Repassivation Behavior of Individual Grain Facets on Dilute Ni–Cr and Ni–Cr–Mo Alloys in Acidified Chloride Solution

The effects of crystal orientation and prior etching on the polarization and repassivation behavior of Ni-Cr and Ni-Cr-Mo alloys have been investigated in acidic chloride environment using dc potentiostatic and ac single-frequency electrochemical impedance spectroscopy. Tests were conducted within the passive region at potentials where local oxide breakdown was possible. Surface morphologies of grains across a wide range of orientations were measured before and after passivation using atomic force microscopy. Oxide growth was monitored on isolated low-and high-index crystallographic planes as a function of repassivation time, enabling the independent measurement of the oxidation and total anodic current densities. Grains exhibited the tendency to either passivate with significant or minimal oxidation or instead resist active passivation and repassivation. The oxidation performance was a function of the crystallographic orientation of the exposed grain surface. The oxides grown on various-orientated surfaces differed in their film growth kinetics, morphology, and steady state thicknesses, even given similar total anodic current densities due to dissimilar oxidation efficiencies. For Ni-11 wt % Cr, only orientations close to (1 0 1) were able to form stable passive films, aided by nanofaceted surfaces with a matchstick-type morphology. On the other hand, all orientations of the Ni-11 wt % Cr-6 wt % Mo alloys formed electrochemically stable oxides films owing to the beneficial influence of Mo on repassivation. Passivation, breakdown behavior, and oxide properties vary with crystal orientation, and this work provides insight into the effects of crystallographic orientation and oxide film morphology on the passivation mechanism of Ni-Cr and Ni-Cr-Mo alloys.