Electrochemical Corrosion and Passivation Behavior of Titanium and Its Ti-6Al-4V Alloy in Low and Highly Concentrated HBr Solutions

Passivation and dissolution behavior of Ti and Ti-6Al-4V alloy was investigated in HBr solutions over a wide concentration range from 0.01 M to 8.0 M using open-circuit potential, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. In the low and high concentration domains, the open-circuit potential for the metal and its alloy was found to increase positively with time due to oxide film thickening on the metal surface. However, there is a certain critical HBr concentration at which the thickening rate of the oxide film on Ti metal has a maximum value. The results were confirmed by EIS measurements at open-circuit potential where a constant phase element model with its complex transfer function were used to analyze the obtained impedance responses. Polarization scans showed that the corrosion current density (i(corr)) for titanium decreases slightly with increasing HBr concentration up to the threshold value before it starts to increase appreciably. On the other hand, for Ti-6Al-4V alloy the value of i(corr) increases gradually with HBr concentration over the whole range, and the alloy dissolves actively due to its poor passivating ability. Interestingly, for the two materials both i(corr) and the critical current density of passivation (i(crit)) decrease with raising temperature, indicating a decrease in the corrosion susceptibility of the passivating oxide film as a result of the decrease in the extent of Br- anion adsorption at higher temperatures. The apparent activation energy for the thickening process of the passive film was calculated from the increase in the spontaneous growth rate (delta(1)) of its inner layer with temperature. The results in general confirmed that Ti metal has a stronger propensity to form passive film in HBr solutions better than its Ti-6Al-4V alloy.