Corrosion of Ti-2 and Ti-7 relevant to nickel acid leach chemistry

The corrosion of commercially pure unalloyed titanium (Ti-2) and titanium with 0.15 wt.% palladium (Ti-7) was studied in acid sulphate solutions with and without chlorides. Both immersion testing and electrochemical techniques were used to characterize the corrosion. Corrosion rates calculated from polarization experiments were related to maximum rates observed during immersion testing; the two values generally agreed within an order of magnitude. The palladium alloyed metal showed corrosion resistance superior to Ti-2. The presence of chlorides enhanced the corrosion, especially of the palladium alloy indicating that palladium was dissolving as a chloro-complex. Corrosion was initiated by pitting near the grain boundary. The final corrosion morphology of the two metals differed significantly. Corroded Ti-2 exhibited a rough morphology with preferential dissolution of the grain material while Ti-7 showed a smooth, bubbly surface when corroded with strong acid and chlorides. The samples were thoroughly characterized.