Morphology, chemical composition, and electrochemical characteristics of colored titanium passive layers

Brightly and uniformly colored passive layers on Ti are formed by applicati on of ac polarization in aqueous NH4BF4. A wide spectrum of well-defined co lors is accomplished by varying the ac voltage. The passive films are stabl e in the ambient and in aqueous chloride, perchlorate, and sulfate solution s. Optical microscopy and scanning electron microscopy analyses indicate th at the passive layers are compact and do not show fractures or cracks. An X -ray photoelectron spectroscopy (XPS) characterization of the colored passi ve layers demonstrates that their surface-chemical composition depends on t he 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). Fl uoride in the film originates from decomposition of NH4BF4, and it accumula tes at the inner metal/passive-film interface. XPS depth profiling shows th at 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 Tafe l plots in the hydrogen evolution reaction (HER) region in 1.0 M aqueous H2 SO4. The polarization curves show that the corrosion potential of the color ed passive layers shifts toward less-negative potentials indicating that th ey are more stable than Ti under the same conditions. The passive region fo r the colored layers resembles that for Ti. The Tafel plots for the HER dem onstrate that the passive layers have higher activity toward the HER than T i. The Tafel relations reveal new features that can be associated with the partial breakdown/decomposition of the passive layers, H absorption, and th e onset of Ti hydride formation.