Role of Alloying Elements in Passive and Transpassive Behavior of Ni-Cr-Based Alloys in Borate Buffer Solution

The role of the alloy composition of binary Ni-xCr alloys (x = 16, 20, 24, 28 wt.%) and ternary Ni-xCr-8Fe alloys (x = 14, 22, 30 wt.%) in borate buffer solution on the passive and transpassive behavior was investigated using successive electrochemical impedance measurements. These measurements supplement conventional polarization curves by enabling monitoring of the evolution of the electrochemical parameters extracted from impedance diagrams (thickness, resistivity at interfaces, etc) during the polarization. The polarization curves and evolution of the impedance parameters revealed that the Cr content and addition of Fe did not significantly affect the behavior of the alloys within the passive potential domain but directly affected the transpassivation process at higher potentials. For the Ni-xCr alloys, a higher Cr content resulted in a higher current-density peak for the transpassivation-second passivation processes. Conversely, the addition of Fe reduced or even prevented the appearance of this current-density peak. X-ray photoelectron spectroscopy analysis provided complementary information to help explain the role of alloying elements in the passivation (and secondary oxidation) mechanisms of Ni-based alloys in borate buffer solution.