EIS for titanium base alloys in aqueous solutions

Electrochemical impedance spectroscopy was used to characterize the passive films on Ti and binary Ti-15Mo, ternary Ti-15 Mo-5A1 alloys in acid (10% H2C2O4-oxalic acid), alkaline (10% KOH) and neutral (30% KCl) solutions. The impedance spectra were obtained at different potential values between -0.75 V and +0.80 V (vs. SCE); these spectra were analysed using a suitable procedure elaborated by Boukamp. Two models of equivalent circuits describing the spontaneous passivation of Ti and its alloys in acid, alkaline and neutral solutions were presented. In acid solution an equivalent circuit with three time constants was fitted; a rime constant is for the passive film and the double layer, the second is for charge transfer reactions and the third for the diffusion processes through the passive layer. In alkaline solution, the same equivalent circuit but with a Constant Phase Element (CPE) instead of Warburg element is available. In neutral solution an equivalent circuit with two time constants was fitted: one for the passive film and the double layer and the second for the charge transfer reactions. In this solution the diffusion process through the passive film does not take place. The models proved that until +0.20 V (vs. SCE) titanium presents a good corrosion resistance but at more positive potentials the alloys are more resistant than base metal in aqueous solutions.