Abstract
In order to understand cathodic reactions of Ti-based alloys, local electronic states of the cathode were simulated by a DV-Xα cluster method. A local electronic cell model was assumed here, in which the anode is the titanium-matrix region and the cathode is the alloying-element containing region. The cathodic reaction is probably enhanced strongly when conduction electrons are localized in the cathode region and discharged readily to H+ resulting in the hydrogen evolution on the cathode surface. It was proposed from the study that the tendency to the electron localization can be evaluated by the density of states near the Fermi level, and the tendency to discharge electrons to H+ is associated with the energy of the Fermi level.
This proposal was confirmed by a series of measurements on the polarization curves of various Ti-0.1 at%M alloys, where M is the alloying element. For example, the cathodes containing Ir, Pt, Ru, Rh and Pd met the proposed conditions for the active cathodic reaction. This is, because they had the high density of states near the Fermi level to promote the electron-localization and also the high Fermi level to allow the smooth electron-discharging. Experimentally for alloys containing these elements, the hydrogen overpotential was small and the cathodic polarization curves shifted to the noble potential side. These results clearly indicated that the cathodic reaction was indeed enhanced, in agreement with the theoretical estimation. Thus, the molecular orbital calculation is very convenient in describing the local electronic states of cathodes in aqueous corrosion.
