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

The corrosion behavior of four different microstructures of Ti-6Al-4V with varying volume fractions of primary a (0, 10%similar to 20%. 40%similar to 50%, and similar to 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 microstructures exhibited spontaneous passivity in 0.9% NaCl at 37 degrees C and active/passive transitions in 1.5 M HCl at 37 degrees 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 alpha) exhibited the worst attack. Analysis of surfaces after 48 h immersion in 3.5 M HCl. revealed that alpha/beta phase boundaries were preferential sites for corrosion attack. Also, beta phase in lamellar structures exhibited preferential dissolution in comparison to alpha phase. The corrosion results are discussed in terms of microstructure and constituent composition. In particular. the extent of partitioning of vanadium to the beta phase during heat treatment varies with the volume fractions of primary a and transform p, which affects the corrosion performance in aggressive environments.