Novel plasma electrolytic oxidation (PEO) coatings on mg alloys for corrosion protection

The aim of this work was to develop novel flash-PEO coatings for AZ31B magnesium alloy with similar or better performance than Cr(VI) conversion coatings (CCC) that could become an alternative corrosion protection system choice for industrial applications. DC flash-PEO coatings were developed in different electrolytes (aluminate-, phosphate-, and silicate-based electrolytes, or their mixtures without/with fluoride) using very short treatments (≤90 s) in order to limit the energy consumption as much as possible. The multidimensional screening, including electrochemical impedance spectroscopy (EIS), specific energy consumption and industrial-type neutral salt spray (NSST), and paint adhesion tests were implemented in order to verify the suitability of PEO coatings’ performance. The energy consumption of the PEO processes remained relatively low for some of the coatings, ~1 kW h m⁻² µm⁻¹, which makes only 5 kW h m⁻² of energy required to produce the overall 5 µm-thick coatings. A correlation was found between the energy consumption of the PEO process with fluorides and the corrosion resistance, demonstrating that the lower the energy consumption of the respective PEO process, the better corrosion resistance of the resulted coating. The overall evaluation of the coatings’ corrosion protection (EIS, NSST, paintability) confirmed that two of the developed PEO coatings (aluminate-phosphate-fluoride and silicate-phosphate-fluoride based ones) could be an adequate substitute for CCC.