Transport of species during plasma electrolytic oxidation of WE43-T6 magnesium alloy

Coating growth on WE43-T6 magnesium alloy during direct current plasma electrolytic oxidation is investigated in alkaline phosphate and silicate electrolytes. The study employs sequential treatments in the two electrolytes, followed by analyses of the coatings and the electrolytes. The coatings comprise mainly an outer, relatively compact layer and an inner, more porous layer. A thin barrier layer is located next to the alloy. The coatings contain magnesium, oxygen, phosphorus, silicon, and alloying element species and include crystalline phases of MgO, Mg-3(PO4)(2), MgSiO3, and Mg2SiO4. Local microstructures suggest melting and solidification in the inner layer. Sequential treatments disclose transport of phosphorus and silicon species, through the outer coating along short-circuit paths. Further, analyses of the electrolytes reveal the presence of coating species in relative amounts suggestive of preferential loss of inner coating material. The efficiency of coating formation is similar to 15 to 20%, with the main reduction due to evolution of oxygen. (C) 2007 The Electrochemical Society.