The morphological evolution of Mg based powders during repeated absorption-desorption reactions with hydrogen has been studied by Scanning Electron Microscopy. The main feature observed is the presence, after several cycles, of surface protrusions probably constituted by the base Mg material without the presence of a catalyst. The effect is present both in catalyzed and non-catalyzed materials and it is considered an indication of the tendency of the base material to exit from the oxide shell surrounding the Mg powder particles. This tendency is confirmed by the observation of empty MgO boxes indicating that the effect can push until a complete expulsion of the base material. This effect can represent the base for an innovative method for cleaning the surface of a tank material by an "in situ" procedure. © 2011 Materials Research Society.