Abstract
Plasma electrolytic oxidation (PEO) coatings were grown on AZ31 Mg alloy in a silicate-based electrolyte containing KF using unipolar and bipolar (usual and soft-sparking) waveforms. The coatings were dual-layered consisting of MgO, MgF2 and Mg2SiO4 phases. Surface morphology of the coatings was a net-like (scaffold) containing a micro-pores network, micro-cracks and granules of oxide compounds. Deep pores were observed in the coating produced by unipolar and usual bipolar waveforms. The soft-sparking eliminated the deep pores and produced the lowest porosity in the coatings. It was found that the corrosion performance of the coatings evaluated using EIS in 3.5 wt. % NaCl solution is mostly determined by the inner layer resistance, because of its higher compactness. After 4 days of immersion, the inner layer resistances were almost the same for all coatings. However, the coatings produced by unipolar and usual bipolar waveforms showed sharp decays in inner layer resistances after 1 week and even the barrier effect of outer layer was lost for the unipolar-produced coating after 3 weeks. The low-frequency inductive loops appeared after a 3-week immersion for all coatings indicated that the substrate was under local corrosion attack. However, both coatings produced by soft-sparking waveforms provided the highest corrosion performance.
Highlights
Plasma electrolytic oxidation (PEO) is an electrochemical surface treatment based on conventional anodizing performed at very high anodic potentials up to the passive oxide breakdown, resulting micro-discharges which develop a two-layered morphology consisting of an outer porous layer and an inner compact one [1]
Due to the higher cathodic current density consumed by more cathodic duty cycle, the overall current density increases with increasing the cathodic duty cycle
PEO coatings were grown on AZ31 Mg alloy in silicate-based electrolyte containing potassium fluoride using unipolar, usual bipolar (10% cathodic duty cycle) and soft-sparking
Summary
Plasma electrolytic oxidation (PEO) is an electrochemical surface treatment based on conventional anodizing performed at very high anodic potentials up to the passive oxide breakdown, resulting micro-discharges which develop a two-layered morphology consisting of an outer porous layer and an inner compact one [1]. This process is basically an oxidation treatment to protect valve metals and their alloys, such as magnesium, aluminum and titanium from wear and corrosion attack [2].
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