Spontaneous GaN/AlN core shell nanowires with high crystal quality were synthesized on sapphire substrates by vapor liquid solid hydride vapor phase epitaxy (VLS-HVPE) without any voluntary aluminum source. Deposition of aluminum is difficult to achieve in this growth technique which uses metal chloride gaseous precursors: the strong interaction between the AlCl gaseous molecules and the quartz reactor yields a huge parasitic nucleation on the walls of the reactor upstream the substrate. We open up an innovative method to produce GaN/AlN structures by HVPE, thanks to aluminum etching from the sapphire substrate followed by redeposition onto the sidewalls of the GaN core. The paper presents the structural characterization of GaN/AlN core shell nanowires, speculates on the growth mechanism and discusses a model which describes this unexpected behavior. (C) 2016 Elsevier B.V. All Tights reserved.