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 pro duces passive films which reveal a wide spectrum of colors. ac polariz ation of Ti electrodes in other electrolytes such as aqueous solutions of H2SO4, Na3PO4, or NaCl does not lead to passive films having color s as well-defined and as bright as those formed in NH4BF4. Their color ation is uniform across the whole surface and is brighter and better d efined than that of films formed by the potentiostatic dc polarization . Coloration depends on the applied voltage and not on the polarizatio n time. An optical microscopy and scanning electron microscopy surface morphology analyses indicate that the Ti passive films are compact an d do not exhibit any cracks or formation of new surface features. In t urn, an atomic force microscopy surface analysis reveals the following surface characteristics: (i) surface roughness of the Ti substrate de creases upon formation of the passive film; (ii) presence of small hem ispherical ''particles'' incorporated into the surface passive film; ( iii) lack of fractures within the passive film. Complementary XPS dept h analysis data show that (i) chemical composition of the Ti passive f ilms is nonuniform and depends on the polarization voltage, (ii) Ti, O , and F are the only identified elements within the passive films, (ii i) most of F accumulates at the inner metal/passive film interface whe reas 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) c orresponds to the O to Ti ratio of N-O/N-Ti = 2; (v) chemical composit ion of the inner layers varies between N-O/N-Ti = 1.5 and N-O/N-Ti = 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 po larization voltage. Potentiostatic ac polarization of Ti electrodes in an aqueous NH4BF4 solution results in formation of multicolored passi ve films whose coloration, morphology, and chemical composition are si gnificantly different from those formed by de polarization.