# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 1 Introduction
The critical elements of electrolytes used for the development of PEO black optical coatings fall into two broad groups: anions like VO\({}^{3-}\), WO\({}^{2-}_{4}\), MoO\({}^{2-}_{4}\), and transition metal cations, including Fe\({}^{2+}\), Cu\({}^{2+}\), Co\({}^{2+}\), Ni\({}^{2+}\) and Zr\({}^{4+}\)[19; 26; 34]. To the best of our knowledge, only two papers have discussed the preparation of black PEO coatings on Grade 2 (G2) titanium. These articles, authored by [PERSON] et al. [5] and [PERSON] et al. [14], both employed a silica-based electrolyte. The former utilizes \(\left[\mathrm{Cu}(\mathrm{NH}_{3})_{4}\right]^{2+}\) as a coloring additive, while the latter, in contrast, did not employ any additives and concentrated on achieving the black color in the coating, which naturally arises from black titanium oxide. Few research groups have contributed to the study of black coatings on Grade 5 (G5) titanium alloy. [PERSON] et al. investigated the effects of various additives in phosphate-based electrolytes for the development of black coatings. Their study consists of several works that provide a comparative analysis of additives (FeSO\({}_{4}\), Co(CH\({}_{3}\)COO)\({}_{2}\), Ni(CH\({}_{3}\)COO)\({}_{2}\) and K\({}_{2}\)ZrF\({}_{6}\)) [33], the effect of different concentrations of FeSO\({}_{4}\)[35] and Co(CH\({}_{3}\)COO)\({}_{2}\)[36] on coating formation. Another research group, led by [PERSON] et al. [37; 38], provided findings on the generation of gray-black coatings using only silicate and phosphate-based electrolytes and no additives. A third group of researchers has also published several papers on black coatings on the same alloy. [PERSON] et al. investigated the effects of various additives (FeSO\({}_{4}\), Co(CH\({}_{3}\)COO)\({}_{2}\), Ni(CH\({}_{3}\)COO)\({}_{2}\) and NaWO\({}_{4}\)) in phosphate-based electrolytes [39], the results of coating preparation without additives using a combination of silicate and phosphate-based electrolytes [40], and the influence of zirconium in two of their papers [41; 42]. A more recent study of G5 titanium alloy can be found in the literature, authored by [PERSON] et al. [9]. This research focuses on the development of black coatings using a complex base electrolyte together with various additives (FeSO\({}_{4}\), Co(CH\({}_{3}\)COO)\({}_{2}\), NH\({}_{4}\)VO\({}_{3}\)). It is important to note that all the aforementioned studies employ pulse mode PEO treatment, frequently with high current density or voltage. The absence of studies involving the direct-current mode of PEO treatment motivated our research. Our second goal was to prepare coatings at a deficient current regime and to try the application of a novel combination of additives, FeSO\({}_{4}\) and \(\left(\mathrm{NH}_{4}\right)_{6}\)Mo\({}_{7}\)O\({}_{24}\), for making black coatings on titanium and its alloy. Additionally, we chose G5 titanium alloy as a substrate, which has a substantial number of reported works on the successful preparation of black PEO coatings on its surface. On the other hand, the lack of research on G2 titanium (commercially \"pure\" titanium) prompted us to use this alloy and compare it to the previously researched G5 alloy. Furthermore, this work investigated the differences between two PEO process times, 30 min and 60 min. The prepared coatings' morphology, chemical and phase composition, black intensity with UV-VIS spectroscopy, and electrochemical corrosion properties were also characterized.