# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 4 Discussion
This particular microstructure might be one of the contributing factors to the increased absorptance in samples subjected to extended PEO treatment times. The nano-pores, on the other hand, are most likely too small to accommodate light waves. Additionally, [PERSON] et al. [14] reported that the black color comes from the titanium ions themselves incorporated into the titanium dioxide. They believe that Ti\({}^{3+}\) and Ti\({}^{2+}\) from the Ti-6 Al-4V substrate are responsible for the black color of TiO\({}_{2}\). Nevertheless, it is known that TiO\({}_{2}\) is mainly used as a white pigment [58; 59; 60]. All PEO-coated samples have better corrosion resistance than bare titanium alloys, G2 and G5, as confirmed by PDP measurements. Based on PDP analysis, the literature [14; 37; 38] also reported improved corrosion performance when using black PEO coatings on titanium alloys in 3.5 wt.% NaCl solution. Despite the PDP curves shifting towards lower current densities and more positive potentials, a higher anodic current is evident in the case of 60-minute samples for both titanium alloys, G2 and G5 (Figure 7). A similar occurrence was noted by [PERSON] et al. [14], who ascribed this phenomenon to the increased formation of rutile due to repeated PEO discharges in the coatings formed at longer treatment times, and [PERSON] et al. [37] noted that the presence of rutile contributes to better corrosion resistance, particularly when exposed to a NaCl solution [49]. Furthermore, upon examining the results of the EIS tests, the distinctions between the samples are apparent. Those produced after a 60-minute PEO process display improved corrosion resistance. This heightened resistance can be attributed to a thicker inner barrier layer and, in general, more compact coatings, as demonstrated by SEM cross-section observations. Compactness, uniformity, and interfacial stability are recognized as crucial factors that dictate the protective effectiveness of oxide coatings. The presence of porosity and inadequate interfacial adhesion between the coatings and the substrate can facilitate the infiltration of corrosive agents into the coating, leading to substrate damage and, consequently, rapid surface pitting [28]. Similar results were found in the literature, where it was reported that the better corrosion resistance for the thin coatings was attributed to the denser, more homogeneous inner (barrier) layer [14; 61]. In addition to SEM observation, the presence of two distinct layers in PEO coatings is also discernible from the appearance of two-time constants on the Bode Phase plots (Figure 8) of PEO-coated titanium alloys. The time constant observed at high frequencies corresponds to the porous (outer) layer, while the second time constant is associated with the inner (barrier) layer. In contrast, untreated samples exhibit only a single time constant. These findings align with previously published research on PEO coatings applied to titanium substrates [28; 62]. The enhancement of the barrier layer is particularly evident in the samples of the G2 titanium alloy. Based on the fitting values of the EIS measurements, sample G2-60 shows even two orders of magnitude higher corrosion resistance in the barrier layer compared to G2-30 (Table 5). Overall, G2 coatings exhibit superior corrosion resistance compared to G5 coatings, and the G2 coating prepared after the prolonged PEO process time of 60 min stands out as the best with the corrosion resistance of 17.12 \(\times\) 10\({}^{7}\)\(\Omega\) cm\({}^{2}\). This can be related to the improved corrosion resistance of the substrate (G2 titanium behaves better than G5 titanium), with an oxide layer resistance of 13.30 \(\times\) 10\({}^{5}\)\(\Omega\) cm\({}^{2}\) and 50.83 \(\times\) 10\({}^{4}\)\(\Omega\) cm\({}^{2}\), respectively. These values are consistent with the corrosion resistance exhibited by similar PEO coatings on titanium alloys. For instance, [PERSON] et al.