Passive film evolution on Ni-Cr-Mo alloys in acidic chloride solution during anodic polarization

• In-situ synchrotron AP-XPS analysis of thin passive films formed on Alloy 59 and Alloy 625 with an adlayer of electrolyte on the surface. • Ex-situ GD-OES analysis of thick transpassive films (tens of nm) on the two Ni-base superalloys formed after polarization at high potentials. • Under transpassive conditions, the passive film thickens significantly and becomes enriched in high valence Mo- and Nb-oxides. Passivity arises from spontaneous formation of a thin protective passive film (oxide/hydroxide layer) on the metal surface, and the stability of passive film is of great importance for corrosion resistance. This study investigates the formation, stability, and transpassive dissolution of passive films on two Ni-based alloys, Alloy 625 and Alloy 59, in acidified NaCl solution, by combining in situ electrochemical synchrotron ambient pressure X-ray photoelectron spectroscopy (AP-XPS) with ex situ glow discharge optical emission spectroscopy (GD-OES) and electrochemical testing, as well as TEM and chemical analyses of the electrolyte. The use of these techniques enables a detailed analysis of chemical states of alloying elements and their distribution within passive films. AP-XPS results reveal a Cr-rich oxide in the thin passive film, which also contains Mo- and Nb-oxides enriched in the near-surface region, and a Cr-hydroxide layer on top of the surface. At increased polarization potentials, low valence Mo- and Nb-components are further oxidized to higher valence components. GD-OES results show that, in the transpassive potential region, the oxide film can grow to several tens of nm thick, while the two Ni-base alloys exhibit quite different behavior. For Alloy 625, Cr is depleted, Ni is almost not present, while Mo and Nb are dominant in the thick transpassive oxide film. In contrast, for Alloy 59 (Nb-free alloy), Cr and Mo remain enriched in the near-surface region and near the base metal, and Ni is present but below 10% in the thick transpassive oxide film. Nb in Alloy 625 forms a stable oxide and inhibits transpassive dissolution, thus contributing to corrosion resistance. By elucidating the fundamental mechanisms governing passivity breakdown, this study provides critical insights for the development of advanced Ni-based materials with enhanced corrosion resistance in aggressive environments.