TiO2 Coatings in Alkaline Electrolytes Using Anodic Oxidation Technique

Abstract

For the construction of a new design of mechanical heart valve a Ti-6Al-4V alloy was selected and coated with TiO2 due to it corrosion resistance and biocompatibility. A required feature on surfaces in contact with blood it is a low level of roughness (Ra ≤ 50 nm) that not favor the formation of blood clots. One technique that can be used to obtain smooth coatings of TiO2 is the electrochemical anodic oxidation technique using pre-spark oxidation voltage. The phenomenon of spark produces porous oxides and roughness higher than desired. The beginning of the spark voltage depends on the electrolyte used. The present work compares the coatings obtained by anodic oxidation of the Ti-6Al-4V alloy applying different voltages (from 10V to 50V), using as electrolyte an aqueous based alkaline solutions of NaOH and KOH, at different concentrations (0.1M, 1M and 2M) and Ca(OH)2 at a concentration of 0.02 M, using a constant current of 50mA to achieve the desired voltage. Morphological analysis of the different oxides is performed using optical microscopy, and roughness measurements using a profilometer. The phases present were analyzed by X-ray diffraction technique with a glancing angle of 1º. By varying the applied voltage coatings of different interference colors were obtained. The morphology and roughness of the obtained oxides varied according to the applied voltage in each experience. With KOH and NaOH 1M the spark occurred at 46V and 41V, respectively. The start voltage of the spark decreased to 34V and 29V, respectively, when the electrolyte concentration increase from 1M to 2M. In pre-spark conditions oxides were of an average roughness of 22 nm. After the spark, the oxides became with roughness of 700 nm, limiting the oxidation conditions for the desired application. Anatase and rutile phases were not detected in the coatings, which would be amorphous or with crystalline fractions undetectable by XRD.