Local electrochemical impedance spectroscopy (LEIS) has been applied to study the corrosion behavior of an as-cast AZ91 magnesium alloy. The results are compared with those obtained from traditional electrochemical impedance spectroscopy and discussed in relation with previously published results. They confirm the specific time-dependent corrosion of the AZ91 magnesium alloy: due to the high aluminum content and to the presence of a eutectic constituent in the alloy, the corrosion begins in the grain body and then spreads to the eutectic areas. This leads to aluminum enrichment of the corrosion product layer. When an aluminum threshold concentration is reached, the properties of the oxide layer become predominantly determined by the properties of the alumina component and there is a change in the impedance diagrams for both global and local impedance spectroscopies. However, the space resolution of the local electrochemical impedance setup was not sufficient to give additional results on the influence of the alloy structure.