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 ove rlayers of Ni-xCr alloys (x = 0-30 at.%) passivated in borate buffer s olutions (pH = 9.2) were determined as a function of the chromium bulk content of the alloy. Depth sputter profiling was performed using bot h Auger electron spectroscopy (AES) and low energy ion scattering spec troscopy (LEIS). Chemical bonding and (oxy-hydroxy) structures of allo ying elements in the passive films were investigated by x-ray photoele ctron (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 dup lex structures where minor external nickel oxy-hydroxide layers are de picted covering an inner protective barrier mainly composed of chromiu m oxide Cr2O3. Only at high bulk chromium contents (> 15 at.%) are com plete Cr2O3 layers built at the interface with the metallic alloy. Ben eath 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.