# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 2 Materials and Methods  
In the present research, Grade 2 (G2) and Grade 5 (G5) titanium alloys were employed as substrates for the Plasma Electrolytic Oxidation (PEO) process. G2 alloy is known as commercially \"pure\" titanium [43] (nominal composition: Ti \(\geq\) 90%, Fe \(\leq\) 0.30%, O \(\leq\) 0.25%, C \(\leq\) 0.08%, N \(\leq\) 0.03%, H \(\leq\) 0.015%, purchased from Acciaierie Valbruna, Vicenza, Italy). G5 possesses a nominal composition of: Ti = 87.6-91%, Al = 5.5-6.75%, V = 3.5-4.5%, Fe \(\leq\) 0.40%, O \(\leq\) 0.2%, C \(\leq\) 0.08%, N \(\leq\) 0.05%, H \(\leq\) 0.015%, and has been purchased from Acciaierie Valbruna, Vicenza Italy. Before treatment, the samples went through the standard metallography preparation. This included polishing with abrasive papers of varying grits (320, 500, 800, 1200, and 4000) and polishing with colloidal silica cloths (from Cloeren Technologies, Padova, Italy). Subsequently, they were cleaned with distilled water and degreased using C\({}_{2}\)H\({}_{6}\)O (Ethanol; Carlo Erba Reagents, Milan, Italy) via ultrasonic treatment for 5 min. The electrolyte used in the process consisted of a phosphate-based solution containing Na\({}_{5}\)P\({}_{3}\)O\({}_{10}\) (Sodium tripolphosphate; Thermo Scientific, Milan, Italy), and two additives, (NH\({}_{4}\))\({}_{6}\)Mo\({}_{7}\)O\({}_{24}\) (Amonium heptamolybdate tetrahydrate; VWR Chemicals, Milan, Italy) and FeSO\({}_{4}\) (Iron (II) sulfate heptabrudrate; Honeywell, Milan, Italy), to achieve a black color. A thermostatic bath was used to keep the solution at room temperature. A TDK-Lambda DC power supply with a capacity of 400 V/8 A, a carbon steel mesh, and a cooling system were used in the PEO process. The titanium substrate served as the anode during the process, while the carbon steel mesh was the cathode. Treatments were performed in galvanostatic mode, with low current density and long treatment durations. Table 1 contains detailed parameter information. Following the PEO treatment, the samples were rinsed with deionized water and dried with compressed air. Optical reflectance measurements were performed with a Jasco V770 UV-VIS-NIR spectrophotometer equipped with an integrating sphere in the 300-2000 nm range. Solar absorptance (\(\alpha\)) was calculated from reflectance spectra in the wavelength range 300-2000 nm according to the following equation:  
\[\alpha=\frac{\int_{300\;\mathrm{nm}}^{2000\;\mathrm{nm}}I_{sol}(\lambda)(1-R( \lambda)d\lambda}{\int_{300\;\mathrm{nm}}^{2000\;\mathrm{nm}}I_{sol}(\lambda)d\lambda} \tag{1}\]  
Here, \(R(\lambda)\) represents the spectral reflectance of the sample at a given wavelength \(\lambda\), while \(I_{sol}(\lambda)\) refers to the incident spectrum [34; 44]. A Cambridge Stereoscan 440 Scanning Electron Microscope (SEM) (Leica Microsystem S.r.l., Milan, Italy) equipped with a Philips PV9800 Energy Dispersive Spectroscopy (EDS) system (Leica Microsystem S.r.l., Milan, Italy) was used to examine the morphological \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Current Density** & **PEO Treatment Time** & **Na\({}_{5}\)P\({}_{3}\)O\({}_{10}\)** & **(NH\({}_{4}\))\({}_{6}\)Mo\({}_{7}\)O\({}_{24}\)** & **FeSO\({}_{4}\)** \\  & **A/cm\({}^{2}\)** & **min** & **g/L** & **g/L** & **g/L** \\ \hline G2-30 & 0.05 & 30 & 108 & 10 & 4 \\ G2-60 & 0.05 & 60 & 108 & 10 & 4 \\ G5-30 & 0.05 & 30 & 108 & 10 & 4 \\ G5-60 & 0.05 & 60 & 108 & 10 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: PEO process parameters (Current Density, PEO Treatment Time, Additives) for preparing the black coatings on titanium alloys, G2 and G5. characteristics of the PEO black coatings as well as the elemental composition of both the surface and cross-section of the samples. Samples were carefully cut, embedded in epoxy resin, and polished using traditional metallographic techniques before performing a cross-sectional analysis.