In situ growth of nano-sized layered double hydroxides （LDH） conversion film on AZ31 alloy was synthesized by a urea hydrolysis method. The formation mechanism of the film was proposed. Firstly, the dissolved Mg2＋ ions deposited into a precursor film consisted of MgCO3 and Mgs（CO3）4（OH）2·4H2O; secondly, the precursor translated into the crystalline Mg（OH）2 in alkaline conditions; finally, the Mg2＋ ions in Mg（OH）z were replaced by A13＋ ions, Mg（OH）2 translated into the more stable LDH structure, simultaneously, the OH- ions in the interlayer were exchanged by CO32-, thus led to the formation of the LDH （Mg6Alz（OHh6CO3·4H2O） film. The results indicated that the LDH film characterized by interlocking plate-like nanostructures and ion-exchange ability significantly improved the corrosion resistance of the AZ31 Mg alloy.
