| Abstract | Brightly and uniformly colored passive layers on Ti are formed by application of ac polarization in aqueous NH4BF4. A wide spectrum of well-defined colors is accomplished by varying the ac voltage. The passive films are stable in the ambient and in aqueous chloride, perchlorate, and sulfate solutions. Optical microscopy and scanning electron microscopy analyses indicate that the passive layers are compact and do not show fractures or cracks. An X-ray photoelectron spectroscopy (XPS) characterization of the colored passive layers demonstrates that their surface-chemical composition depends on the ac polarization voltage. The main constituents of the passive layers are Tiz+, O2-, and F- (z varies from 4 to 2 depending on the film's depth). Fluoride in the film originates from decomposition of NH4BF4, and it accumulates at the inner metal/passive-film interface. XPS depth profiling shows that the higher the ac voltage applied, the thicker the passive film formed. Electrochemical properties of the colored Ti passive layers are determined by recording polarization curves in the −0.8 to 3.2 V range as well as Tafel plots in the hydrogen evolution reaction (HER) region in 1.0 M aqueous H2SO4. The polarization curves show that the corrosion potential of the colored passive layers shifts toward less-negative potentials indicating that they are more stable than Ti under the same conditions. The passive region for the colored layers resembles that for Ti. The Tafel plots for the HER demonstrate that the passive layers have higher activity toward the HER than Ti. The Tafel relations reveal new features that can be associated with the partial breakdown/decomposition of the passive layers, H absorption, and the onset of Ti hydride formation. |
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