A New Procedure of Formation of Multicolor Passive Films on Titanium - Compositional Depth Profile Analysis

Potentiostatic ac polarization of Ti in an aqueous NH4BF4 solution produces passive films which reveal a wide spectrum of colors. ac polarization of Ti electrodes in other electrolytes such as aqueous solutions of H2SO4, Na3PO4, or NaCl does not lead to passive films having colors as well-defined and as bright as those formed in NH4BF4. Their coloration is uniform across the whole surface and is brighter and better defined than that of films formed by the potentiostatic dc polarization. Coloration depends on the applied voltage and not on the polarization time. An optical microscopy and scanning electron microscopy surface morphology analyses indicate that the Ti passive films are compact and do not exhibit any cracks or formation of new surface features. In turn, an atomic force microscopy surface analysis reveals the following surface characteristics:  (i) surface roughness of the Ti substrate decreases upon formation of the passive film; (ii) presence of small hemispherical “particles” incorporated int...