The influence of carbon dioxide on the atmospheric corrosion of some magnesium alloys in the presence of NaCl

The influence of ambient concentrations of carbon dioxide on the NaCl-induced atmospheric corrosion of high purity die-cast MgAl alloys (AM20 and AM60) and MgAlZn alloy (AZ91) is reported. Samples were exposed four weeks in a carefully controlled synthetic atmosphere. Relative humidity was 95%, the temperature was 22.0 deg C, and the concentration of CO sub 2 was < 1 or 350 ppm. Sodium chloride was added before the exposures (70 mu g/cm exp 2 ). The corrosion products were analyzed by gravimetry, ion chromatography, X-ray diffraction, and environmental scanning electron microscopy. Mass gain and metal loss results are reported. As expected, the combination of high humidity and NaCl is very corrosive toward the magnesium alloys investigated. The NaCl-induced corrosion is inhibited by ambient concentrations of CO sub 2 . Exposures in the absence of CO sub 2 give rise to heavy pitting; Mg(OH) sub 2 , brucite, is the dominant corrosion product. In the presence of CO sub 2 , a uniform layer of hydrated magnesium hydroxy carbonate, Mg sub 5 (CO sub 3 ) sub 4 (OH) sub 2 -5H sub 2 O, forms. In both environments, the corrosion rates increased in the order AZ91 < AM60 < AM20.