Electrochemical Study on Enhanced Corrosion Resistance of Titanium Nitride Films Prepared by Ion Mixing and Vapor Deposition Method

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TiN films prepared by dry coating process such as physical vapor deposition exhibit superior wear and good corrosion resistance, and furthermore their appearance is attractive gold-like color. The most important point in coating corrosion-resistant films is to make no micro-defects because the corrosion of film starts form a defective part such as a perforating pinhole. In order to minimize pinhole defect density in a TiN film, the process parameters must be adequately controlled, and a quick and quantitative evaluation method of pinhole defect must be developed. Incidentally, the critical passivation current density (CPCD) method makes use of the fact that the critical passivation current density of metals exhibiting active-passive transition varies in proportion to the metal surface area. When a coating is applied over this sort of metal substrate, the metal part at the bottom of the perforating pinhole is subjected to active dissolution in the range of the active potential region. T … More hus the intensity of the measured critical passivation current density, becomes proportional to the exposed substrate area at the bottom of pinholes. In this study, therefore, the electrochemical CPCD method was used to quantitatively evaluate the pinhole defect area for the TiN-coated stainless steels prepared by ion mixing and vapor deposition method.The performance of ion mixing and vapor deposition machine used was studied from the aspect of evaculation, ion beam, substrate temperature and film thickness, and the TiN film prepared was evaluated successfully by SEM, XRD, AES and XPS methods. The TiN films were homogeneous and with the characteristic gold-like color. They exhibited the columnar structure with the preferential orientations, and contained more or less pinhole defects. Here, the electrochemical evaluation of pinhole defects is focused on the TiN films deposited onto stainless steels. The critical passivation current density IィイD2critィエD2 in the TiN films prepared at normal temperature of substrate decreased with increasing film thickness. Above1.5μm in thick films, however, there was an increasing tendency in the IィイD2critィエD2 with cracking and/or peeling. On the other hand, the IィイD2critィエD2 in the TiN films prepared at 300℃ decreased considerably with increasing film thickness, and the resulting corrosion resistance was improved by TiN-coating. The area ratio of pinhole defects was evaluated by the ratio of IィイD2critィエD2 of a coated and a non-coated specimens, i.e., by CPCD method. It is demonstrated that such electrochemical measurement is superior as an evaluation method for the pinhole defects of corrosion-resistive coating films. Less