Influence of microstructure on corrosion behavior of Ti–5%Ta–1.8%Nb alloy

This paper presents the results of a study on the influence of microstructure on the corrosion behavior of a a–b Ti–5%Ta–1.8%Nb alloy—a candidate material for use in high concentrations of boiling nitric acid. The ‘‘as cast’’ alloy had a lamellar structure and showed a corrosion rate of about 1.5 mpy. Thermo-mechanical processing of the cast alloy resulted in a structure of predominantly of equiaxed a with random distribution of fine b particles. This ‘‘reference’’ structure was further modified employing different heat treatments similar to that for commercial titanium alloys such as mill annealing, solution treatment and aging or over aging treatments. Corrosion rates evaluated in boiling nitric acid in the liquid, vapor and condensate phases, showed low values ~1 mpy. Of these, the lowest corrosion rate (~0.03 mpy) was exhibited by the structure with minimum amount of b phase, distributed in an equiaxed a matrix. This structure was obtained by aging of the solution treated ‘‘reference’’ alloy. Hence, solution treatment high in the a + b phase field followed by aging at a temperature low in the a + b phase field has been identified as the optimum treatment to obtain a microstructure with superior corrosion resistance.