Role of CO₂ in the initial stage of atmospheric corrosion of AZ91 magnesium alloy in the presence of NaCl

The effect of CO₂ and NaCl on the initial stage of atmospheric corrosion of AZ91 magnesium alloy was studied. The observation of surface morphology by optical microscopy (OM), scanning electron microscopy (SEM) and the analysis of corrosion products by X-ray diffraction (XRD) were integrated to investigate corrosion evolution. The results showed that NaCl stimulated the corrosion by promoting the formation of thin electrolyte film, increasing the conductivity and breaking the protective film in the absence of CO₂. The morphology of the corroded samples with deposited NaCl was more homogenous in the presence of CO₂. It was suggested that NaCl-induced corrosion was inhibited in the presence of CO₂ by the formation of slightly soluble corrosion products containing hydroxy carbonates and hydroxy chlorides that provided a partly protective layer on the surface of the magnesium alloy.