By adding 3% Ca in AZ91D, the reticular Al2Ca phase on grain boundaries formed and the corrosion rate decreased to 14.1% of the AZ91D corrosion rate in 5 wt % NaCl solution. The corrosion morphologies showed that the reticular Al2Ca phase effectively confined the development of corrosion from grain to grain and hence significantly reduced the corrosion rate. The Auger depth profiles indicated the corrosion films on AZ91D + 3% Ca had a three-layered structure: a thick Al enrichment outer layer, an intermediate layer, and a thin protective inner layer. The thin inner layer should consist of an Al2Ca phase, which acted as an effective corrosion barrier. (c) 2006 The Electrochemical Society.