Characterization of surface products on AZ31 magnesium alloy in dilute NaCl solution

In earlier work, we have drawn a corrosion map of AZ31 Mg alloy in dilute NaCl solutions and determined two zones-corrosion and passivation zones. In this paper, the surface products formed on AZ31 Mg alloy polarized in both zones were characterized in detail by X-ray photoelectron spectroscopy (XPS). MgO, Mg(OH)2, and MgCO3 were found in the surface products. Based on these results and previous ones by XRD, Mg(OH)2, Mg5(CO3)4(OH)2.8H2O, and MgO phases were presented in the corrosion zone and the latter two ones were also found in the passivation zone. In the corrosion zone, the higher hydration of the surface components gave a continuous corrosion attack of the metallic surface and degraded the passivation films. In the passivation zone, chloride-induced corrosion was retarded by the formation of the magnesium hydroxyl carbonate product, which provided a protective layer on the material. The adsorption of carbonate played a dual role in the solution. One is that the formation of the soluble salt with co-adsorption of CO32- and Cl- ions accelerated the dissolution of the passivation film. The other is that the formation of the carbonate-containing product served as a protective layer on the surface.