The corrosion behavior of Ti-6Al-3Nb-2Zr-1Mo alloy: Effects of HCl concentration and temperature

Investigation about the corrosion behavior of Ti alloys in different ambient environment is of great significance for their practical application. Herein, we systematically investigate the corrosion behavior of a newfound Ti-6 Al-3 Nb-2 Zr-1 Mo(Ti80) alloy in hydrochloric acid(HCl) ranging from 1.37 to 7 M, and temperature ranging from 25 to 55℃, by means of electrochemical measurements, static immersion tests and surface analysis. Results manifest that increasing either HCl concentration or temperature can accelerate the corrosion of Ti80 alloy via promoting the breakdown of native protective oxide film and then further facilitating the active dissolution of Ti80 matrix. According to potentiodynamic polarization curves, Ti80 alloy displays a spontaneous passive behavior in 1.37 M HCl at 25℃, compared to a typical active-passive behavior under the other conditions. As indicated by cathodic Tafel slope, the rate determining step for cathodic hydrogen evolution reaction is likely the discharge reaction step. The apparent activation energies obtained from corrosion current density and maximum anodic current density for Ti80 alloy in 5 M HCl solution are 62.4 and 55.6 kJ mol-1, respectively, which signifies that the rate determining step in the corrosion process of Ti80 alloy is mainly determined by surface-chemical reaction rather than diffusion. Besides, the electrochemical impedance spectroscopy tests demonstrate that a stable and compact oxide film exists in 1.37 M HCl at 25℃, whereas a porous corrosion product film forms under the other conditions. Overall, the critical HCl concentration at which Ti80 alloy can maintain passivation at 25℃ can be determined as a value between 1.37 and 3 M. Furthermore, the corroded surface morphology characterization reveals that equiaxed α phase is more susceptible to corrosion compared to intergranular β phase due to a lower content of Nb, Mo, and Zr in the former.