Characterization of plasma electrolytic oxidation of magnesium alloy AZ31 in alkaline solution containing fluoride

Plasma electrolytic oxidation (PEO) of magnesium alloy AZ31 in water solution containing 4 g/L Na2SiO3 ⋅ 5H2O + 4 g/L KOH + 0.5 g/L NaF was investigated. Scanning electron microscopy was used to characterize morphology of the formed surface coatings, while the crystallinity of the samples was analyzed by X-ray diffraction. Spectral luminescence measurements during PEO of magnesium were taken. The spectra consisted of atomic/ionic lines originating from the elements present both in the substrate and the electrolyte. Besides, the v′ = v″ sequence of the B1Σ+ - X1Σ+ emission transition of MgO, and two broad peaks assigned to the unresolved v′ = v″ and v′ = v″ + 1 sequences of the A2П - X2Σ+ band system of MgF were identified. The measured intensity distributions within these band sequences were utilized for determination of the plasma temperature, found to be (8500 ± 1000) K. The composition of plasmas containing Mg, O, H, and F was calculated in the temperature range up to 12000 K. The results of these calculations explain the appearance of observed spectral features and support the assumption of high-pressure conditions in the microplasmas built during the PEO process.