Role of particle type and concentration on characteristics of PEO coatings on AM50 magnesium alloy

Composite ceramic coatings were formed on a magnesium alloy by AC plasma electrolytic oxidation using a phosphate-based electrolyte with α-Al2O3, monoclinic ZrO2 or CeO2 particles in suspension. Effects of particles concentration (2, 5 and 10 g/L) on the electrical response, composition/morphology and corrosion behaviour of the coatings were investigated. Findings revealed successful incorporation of particles, which were preferentially located in the outer coating layer. Due to high temperatures at the locations of microdischarges, zirconia particles underwent transformation from monoclinic to tetragonal structure. The majority of alumina and ceria particles remained unaffected, although some of the alumina particles possibly formed MgAl2O4 by reaction with the substrate. Porosity and thickness of the coatings tended to decrease with increasing particles concentration in the electrolyte. Coatings formed in the electrolytes containing CeO2 particles revealed the best long-term corrosion performance in 0.5 wt% NaCl solution.