Effect of heat treatment on the corrosion behaviour of Ti–5Ta–1.8Nb alloy in boiling concentrated nitric acid

The effect of heat treatment i.e., stress relieving (SR), mill annealing (MA), solution treatment (ST), aging (AG) and overaging (OA) on the corrosion behaviour of Ti–5%Ta–1.8%Nb alloy in nitric acid medium has been studied. The corrosion rates in liquid, vapour and condensate phases of boiling 11.5 M nitric acid determined by weight loss method for Ti–5Ta–1.8Nb was much lower compared to that of commercial purity titanium. The corrosion rate in condensate phase was higher than that in liquid and vapour phases. This has been explained by considering the formation of protective film on the sample exposed to the liquid and vapour phase and the inhibiting effect of titanium ions in the liquid phase. The microstructures after the various heat treatments consisted of different proportions of equiaxed and acicular α, martensite (α′) and retained β phases. The homogeneous equiaxed α phase in OA and AG conditions provided good corrosion resistance compared to acicular α or α′ phase in other heat treated conditions. Potentiodynamic anodic polarisation studies indicated spontaneous passivation in 11.5 M nitric acid.