On the Passivity of Titanium

Passivation and activation of titanium in deaerated non-oxidizing acid solutions were studied in order to investigate on the passivity of titanium. Critical potential for passivity and Flade potential shift towards noble direction with increasing chloride and hydrogen ion concentration, thus passivation becoming difficult. The critical potentials are expressed as E=E0+mlog[Cl-]-n(RT/ 0.4343F)pH, where R, the gas constant, T, absolute temperature, F, Faraday constant, E0 and m are constants. The constant n varies from 0.9 to 3.0 with environmental conditions such as temperature and kind of anion. Such different pH dependence of the critical potentials is attributed to the presence of different equilibrium surface reactions corresponding to passivation or activation for the respective corrosion conditions. The equilibrium reactions are considered to be Tia++2H2O=TiO2+4H++(4-a)e, 2Tib++3H2O=Ti2O3+6H++(6-2b)e, Ti+2H2O=TiO2+4H++4e, 2Ti+3H2O=Ti2O3+6H++6e. When the critical potentials expressed as equilibrium potentials of two former reactions, passivation or stability of passivity is affected directly by titanium ion. In such cases, as demonstrated for titanium-palladium alloy, corrosion resistance of titanium depends on the concentration of titanium ion in corrosive media.