Hydroxyl transport mechanisms upon passivation of Cr-Fe-Co-Ni-Mo multi-principal element alloy surfaces investigated by isotopic labelling.

[Display omitted] • Interfacial passivation mechanisms studied on Cr- and Mo-containing alloy surfaces. • D 2 O labelling applied to study the growth of nanometer-thick passive oxide film. • D-labelled species mostly detected in outer hydroxide layer of passive film. • Minor inward hydroxyl diffusion from electrolyte during anodic passivation. • Key role of hydroxyls initially in native oxide in forming Cr hydroxide. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with deuterium labelling (D 2 O), and X-ray photoelectron spectroscopy (XPS) were applied to investigate the interfacial transport mechanisms of hydroxyls between aqueous electrolyte and outer part of the protective oxide film during passivation of Cr 15 Fe 10 Co 5 Ni 60 Mo 10 multi-principal element alloy surfaces. A ToF-SIMS signal treatment methodology was developed to retrieve the in-depth distribution of deuterated hydroxides in the nanometer-thick surface oxide film. After immersion at free potential or anodic passivation in acidified D 2 O, the initial bilayer structure is retained. The use of heavy water electrolyte has no effect on the composition alterations induced by passivation. Deuterated species were detected mostly in the outer layer of the surface oxide film. Pure inward hydroxyl diffusion from electrolyte/oxide to oxide/metal interface is excluded from being primarily responsible for oxide film growth induced by anodic passivation. The hydroxyls from the dissolving hydroxide species are retained in the oxide film and participate in forming Cr hydroxide, thus reducing the hydroxyl surface uptake from the electrolyte. This work provides deeper insight into the mechanisms of oxide growth and corrosion protection induced by anodic passivation. [ABSTRACT FROM AUTHOR]