Characteristics of colored passive layers on titanium: morphology, optical properties, and corrosion resistance

Electrochemically formed colored passive layers on titanium and their optical, surface morphology, and corrosion properties are presented and discussed. With the application of progressively higher AC voltages (VAC) during preparation of these passive layers, they are found to become more protective of the underlying metal, as determined from corrosion resistance measurements employing electrochemical polarization curve and inductively coupled plasma mass spectrometry experiments. The passive layers on titanium were found to be uniform in their surface morphology with no apparent cracks or pits. Surface morphology, and its relation to optical properties, was also investigated using visible light microscopy, profilometry, and near-infrared ultraviolet visible reflectance spectroscopy measurements. A correlation between the light reflected from the entire sample surface and the coloration of surface grains was also observed through these measurements. The reflectance spectra showed a red-shift of wavelength maxima (λmax) values as AC voltages and, therefore, thicknesses were increased. Overall, these passive layers are protective of an already remarkable metal, and with greater knowledge of the properties of colored protective layers, their potential may be employed in a wide range of applications.