The nature of the surface film formed spontaneously on Mg exposed to 1 M NaOH was investigated using scanning transmission electron microscopy, coupled with energy dispersive spectroscopy. Thin foils containing the surface film in cross-section were prepared by focused ion beam milling. The surface film formed is found to consist mainly of a crystalline MgO layer that has been hydrated to Mg(OH)(2) to a variable degree. Although the film exhibits excellent corrosion resistance, it is not stable. The film tends to experience an irregular breakdown/repair process, which is characterized by large irregular potential drops (about 1 V) under open-circuit conditions. The breakdown/repair process is believed to involve the hydration-induced stress-rupture of the MgO film at discrete sites and the subsequent formation of a Mg(OH)(2) self-healing corrosion product nodule. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.018302jes] All rights reserved.