**1. Introduction**

Titanium alloys are widely applicable in marine, aerospace, chemical and biomedical industries due to their high specific strength, corrosion resistance and excellent biocompatibility [1–4]. However, their advanced properties are limited due to the lack of the protective surface properties. One of the most promising environmentally friendly processes of coating formation is micro arc oxidation (MAO).

MAO is an electrochemical processing of ceramic oxide coating formation on the metallic substrate by the appearance of dielectric breakdown and plasma discharges. The basic principle of the MAO technology is an application of a high voltage between the treated metal and the electrode. During the process, micro-arc discharge migration points appear on the surface lead to the oxide coating formation [5–8]. Usually, MAO treatment is suitable for the enhanced mechanical properties and corrosion resistance of the alloys' surface [9–11].

Ti-6Al-4V is one of the most commonly applicable titanium alloys because of its outstanding properties [12]. This Ti alloy contains both aluminum and vanadium; Al stabilizes α phase and has the HCP lattice, and V stabilizes β phase and has the BCC lattice. Consequently, this alloy exhibits advanced mechanical properties as α phase provides a high strength while β phase provides high ductility. Moreover, Ti-6Al-4V alloy is one of the main metallic biomaterials due to its high biocompatibility [13–15]. Additional treatment of this alloy by MAO process will lead to the production of an oxide coating with excellent corrosion resistance for applications in wide range of industries.

The majority of research works have been devoted to studying the electrolyte chemical composition effect on the structure and the properties of the created coatings [16–19]. However, the influence of the process electrical parameters affects also the formation of the coating, resulting in different surface properties [20,21].

Therefore, the goal of the present work is an investigation of the current pulse frequency effect of the MAO process on the coating formation on Ti-6Al-4V alloy. The obtained coating morphology, and the chemical and the phase compositions were also examined in details in the work. Furthermore, the influence of the current frequency on the corrosion resistance of the produced alloys' surface was also examined.
