# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 5 Conclusions  
Effective black ceramic coatings were successfully obtained on G2 and G5 titanium alloy with the PEO process. The coatings were prepared to work in direct-current mode at a constantly low current density of 0.05 A/cm\({}^{2}\). The difference between coatings generated at 30 min and 60 min was investigated. A novel combination of additives (FeSO\({}_{4}\) and \(\left(\mathrm{NH_{4}}\right)_{6}\mathrm{Mo_{7}O_{24}}\)), not previously documented in the literature, was employed as coloring agents. The following conclusions can be drawn after examining the optical, morphological, structural, chemical, and electrochemical characterization.  
1. All the coatings generated possess a near 10% reflectance in the visible spectrum and a peak reflectance of around 35% in the NIR range. Under all tested conditions, the calculated absorptance of the coatings exceeds 0.8, reaching a maximum peak of 0.86 in the instance of PEO coatings prepared following a 60-minute duration.
2. The surface SEM images clearly demonstrate that the nano-pores are stacked and connected after 60 min of the PEO process, forming micro-pores and a particular channel structure. Since this morphology is suitable for trapping light waves, the coatings provide a deeper black color. Cross-sectional SEM micrographs show no difference in coating size after the PEO process is extended, but there is a difference in homogeneity. After 60 min, more uniform coatings were obtained with a thicker barrier layer.
3. According to EDS analysis of the surface and cross-section, coatings are rich in oxygen and phosphorus, with modest amounts of iron, molybdenum, and sodium. The presence of TiO\({}_{2}\) in two different mineral forms, anatase, and rutile, Na\({}_{3}\)PO\({}_{4}\), \(\gamma\)Fe\({}_{2}\)O\({}_{3}\) and Na\({}_{2}\)MoO\({}_{4}\), is confirmed by XRD and XPS.
4. From the PDP, the noticeable shift of the current density towards lower current densities demonstrates the improved corrosion resistance of the PEO-treated samples compared to the untreated titanium alloys. The improvement is noticeable in G2 and G5 titanium alloy and at both PEO process times.
5. The EIS confirms that the coatings on G2 titanium are more corrosion-resistant than G5. The internal barrier layer of G2 titanium, in particular, exhibits significantly higher resistance, and G2-60 stands out as the best coating in terms of corrosion resistance, with an internal barrier resistance of 17.12 \(\times\) 10\({}^{7}\)\(\Omega\).  
Conceptualization, L.P. and M.D.; methodology, L.P. and K.B.; software, L.K. and M.M.N.; validation, L.K.; formal analysis, L.K., M.M.N. and E.C.; investigation, L.K., M.M.N., E.C. and L.P.; resources, K.B., M.D. and M.M.N.; data curation, L.K., M.M.N., E.C. and L.P.; writing--original draft preparation, L.K.; writing--review and editing, L.P., M.M.N. and E.C.; visualization, L.K.; supervision, K.B., M.D. and L.P.; project administration, K.B. and M.D.; funding acquisition, K.B. and M.D. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the fact that are part of an ongoing study.  
XPS measurements were performed with an ESCALAB(tm) QXi Spectrometer funded by \"Sviluppo delle infrastrutture e programma biennale degli interventi del Consiglio Nazionale delle Ricerche (2019)\".  
The authors declare no conflict of interest.