An apparatus for the anodic treatment of aluminum alloy pistons used in internal combustion engines. The apparatus comprises at least two members applied symmetrically to the lateral cylindrical surface of the piston which are connected to a source of positive direct current. An electrode connected to a source of negative direct current and having at least one aperture therein is positioned adjacent the head portion of the piston which is to be treated. A deflector of electrically insulating material disposed generally parallel to and spaced from the electrode and having a circumference curved toward the electrode is provided with a central passage therethrough which receives the head portion of the piston, with the head portion being disposed opposite the electrode. The deflector surface facing the electrode is provided with a flexible means for sealing the deflector to the head portion and a regulated flow of anodizing electrolyte is provided through the aperture in the electrode, in the direction of the deflector and the head portion. This apparatus can be applied to the high speed production of barrier coatings which are confined to the piston heads and which can prevent development of heat stresses which could adversely affect the satisfactory functioning of the piston. The coatings are produced without the use of masks or treatments other than the anodic treatment.