“…Plasma electrolytic oxidation (PEO) processes are considered to be among the most environmental-friendly surface treatments of aluminum, magnesium, and titanium alloys, providing good wear and thermal and corrosion resistance. − The coatings have potential for applications in a wide range of industry sectors, including aerospace, automotive, textile processing, electronic components, energy, oil and gas, and leisure and sports products . The coatings are further being developed for catalysis, − biocompatibility, , drug release systems, sensors, decorative finishing, and as nanostructured and nanocomposite coating materials. , Furthermore, PEO of zirconium and zirconium alloys is of interest for thermal barriers, for improved wear and corrosion resistance in nuclear applications, and for implants. − Porous ceramic coatings are formed during PEO with the assistance of dielectric breakdown at the sites of short-lived microdischarges. , The conditions employed for carrying out PEO, such as surface pretreatment, electrolyte composition, current density, waveform, and time of the treatment, have significant effects on the discharge characteristics and, hence, affect the morphology, density, and composition of the coating. ,− Typically, the PEO coatings consist of an interfacial barrier-like film, up to 1000 nm thick, an inner denser layer, and an outer porous layer containing large cavities . PEO processes can form coatings across a wide range of thicknesses up to several hundreds of microns, − commonly with a linear dependence on treatment time …”