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

Abstract

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 into the surface passive film; (iii) lack of fractures within the passive film. Complementary XPS depth analysis data show that (i) chemical composition of the Ti passive films is nonuniform and depends on the polarization voltage, (ii) Ti, O, and F are the only identified elements within the passive films, (iii) most of F accumulates at the inner metal/passive film interface whereas the F concentration in the outer and inner sections of the films is 4−5 atom %; (iv) composition of the topmost surface layers (7 nm) corresponds to the O to Ti ratio of NO/NTi = 2; (v) chemical composition of the inner layers varies between NO/NTi = 1.5 and NO/NTi = 0.75 and is different in the case of each passive film; (vi) different stoichiometries between Ti, O, and F are obtained by varying the ac polarization voltage. Potentiostatic ac polarization of Ti electrodes in an aqueous NH4BF4 solution results in formation of multicolored passive films whose coloration, morphology, and chemical composition are significantly different from those formed by dc polarization.