Studies on transition of titanium from active into passive state in phosphoric acid solutions

摘要

Studies on active-passive transition of titanium in phosphoric acid solutions of 0.5-4 M revealed the unusual surface chemistry with the electrolyte concentration. The passive film was affected by the applied anodic potential, but under galvanostatic conditions at low current densities (0.1-0.6 Am~(-2)) the slope of dE/dt always showed the minimum at the concentration ～2 M, which resulted in a coating of an oxide film by a gel-like layer. The active-passive transition was a process in which an inhibiting effect of phosphate ions on a dissolution of oxide layer was observed. Capacitance and SEM/EDS examination gave the evidence of two-layered surface film. To explain all the above results, an oxide pre-film model and the mechanism of two-layered surface film formation on titanium in phosphoric acid solutions are proposed.