Plasma Electrolytic Oxidation of AZ91 Mg Alloy in the Sodium Stannate Electrolyte

The effect of sodium stannate on Plasma electrolyte oxidation (PEO) process of AZ91 Mg alloy was systematically investigated. The oxide layer formed on an AZ91 Mg alloy created using an electrolyte containing sodium stannate formed a much thicker layer consisting of: MgO, SnO, Mg 2 SiO 4 , and MgF 2 . The result of EDS analysis shows a tin enriched zone at the oxide layer/substrate interface. Corrosion resistance of the sample coated from the electrolyte containing sodium stannate was determined to be superior to that of the sample coated from the electrolyte without sodium stannate. As a result of salt spray test for 120 hrs, severe filiform corrosion is observed on the thin oxide layer of AZ91 Mg alloy without sodium stannate. However, the sample of the electrolyte containing sodium stannate shows localized pitting corrosion. Tin ions in the electrolyte influence on the growth rate of the oxide layer, especially resulting in a formation of dense barrier layer. The co-existence of dense MgO and SnO in the oxide layer could improve the corrosion resistance of the AZ91 Mg alloy.