Corrosion of magnesium in humid air

Abstract

The influence of ambient concentrations of carbon dioxide on the atmospheric corrosion of magnesium has been studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM), revealing the development and growth of corrosion products. The surfaces investigated by AFM were prepared by ultramicrotomy, using a diamond knife, to generate a smooth finish without using water or lubricant. Sputter-deposited Mg films were also studied with mechanically polished samples used to monitor the overall corrosion process. The exposures were performed at 22.0 °C in a synthetic environment with precise control of relative humidity (95%) and CO 2 (0 or 350 ppm). Corrosion is localized in the absence of CO 2 and is related to noble inclusions in the metal matrix. After corrosion product removal, relatively deep pits are evident. In contrast, pitting is inhibited in the presence of CO 2 with relatively uniform corrosion product development; further, the noble inclusions have no effect on the distribution of corrosion. The inhibitive effect of CO 2 is also observed in the long-term exposures, showing that CO 2 reduces the average corrosion rate. Mechanisms are introduced to explain the effects of CO 2 and the roles of the noble inclusions on the corrosion behaviour.