Effect of platinum group metal addition on microstructure and corrosion behaviour of Ti–47·5 at-%Al

Plain and alloyed titanium aluminides of composition Ti–47·5 at-%Al were prepared with the addition of 1·0 at-% platinum group metals (PGMs). The as cast alloys were subjected to potentiodynamic scans in 5, 15 and 25 wt-%HCl solutions at room temperature, and the PGM containing alloys were assessed for their abilities to spontaneously passivate by cathodic modification. Plain titanium aluminide had a duplex microstructure consisting of lamellar (α2 and γ alternating lamellae) and γ-TiAl phase grains. The introduction of 1·0 at-%PGMs (platinum, palladium and iridium) led to the formation of a new phase, developing more in the γ-TiAl phase grains and a general improvement of corrosion resistance by increasing the corrosion potential to nobler values. Platinum group metal additions to plain TiAl resulted in the corrosion potentials falling in the passive region of plain TiAl, indicating spontaneous passivation of PGM alloyed TiAl in 5 and 15 wt-%HCl solutions. In 25 wt-%HCl solution, the addition of PGMs shifted the cathodic process in the transpassive or active region of plain TiAl, resulting in either case in the dissolution of the alloy due to the absence of an extended passivation region. The cathodic modification of PGM alloyed TiAl occurred as a result of PGM accumulation on the surface of the TiAl alloys, which simultaneously improved the hydrogen evolution efficiency and inhibited anodic dissolution.