Investigation of Chemical Species in the Corrosion Barrier Layer on Thermal CO₂ Treated AZ91D Magnesium Alloy with Lithium Nitrate

A unique corrosion barrier layer, formed by thermal CO₂ treatment with LiNO₃ on AZ91D, was characterized to reveal the chemical species and their spatial distribution using the combination of scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS), and electron energy loss spectroscopy (EELS) techniques. Two distinct and spatially segregated phases were identified as Li compound(s) with MgCO₃ and MgO in the surface barrier layer. Based on the characterization results, it is presumed that the reaction of Mg oxide/hydroxide with LiNO₃ and CO₂ occurred preferentially in some local areas where Li compound(s) with MgCO₃ were present, but the unreacted areas contained MgO in the surface barrier layer.