Gel-like layer development during formation of thin anodic films on titanium in phosphoric acid solutions

Anodising of commercial pure titanium in phosphoric acid solutions at different concentrations (0.5-4 M) have been investigated using galvanostatic and potentiodynamic polarisation techniques. Under galvanostatic conditions at low current densities (0.1-0.6 A m⁻²), the chrono-potentiometric curves always show a linear section in the early stage of the process. The slope of the linear section, dE/dt, decreases initially and then increases with increasing H₃PO₄ concentration, with the minimum slope at a concentration, ∼2 M. Under potentiodynamic conditions at this concentration the samples exhibit a different anodic behaviour. The development of gel-like layer during formation of thin anodic films on titanium in H₃PO₄ solutions is proposed to account for experimental observations.