**4. Conclusions**

In the current work, ceramic oxide coating on the Ti-6Al-4V alloy using MAO processing has been produced. The effect of different current pulse frequency on the coating morphology, phase composition and corrosion resistance has been studied. It was found that the application of different current frequency affected the porosity and compaction of the formed oxide coating.

Surface morphology and cross-sectional investigation by electron microscopy showed that the higher the applied current pulse frequency, the more compact and less porous the obtained coating is. After processing with a pulse current of 1000 Hz, the coating with a thickness of 1 μm and with a low porosity has been obtained. PIXE analysis revealed the appearance of titanium oxide coating on treated alloys. Two phases of titanium oxide, anatase and rutile were identified by XRD. Raman spectroscopy investigation detected the appearance of anatase phase on the surface of the examined samples. Rutile appeared in inner layer of the coating as the result of the partial transformation of the anatase phase.

Potentiodynamic polarization curves have evaluated that the corrosion resistance of the produced coating 40 times higher compared to the untreated alloy. Moreover, it was found that the higher the current pulse frequency, the higher the corrosion resistance of the obtained coating resulting in the formation of compacted and low porous oxide coating.

**Author Contributions:** Conceptualization, A.S., A.K. and K.B.; formal analysis A.S., A.K. and K.B.; investigation, A.S.; writing—original draft preparation A.S. and K.B.; writing—review and editing, A.S. and K.B.; supervision K.B.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
