AZ31 Mg alloy was anodized with a pulse potential between the transpassive (10 V-Ag/AgCl) and active regions (-1.35 V-Ag/AgCl) in 2 M NaOH aqueous solution at 303 K. Optimal conditions for the pulse anodizing were a duty ratio of 91%, a frequency of 0.09 Hz, and an anodizing time of 600 s. Pulse anodizing caused a remarkable 11-fold decrease in the surface porosity and a 1.6-fold increase in the film thickness from those obtained under a constant potential of 10 V-Ag/AgCl. Furthermore, an Al-enriched crystalline oxide layer was formed on the outer surface of MgO, which improves the corrosion resistance of the Mg alloy in neutral solutions. In consequence, the pitting potential E-pit increased to -1.36 V-Ag/AgCl and the corrosion current density i(corr) decreased to 60 +/- 10 mu A cm(-2), resulting in an approximately 3-fold decrease in the corroded area after anodic breakdown and salt spray tests. (C) 2013 The Electrochemical Society. All rights reserved.