Modification of the Electrochemical Characteristics of Colored Passive Layers on Titanium by RuO_2, IrO_2, and RuO_2-IrO_2 Doping

摘要

AC polarization of Ti in aq. NH_4BF_4 results in formation of passive films that reveal a wide spectrum of well-defined colors. Such layers on Ti can be subsequently modified by their doping with RuO_2, IrO_2, and RuO_2+IrO_2. This method is based on thermal oxidative decomposition of the respective Ru or/and Ir chlorides. Such formed Ru or/and Ir oxides are incorporated into the colored apssive layers, thus modifying their chemical, physical and electronic properties. The morphology of such modified films is examined by SEM and the results show that they possess distinct features. In the case of the RuO_2-modified layer, the surface appears free of cracks or fractures. On the other hand, the IrO_2 and RuO_2+IrO_2 modified passive layers are cracked and fracturec. Electrochemical behavior of the modified films is examined by recording polarization curves and Tafel steady-state plots in the hydrogen evolution reaction region. important electrochemical parameters such as the corrosion potential, corrosion current density, extent of surface passivation, Tafel slope and exchange current density are determined and compared. The chemically modified passive layers on titanium show superior electrocatalytic properties towards the hydrogen evolution reaction with respect to the pure Ti or colored layers, yet their applicability is directly related to the surface morphology. The RuO_2 modified filmd might become a Ti-based cathode due to the lack of surface crack or fracture.