Corrosion Behavior and Passive Film Characteristics Formed on Ti, Ti[sub 3]SiC[sub 2], and Ti[sub 4]AlN[sub 3] in H[sub 2]SO[sub 4] and HCl

Abstract

We report on the chemical and electrochemical corrosion of Ti, Ti 3SiC2, and Ti4AlN3 in HCl and H 2SO4 solutions. The chemical corrosion of Ti 3SiC2 under conditions similar to those used industrially for the electrolysis of HCl (15% HCl, 65°C) is significantly better than that of pure Ti, indicating that it could be a promising substrate for dimensionally stable anodes. The enhanced resistance is attributed to the formation of a thin, passivating SiO2-based layer. The same layer protects Ti3SiC2 during its anodic oxidation. After a 4 day period, during which the rate of growth of the SiO2-based layer is initially high, but then slows to a steady rate of about 130 μm/yr cm 2 is established, which translates to a Ti dissolution rate of ∼9 × 10-4 mol/yr cm2. Analysis of Mott-Schottky plots and electrochemical impedance spectra show that the passive films that form onto Ti and Ti3SiC2 in 1 M HCl and 1 M H 2SO4 at room temperature are n-type semiconductors dominated by space-charge capacitances. The films forming on Ti 3SiC2 have significantly lower space-charge-layer thicknesses and higher donor densities than those that form on Ti. Preliminary results for Ti4AlN3 indicate that the passive film formed is a p-type semiconductor. © 2004 The Electrochemical Society. All rights reserved.