Effects of Heat Treatment on Microstructures and Properties of Cold Rolled Ti-0.3Ni Sheets as Bipolar Plates for PEMFC

Abstract

As promising materials for bipolar plates substrate, as-cold rolled Ti-0.3Ni (wt.%) sheets were heat treated with three different processes in this work. As-cold rolled sheets consist of α matrix and dispersed Ti2Ni intermetallic precipitates, and typical Widmanstatten microstructure can be observed after heat treatment. Lamellar Ti2Ni precipitates inside the colonies. Elongation of as-cold rolled sheets equals less than 7% while this value rises up to around 20%, and tensile strength decreases by more than 47% after heat treatment. Open circuit potentials of as-cold rolled sheets treated at 950 °C for 1 h followed by wind cooling (950 °C/1 h/WC), sheets aged at 500 °C for 3 h followed by air cooling (950 °C/1 h/WC + 500 °C/3 h/AC), and sheets treated at 950 °C for 1 h followed by furnace cooling (950 °C/1 h/FC) equals −0.536 V, −0.476 V, −0.486 V, −0.518 V, respectively. A potentiodynamic polarization test reveals that all of the specimens exhibit typical active–passive transition behavior. Sheets treated at 950 °C/1 h/WC possess the lowest corrosion current density (155.4 μA·cm−2). Results of electrochemical impedance spectroscopy (EIS) show that 950 °C/1 h/WC treated sheets possess the largest polarization resistance (Rpol), 122.6 Ω·cm2. Moreover, steady-state current densities (Iss) increase in the order of 950 °C/1 h/WC, 950 °C/1 h/WC + 500 °C/3 h/AC, 950 °C/1 h/FC according to the results of potentiostatic polarization. This can be attributed to various amounts of Ti2Ni precipitation caused by different cooling rates.