THE ANODIC BEHAVIOR OF ION-BEAM MIXED SURFACE ALLOYS

Surface alloys of several hundred ångstroms thickness have been prepared by ion beam mixing Pd into titanium, Ti into iron, and Mo into aluminum. Each of these systems consists of a more noble component mixed into a more active substrate. The ion beam mixed Pd-Ti surface alloy has a corrosion rate of 1/400 that of unalloyed titanium in H2SO4 at 100°C, whereas ion beam mixing of Ti into iron has a modest effect on the corrosion rate in 0.1 M NaCl at 25°C. In both cases the results are explained by superposition of the anodic and cathodic polarization curves for the pure constituents. In the Pd-Ti surface alloy in 1 M H2SO4 at 100°C, the cathodic curve for Pd intersects the anodic curve for Ti in the passive region of Ti; but for the Ti-Fe surface alloy in 0.1 M NaCl, the cathodic curve for Ti intersects the anodic curve for iron in the active region of Fe. Ion beam mixed surface alloys of Mo-Al were prepared by either ion implanting or vapor depositing molybdenum on to an aluminum substrate and then mixing with an inert or reactive element. The pitting potentials of the ion beam mixed samples were 0.100 to 0.650 V higher than the pitting potential of pure aluminum and are also higher than for the corresponding unmixed alloys prepared either by ion implantation or vapor deposition of Mo. The improvement in pitting resistance of the Mo-Al surface alloys is explained in terms of the pH of zero charge model of passive film breakdown. © 1990.