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

Abstract

Plasma electrolytic oxidation (PEO) of magnesium alloy AZ31 in water solution containing 4g/L Na<inf>2</inf>SiO<inf>3</inf> · 5H<inf>2</inf>O+4g/L KOH+0.5g/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 A²Π - 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 12000K. 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.