Applications of Ion Implantation to Metallic Corrosion

The Bureau of Mines is conducting research on the corrosion behavior of alloys formed by ion implantation with the ultimate goal of conserving strategic alloying materials such as chromium and nickel. These alloys were fabricated by the implantation of 25-keV chromium, nickel, and aluminum ions into polycrystalline iron to fluences ranging from 1 to 5 × 1016 ions/cm2. The alloy distribution as a function of depth (depth profile) has been determined for the ion-implanted alloys, and the results have been compared with theoretical predictions. Changes in the profiles due to annealing were investigated, and the values that were obtained for the diffusion coefficients were compared with published values for the volume diffusion coefficients. The resistance of these alloys to environmental attack has been evaluated both by determining their anodic polarization behavior under potentiostatic conditions in a buffered boric acid solution and by determining their gaseous oxidation characteristics. Results of the electrochemical studies have shown that the general corrosion resistances for the ion-implanted alloys were comparable to those of nominally equivalent bulk alloys and that their pitting corrosion resistance was superior to that of iron, although generally not as good as that for most equivalent bulk alloys. Gaseous oxidation studies have shown that ion-implanted and bulk iron-chromium alloys exhibit essentially identical oxidation kinetics, with a much higher rate of oxidation observed for iron. The influence of radiation damage on the gaseous oxidation of iron has been studied by implanting iron ions into iron.