**1. Introduction**

Micro arc oxidation (MAO) is one of the most promising methods for the surface treatment of metals and alloys, and has recently received wide acceptance from various branches of industry. MAO is generally used to produce multipurpose wear-, corrosion-, and heat-resistant dielectric and decorative coatings on valve metals, such as Al, Mg, Ti, Ta, Nb, Zr, and Be [1–5].

Currently, titanium is an appealing metal due to its high specific strength [6], corrosion resistance [7], and excellent biocompatibility [8]. The use of MAO favors adapting surface composition, crystallographic structure, and morphology to achieve a large functionality that cannot be provided by the parent metal. The versatility of coatings obtained by MAO on Ti alloys, together with the simplicity and low cost of this treatment method, stimulated numerous attempts to coat titanium by the MAO [9–12] approach for various applications, including tribological, biomedical, dielectric, and photovoltaic coatings. Ti-6Al-4V is the most widely used Ti alloy as it contains stabilizer elements for both α and β phases for good creep and strength, respectively.

The mechanism of the MAO process is based on the anodizing electrochemical reaction, which occurs on a metallic surface and is accompanied by microarc discharge to form an oxide ceramic surface layer with a particular morphology and phase composition [13,14].

The formation of the coating results in numerous difficulties that affect different factors of the layer quality. Among those factors are the chemical composition, the concentration and temperature of the electrolyte, the duration of the treatment process, the chemical composition and structure of the substrate, and the electrical parameters of the MAO process [15–18]. The following factors affecting MAO in an aqueous electrolyte are considered as undesirable: the necessity of forced cooling of the treatment bath, an increased current density, the thermal dissociation of the electrolyte, the formation of compounds in the ceramic coating, and the low growth rate. Those issues can be solved by replacing an aqueous electrolyte with molten salts. The application of molten salt as an electrolyte in the MAO process has been reported by us in an earlier work on Al alloy surface treatment [19,20].

In the present work, the formation of a ceramic coating on Ti-6Al-4V alloy in a mixture of molten nitrate salts by MAO process was obtained. The chemical and phase composition of the obtained coating as well as its morphology and corrosion resistance are investigated and illustrated.
