Development of a model for the anodic behavior of T60 titanium in chlorinated and oxygenated aqueous media. Application to the specific conditions of hydrothermal oxidation (1 MPa
Résumé
This work evaluates the anodic electrochemical behavior of titanium metal in hydrothermal oxidation conditions (up to 400 °C and 28 MPa) in chlorinated media in order to estimate the supercritical water oxidation reactors reliability for the treatment of less than 10% organic-waste waters. The titanium room temperature dissolution mechanism in chlorinated acidic medium (pH<0) is not fundamentally modified by oxygen. Deduced from the ‘current–potential' and ‘valence–potential' curves, it is based on four crucial elementary steps leading to two branches: a so-called active branch corresponding to a trivalent dissolution (its effect is inversely proportional to the pH), and a passive branch (TiO2 oxide formation with a very limited tetravalent dissolution). In hydrothermal oxidation (pH>1), only the second branch is effective. The titanium protection is directly related to the oxide stability in high pH systems. The mechanism model is expressed in terms of ‘current–potential' laws, which provide kinetic parameters using optimization calculations. The different elementary steps reaction rates were estimated as well as the evolution of the reaction intermediates coverage ratios with the potential. The quantification of each elementary step was performed to understand and/or orient the materials behavior according to different factors (pH, chloride ions contents, potentials...).