Fluoride effect on plasma electrolytic oxidation coating formed on Mg-Al alloy in alkaline electrolytes

Plasma electrolytic oxidation (PEO) is an effective protection technique developed for light alloys, such as aluminum, magnesium and titanium alloys. PEO electrolytes often contain fluorides, yet their functionality is not well understood during the oxide layer formation and growth. In the present work, we tracked the fluoride evolvement contained in PEO ceramic protective layer formed on AZ91 magnesium alloys. It is found that a fluoride rich layer with 1–2 μm thickness is often formed at the coating/substrate interface and nanocrystalline MgO islands with size of 20–60 nm are found to be embedded in amorphous coating substrate.