Corrosion Behavior of Ti-6Al-4V with Different Thermomechanical Treatments and Microstructures

Abstract

The corrosion behavior of four different microstructures of Ti-6Al-4V with varying volume fractions of primary α (0, 10%~20%, 40%~50%, and ~90%) was investigated in sodium chloride (NaCl) and hydrochloric acid (HCl) solutions. The microstructure constituent morphologies and distributions were characterized with optical and scanning electron microscopy (SEM) while constituent compositions were determined with energy-dispersive x-ray (EDX) analysis. All four micro-structures exhibited spontaneous passivity in 0.9% NaCl at 37°C and active/passive transitions in 1.5 M HCl at 37°C. Very little difference in corrosion rate was observed using potentiodynamic polarization curves and weight-loss tests. However, SEM analysis of corroded surfaces indicated that the fully lamellar structure (0% primary α) exhibited the worst attack. Analysis of surfaces after 48 h immersion in 3.5 M HCl revealed that α/β phase boundaries were preferential sites for corrosion attack. Also, β phase in lamellar structures exhibited preferential dissolution in comparison to α phase. The corrosion results are discussed in terms of microstructure and constituent composition. In particular, the extent of partitioning of vanadium to the β phase during heat treatment varies with the volume fractions of primary α and transform β, which affects the corrosion performance in aggressive environments.