The Effect of Fluoride Ions on Plasma Electrolytic Oxidation Coatings Formed on Magnesium Alloys

Plasma electrolytic oxidation (PEO) is an effective method for developing protective conversion coatings on magnesium and its alloys. The efficiency of this process is governed by several factors, including electrical parameters and electrolyte composition. Typically, PEO has been performed in alkaline silicate or phosphate solutions, often enriched with simple fluoride ions (F^-) to improve the mechanical and anticorrosive properties of the resulting layers. Recently, the positive impact of complex fluoride ions (such as ZrF₆^2-, TiF₆^2-, SiF₆^2-, AlF₆^3- and PF₆^-) on the properties of conversion coatings on magnesium substrates has been shown. This review analyzes how these complex precursors influence the phase composition, morphology, and corrosion resistance of coatings. Furthermore, these parameters are compared with those obtained by using electrolytes composed of a mixture of simple fluorides and additional constituents that provide the incorporation of the appropriate elements into the coating structure (phosphorus, silicon, zirconium, titanium, or aluminum). It was indicated that when using complex fluoride salts, an important aspect is the stability of the electrolyte, which depends on pH and the presence of stabilizing additives. Ensuring that the decomposition of the complex fluoride occurs during the PEO process increases the amount of fluorine incorporated into the layers formed, leading to the formation of a coating with enhanced properties.