Passivity and its breakdown in Al-based amorphous alloys

The effect of Cl− ions (0–1 M NaCl) on the anodic behaviour of two series of sputter-deposited Al–Ta (11–46 at.%) and Al–Nb (13–46 at.%) amorphous alloys was investigated in a neutral electrolyte by using electrochemical methods and the results were compared with those for Al–Mo and Al–W alloys. In solutions containing Cl− the alloys broke down at and above the pit nucleation potential Enp. They exhibited a stable passivity within a much wider (ΔE > 1 V) potential range than crystalline Al. The anodically formed oxide films in Cl−-free electrolyte were characterized by surface analytical measurements including high-energy resolution AES. Galvanostatic polarization was used to produce thicker anodic films than those formed under natural conditions, to facilitate detailed examination of their depth composition profiles. It was found that the anodic films grown on Al–Ta and Al–Nb alloys had a homogeneous structure in contrast to those on Al–Mo and Al–W. Transition metal Ta or Nb was detected at any depth of the composition profile. A relative enrichment of refractory metal within the inner part of the anodic film was not found, for these alloys. Tentative mechanisms of enhanced stability of the passive state of the Al-based amorphous alloys (AAs) are discussed.