicating a black appearance. Similar outcomes can be found in the existing literature [13], where black coatings are typically reported to have solar absorptance values ranging from 0.75 to 0.90. [PERSON] et al. [39] stated that the resulting coatings with an average solar absorbance value of 0.76 were deemed appropriate for application in optical components aboard spacecraft. Notably, the coatings have significant differences, especially when examining the same titanium alloy but subjected to a longer PEO process duration. The coating's absorptance increases as the process time is extended. Coatings produced during the 60-minute PEO process absorb up to 86% of radiation, and this difference is visible in stereo microscope images. A more expressive black color is confirmed compared to 30-minute PEO coatings. The blackness of the coatings is affected by anions such as VO\({}^{3-}\), WO\({}_{4}^{2-}\), MoO\({}_{4}^{2-}\) and transition metal cations such as Fe\({}^{2+}\), Fe\({}^{3+}\), Cu\({}^{2+}\), Co\({}^{2+}\), Ni\({}^{2+}\) and Zr\({}^{4+}\), according to the literature [19; 26; 34]. Regarding our present study, additives based on molybdenum and iron were used. XRD analysis showed that Fe\({}^{3+}\) and MoO\({}_{4}^{2-}\) were successfully incorporated \begin{table}
\begin{tabular}{c c c} \hline  & **G2** & **G5** \\ \hline Rs \(\left[\Omega\,\mathrm{cm}^{2}\right]\) & 3.61 & 4.89 \\ Ro \(\left[\Omega\,\mathrm{cm}^{2}\right]\) & 13.30 \(\times\) 10\({}^{5}\) & 50.83 \(\times\) 10\({}^{4}\) \\ Yo\({}_{\mathrm{o}}\)\(\left[\mathrm{S}\,\mathrm{s}^{4}\mathrm{cm}^{-2}\right]\) & 28.71 \(\times\) 10\({}^{-6}\) & 33.71 \(\times\) 10\({}^{-6}\) \\ \(\alpha_{\mathrm{o}}\) & 0.85 & 0.84 \\ \(\chi^{2}\) & 11.59 \(\times\) 10\({}^{-3}\) & 10.10 \(\times\) 10\({}^{-3}\) \\ \hline \end{tabular}
\end{table} 
Table 6: Results of the experimental EIS data fitting with the equivalent electrical circuit for the as-received samples. \begin{table}
\begin{tabular}{c c c c c} \hline  & **G2-30** & **G2-60** & **G5-30** & **G5-60** \\ \hline Rs \(\left[\Omega\,\mathrm{cm}^{2}\right]\) & 11.59 & 3.90 & 5.40 & 9.67 \\ Rp \(\left[\Omega\,\mathrm{cm}^{2}\right]\) & 36.07 \(\times\) 10\({}^{4}\) & 13.07 \(\times\) 10\({}^{2}\) & 28.82 \(\times\) 10\({}^{3}\) & 22.72 \(\times\) 10\({}^{5}\) \\ Rb \(\left[\Omega\,\mathrm{cm}^{2}\right]\) & 26.47 \(\times\) 10\({}^{5}\) & 17.12 \(\times\) 10\({}^{7}\) & 22.68 \(\times\) 10\({}^{4}\) & 51.19 \(\times\) 10\({}^{4}\) \\ Yo\({}_{\mathrm{p}}\)\(\left[\mathrm{S}\,\mathrm{s}^{4}\mathrm{cm}^{-2}\right]\) & 10.00 \(\times\) 10\({}^{-6}\) & 16.09 \(\times\) 10\({}^{-7}\) & 13.20 \(\times\) 10\({}^{-6}\) & 15.70 \(\times\) 10\({}^{-6}\) \\ \(\alpha_{\mathrm{p}}\) & 0.73 & 0.79 & 0.63 & 0.69 \\ Yo\({}_{\mathrm{o}}\)\(\left[\mathrm{S}\,\mathrm{s}^{4}\mathrm{cm}^{-2}\right]\) & 80.62 \(\times\) 10\({}^{-7}\) & 30.12 \(\times\) 10\({}^{-7}\) & 26.50 \(\times\) 10\({}^{-6}\) & 44.05 \(\times\) 10\({}^{-6}\) \\ \(\alpha_{\mathrm{b}}\) & 0.86 & 0.86 & 0.72 & 0.87 \\ \(\chi^{2}\) & 82.99 \(\times\) 10\({}^{-3}\) & 56.82 \(\times\) 10\({}^{-4}\) & 12.00 \(\times\) 10\({}^{-4}\) & 18.67 \(\times\) 10\({}^{-4}\) \\ \hline \end{tabular}
\end{table} 
Table 5: Results of the experimental EIS data fitting with the equivalent electrical circuit for the PEO-coated samples. into all prepared coatings. XPS and EDS analysis further confirmed the presence of these species. As well as cations and anions, the black color of the coatings is influenced by the microstructure. Irregular surfaces and micro-pores of the coatings constitute optical traps, increasing multiple reflections of incident light on the surface of the coatings and resulting in a decrease in the reflectivity [9; 41]. Micro-pores dimensions correspond to the infrared spectral range. The surface SEM examination reveals the formation of a channel-like structure, which arises from the interconnection of nano-pores after 30 min.