Anodic Dissolution and Passivation of Titanium in Acidic Media: III . Chloride Solutions

Abstract

The electrochemical behavior of titanium in deoxygenated acidic chloride solutions, with and without additions of Ti(III) and Ti(IV) ions, has been determined and compared with that observed in acidic sulfate media. A monolayer mechanism has been developed for the active‐state dissolution and passivation of titanium in acidic media. According to this mechanism, the metal is oxidized in a sequence of one‐electron charge‐transfer reactions which results in the formation of adsorbed reaction intermediates corresponding to each of the relevant valence states of titanium (+1, +2, +3, +4) and which leads to Ti(III) ions in solution in the active and active‐passive transition regions. The mechanism is in quantitative agreement with the experimental results for both chloride and sulfate media. In acidic chloride solutions, as well as in acidic sulfate solutions, the rates of oxidation at a passive titanium surface of Ti(III) ions in solution to Ti(IV) and of reduction at an active‐state surface of Ti(IV) ions in solution to Ti(III) are directly proportional to the concentrations of Ti(III) and Ti(IV), respectively. The reduction of Ti(IV) at an active surface is responsible for the fact that, at a critical concentration of Ti(IV), an active‐state surface passivates. In localized corrosion systems, active and passive surfaces are in simultaneous contact with the electrolyte within the occluded cell, and the aforementioned oxidation and reduction reactions serve to couple the active‐state and passive‐state electrochemical systems. Such electrolyte‐coupled active‐passive systems are capable of generating the critical concentration of Ti(IV) required to passivate the active‐state surface, a fact which explains random spontaneous cessation of localized corrosion (self‐healing). The critical concentration of Ti(IV) is much greater in chloride solutions than in sulfate media and takes much longer to attain. Consequently, the halide ion functions as a promoter of localized corrosion.