硫酸および塩酸溶液中での不動態チタンの自己活性化に及ぼすCr(II), Ti(III), Ti(IV)イオンと温度の影響

Abstract

The effect of three kinds of cations such as Cr(II), Ti(III), and Ti(IV) ions on the stability of passivated titanium in sulfuric and hydrochloric acid solutions of 1 kmol·m−3 was investigated by immersion tests. The open-circuit reduction of the passive film was accelerated by Cr(II) and Ti(III) ions, and retarded by Ti(IV) ion. In relatively dilute solutions of the cations, the reciprocal of the self-activation time exhibited a linear dependence on the cation concentration. A galvanostatic reduction test was also conducted with an electronic current regulation device and showed that the inverse value of the self-activation time increased linearly with the increase of the applied cathodic current. These results were compatible with each other and the cation concentration-applied current plot derived from them was used to calculate the rate constant of the ions in the reduction of the passive film. The value of the rate constant so obtained was 5×10−7 m·s−1 for Cr(II), 2×10−8 m·s−1 for Ti(III) and 2.4×10−7 m·s−1 for Ti(IV) in the sulfuric acid solution, and 1×10−8 m·s−1 for Ti(III) in the hydrochloric acid solution. The rate of the self-activation of the passivated titanium in the sulfuric acid solution was also estimated to be 0.74×10−2 A·m−2, twice the value in the hydrochloric acid solution. Additions of Ti(IV) ions of 3 mol·m−3 or above prevented the self-activation in the sulfuric acid solution, attributed to the inhibition of the film dissolution. These observations explained a relatively rapid initiation and cessation of crevice corrosion in sulfuric acid solutions. The potential-time curves at temperatures of 313 K to 328 K yielded a linear relation between logarithm of the rate of the self-activation and the inverse value of temperature, and from the slope of the straight line activation energy for the anodic dissolution of the metal was estimated to be 78 kJ·mol−1.