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

Abstract 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, d E /d t , decreases initially and then increases with increasing H 3 PO 4 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 H 3 PO 4 solutions is proposed to account for experimental observations.