Analytical and electrochemical study of passive films formed on nickel—chromium alloys: Influence of the chromium bulk concentration

Depth composition and chemical states of elements constituting the overlayers of NixCr alloys (x = 0–30 at.%) passivated in borate buffer solutions (pH = 9.2) were determined as a function of the chromium bulk content of the alloy. Depth sputter profiling was performed using both Auger electron spectroscopy (AES) and low energy ion scattering spectroscopy (LEIS). Chemical bonding and (oxy‐hydroxy) structures of alloying elements in the passive films were investigated by x‐ray photoelectron spectroscopy (XPS). The electrochemical study mainly consists in establishing the cathodic reduction kinetics of the passive layers to characterize the resistance of the internal Cr₂O₃ barrier to reduction. Very thin films (less than 2.5 nm) were obtained in those conditions showing duplex structures where minor external nickel oxy‐hydroxide layers are depicted covering an inner protective barrier mainly composed of chromium oxide Cr₂O₃. Only at high bulk chromium contents (> 15 at.%) are complete Cr₂O₃ layers built at the interface with the metallic alloy. Beneath the film, in the underlying matrix, a metallic nickel enrichment combined with a chromium depletion is observed, which seems to confirm, as for FeCr alloys, a mechanism by which Cr oxidizes preferentially in this medium during the first steps of the film growth.