The initial steps of atmospheric corrosion on magnesium alloy AZ91D

Magnesium, with its low density, is a very interesting metal in applications in which weight is important. In this work the initiation of corrosion attacks on magnesium alloy AZ91D has been investigated under atmospheric conditions, using confocal laser scanning microscopy and SKPFM. The corrosion attack starts in the alpha phase in larger grains and is under atmospheric conditions initiated at the boundary between the alpha phase and the eutectic alpha-/beta phase. The SKPFM measurement shows that a high Volta potential difference exists between the larger grains and the surrounding eutectic alpha-/beta phase and the beta phase. A microgalvanic element is formed in the thin adlayer on the surface, with anodic Mg dissolution in the alpha phase and the cathodic reaction primarily in the eutectic alpha-/beta phase. The initiation in the larger grains could be explained by the smaller aluminum content in the larger grains due to the solidification process. Even though intermetallic Al-Mn particles show a high Volta potential difference with the connecting alpha phase, they are not involved in the initiation of the corrosion. This is explained by the position within the microstructure of Al-Mn particles embedded in the beta phase and located away from the alpha phase.