The role of electronic structures of passive film in corrosion resistance of transition-metal based alloys

• Project/Area Number: 08650827
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Structural/Functional materials
• Research Institution: Suzuka National College of Technology
• Principal Investigator: EZAKI Hisakazu Suzuka National College of Technology, Department of Materials Science and Technology, Associate Professor, 材料工学科, 助教授 (80160357)
• Project Period (FY): 1996 – 1997
• Project Status: Completed (Fiscal Year 1997)
• Budget Amount: ¥1,900,000 (Direct Cost: ¥1,900,000); Fiscal Year 1997: ¥400,000 (Direct Cost: ¥400,000); Fiscal Year 1996: ¥1,500,000 (Direct Cost: ¥1,500,000)
• Keywords: Transition Metal based alloy / Titanium alloy / Photoelectrochemistry / Anodic polarization / Photocurrent / Passive film / Electronic structure / Corrosion / 遷移金属

Research Abstract

The purpose of this study is to clarify the relations between corrosion resistance of transition-metal based alloys and electronic structures of passive films formed on them. In this study, attention was focused on the passive films of Ti alloys. The electronic states of passive films were examined for Ti and Ti alloys containing 5at% of 3d (V,Cr, Mn, Fe, Co, Ni, and Cu), 4d (Nb and Mo) and 5d (Ta) with the photoelectrochemical technique. In addition, the electronic structures of passive films were calculated by using the molecular orbital calculation method. The calculation results on the electronic structures of passive films were compared with the experimental results.
The positive photocurrent was observed when the passive film of each alloy was exposed to the light. This means that the passive films of Ti alloys have the characteristics of n-type semi-conductor. Such characteristics did not change in accordance with the alloying elements. The shape of photocurrent spectra obtained by changing the wave-length of incident light also did not change in accordance with alloying elements. The measured band-gap of passive films was about 4. OeV.In contrast to the above fact, the flat-band potential which represents the degree of band bending changed largely depending on the alloying elements. The flat-band potential decreased by the addition of Co, Cu, Nb and Ta, whereas it increased by the addition of Fe and Mo. It was found that the current density necessary for the passivation of alloy, which is a measure of the corrosion resistance of alloy, was well related to the flat-band potential of alloy. The alloy with less-noble flat-band potential has a small current density for the passivation. The values of calculated band-gap of the passive films were in good agreement with those obtained in the experiments. However, there was no obvious relation between the flat-band potential and the calculated Fermi energy level which was expected to be responsible for the band-bending. Further theoretical consideration would be necessary in this regard.