*2.2. Characterization Techniques*

The morphology examinations of the obtained coatings were done on the cross section of the treated specimen by TESCAN MAIA3 scanning electron microscopy (SEM) (Brno, Czech Republic) equipped with an energy dispersive X-ray spectroscopy (EDS) system by Oxford Instruments (Abingdon, UK) with an X-MaxN detector. The phase analysis of the coating was determined by the X'Pert Pro diffractometer (PANalytical B.V., Almelo, The Netherlands) with Cu<sup>α</sup> radiation (λ = 1.542 Å) at the grazing incidence mode (angle of 3◦) with a 2θ range from 30◦ to 80◦ (step size of 0.03◦) at 40 kV and 40 mA.

The corrosion behavior of the treated and untreated specimens was examined by a potentiodynamic polarization test in a 3.5 wt % NaCl (Sigma-Aldrich Co.) solution by PARSTAT 4000A potentiostat/ galvanostat (Princeton Applied Research, Oak Ridge, TN, USA). A three-electrode cell configuration was used for the corrosion test, wherein a Pt sheet acted as a counter-electrode and saturated Ag/AgCl (Metrohm Autolab B.V., Utrecht, The Netherlands) acted as a reference electrode. The polarization resistance of a sample was detected at the range of ±250 mV with respect to the recorded corrosion potential at a scan rate of 0.1 mV/s. Prior to the potentiodynamic polarization test, the samples were kept in the 3.5 wt % NaCl solution for 60 min in order to reach the steady state of a working electrode.

### **3. Results and Discussion**

### *3.1. MAO Process Characterization*

Plots of voltage and current as a function of time during the MAO process are presented in Figure 2a,b, respectively.

**Figure 2.** Plot of electric parameters of the MAO process applied on alloy Ti-6Al-4V: (**a**) voltage as a function of treatment time; (**b**) current as a function of treatment time.

Here, we sought to establish optimal conditions for plasma-mediated oxidation of Ti alloy. Briefly, the sample was immersed in a molten salt electrolyte in a nickel crucible, and the voltage was applied so that the sample served as a positive pole and the crucible as a negative pole. During the first few seconds of the process, a double electric layer was formed, followed by the charging, accompanied by the adsorption of gas bubbles and the formation of an amorphous film on the specimen surface (Figure 2a area 0–1). With the increase in the treatment time, a thicker oxide layer was formed, followed by a dielectric breakdown (Figure 2a area 1–2), which was accompanied by the formation of sparks on the specimen surface. It can be noted that during the MAO process, the voltage turns to the stationary mode after 300 s, meaning that the sparking process moves into the so-called micro arc oxidation mode (Figure 2a, area 2–3).

The process applied in molten salt is conducted at significantly lower potentials, about 22 V compared to the potentials of 300–600 V in the process conducted in aqueous electrolyte [21]. The current values of both processes are in the same range [22]. Those parameters indicate that the MAO process in molten salt is a more energy-efficient process and therefore is more economically beneficial.
