Microstructural effects on corrosion behaviour of investment cast Ti-6.5Al-3.4Mo-1.7Zr alloy

Ti-6.5Al-3.4Mo-1.7Zr was cast as rods in a graphite mould using vacuum induction furnace. The cast rods were hot swaged and then heat treated using two regimes, which resulted in fine and coarse lamella structures. The influence of these different structures on the corrosion behaviour of alpha + beta Ti alloy samples was investigated. Potentiodynamic polarisation in 3.5% NaCl solution showed that the swaged condition exhibited the highest corrosion resistance compared to the others. After isothermal oxidation for different times (2, 4, 8 and 16 h), maximum weight gain was obtained after 8 h and the corrosion resistance of the swaged material was significantly higher than the other conditions. Thereby, isothermal oxidation enhanced markedly the corrosion resistance of the studied Ti alloy. Among various conditions of the isothermally oxidised samples, the as cast condition showed the highest value of pitting potential.