2, Queen's University, Kingston, Ontario, Canada
Ti and its alloys have been widely used as biomedical materials and have been found application in artificial bones, dental and/or orthopedic implants, etc. because they have suitable chemical, physical, mechanical, and physiological properties. Their biological corrosion resistance to body fluids and biocompatibility with human tissues especially make them excellent implant materials. In addition, Ti is a relatively light metal that possesses good ductility, high tensile modulus and fatigue strength, and an elastic modulus similar to that of human bones. In order to improve bone-bonding ability of Ti implants, it is necessary to apply a suitable surface treatment. In this work, formation of nano-sized porous layers on Ti by asymmetric alternating current (AC) anodizing in sulfuric acid aqueous solution has been studied by using electrochemical techniques and scanning electron microscopy (SEM). When Ti is oxidized in 1 M H2SO4 aqueous solution for 1 min by AC electrolysis, vigorous gas evolution proceeds with the spark discharge on the Ti surface. As a result, a nano-sized porous layer cannot obtain on a Ti surface and the spark discharge gives some damage on the surface. The gases evolved on both electrodes during AC electrolysis consist of hydrogen and oxygen. These gases act as insulators which cause a breakdown phenomenon to proceed at evolved gas layers near Ti and Pt surfaces. Therefore, the spark discharge is observed on both the electrode surfaces during AC electrolysis. Applying a very high voltage between the Ti and Pt counter electrodes thus cannot be applied to obtain a nano-sized porous layer. In order to inhibit the spark discharge, the cathodic current on a Ti electrode, which corresponds to the rate of the hydrogen evolution reaction, is limited using a special electrical circuit. The electrical circuit consists of a variable resistance and two diodes. It inhibits the hydrogen evolution during AC electrolysis and therefore prevents the spark discharge on the Ti surface. Accordingly, a nano-sized porous layer is formed on the Ti surface. The formation of porous layer is dependent on electrical resistance value of the special electrical circuit. As formation of a nano-sized porous layer becomes insufficient when the electrical resistance is high, a low electrical resistance could be used to obtain a sufficient porous layer. The nano-sized porous layers thus obtained have the same morphological features as those formed by direct current (DC) anodization in 1 M H2SO4 solution. The porous layer obtained at low voltage by asymmetric AC electrolysis with the special electrical circuit is compared to those that are obtained by DC anodization voltage. The absorbed hydrogen near Ti surface during asymmetric AC electrolysis greatly influences the formation of a nano-sized porous layer.