Anodic Behavior of Titanium and Commercial Alloys in Sulfuric Acid

Abstract

Abstract The anodic polarization behavior of commercially pure alpha titanium 75A (annealed), beta alloy 13V-llCr-3Al (annealed) and alpha-beta alloy 6Al-6V-2Sn (heat treated to strength levels of 140, 160 and 180 ksi) in hydrogen saturated 5, 12 and 20 percent sulfuric acid solutions was investigated at 20, 35, 50 and 65 ± 1 C, using a potentiostatic technique. All specimens exhibited active to passive transition at temperatures above 20 C and acid concentrations greater than 5 percent Titanium alloy 13V-11Cr-3Al showed transpassive behavior which was attributed to the 11 percent chromium content Increasing acid concentration increased me critical current for passivity and shifted the critical potential for passivity in the noble direction. Increasing the temperature served only to increase the critical current for passivity. Temperature coefficients and activation energies were the right order of magnitude for a reaction controlled by reactivity at the metal surface. For the alpha beta alloy 6Al–6V–2Sn the critical current for passivity increased as the ratio of amount of alpha to beta phase increased with increasing strength level. The dissolution is accelerated, probably, by the galvanic effects and the unfavorable area ratio. Ti 75A, a commercially pure metal and Ti 13V-11Cr-3Al, a homogeneous single phase alloy, had a smaller critical current for passivity values man did Ti 6Al-6V-2Sn, a multiphase alloy, which was expected. The effect of Fe3+, Cu2+ and Cl− on polarization of Ti 6Al-6V-2Sn (180 ksi) was investigated also. Fe3+ and Cu3+ inhibited the anodic dissolution reaction and Cl− facilitated passivation.