Effect of pH on the Behaviour of Some Titanium Alloys with Biphasic Structure

In this paper, the thermal and plastic deformation treatments were applied to Ti6Al4V alloy and its behaviour in Ringer 2 solution of different pH values (2.49; 6.9; 8.91), simulating the hardest conditions that can appear in human body, on the life time of an implant (10-20 years or more) was studied. The behaviour of the untreated Ti6Al4V alloy and its five types of thermal-mechanical treated alloys was estimated from the corrosion rates, ion release, variation in time (500 hours) of the open circuit potentials E-oc and of the potential gradients Delta E-oc (pH) due to the pH non-uniformity of Ringer 2 solution. Very low corrosion rates in the "perfect stable" or very stable" range were determined. Ion release revealed very low values under permitted limits, therefore much reduced citotoxic effects. The open circuit potentials were ennobled at the beginning, then slowly decreased, tending to about constant values, showing the existence of the passive films on the studied material surfaces and of some dissolution, repassivation and absorption processes and their tendency to become stable. Open circuit potential values are more electropositive than those of titanium, so, more stable, more resistant films due to the both passivation effects of the alloying elements and the stability effect induced by the thermo-mechanical treatments. Potential gradients due to the pH nonuniformity of the Ringer solution have low values that can not generate galvanic corrosion (for 500 exposure hours).