Voltammetric, optical, and spectroscopic examination of anodically forced passivation of cobalt-tungsten amorphous alloys

Passivation and corrosion of thin layers of amorphous Co-W 77.5W-22.5 (at%) alloy were investigated using potentiometry with linear current change, voltammetry, and anodic current pulsation. The state and the composition of the anodized alloy surface were determined by x-ray diffraction, electron microprobe spectroscopy, and optical microscopy. Investigations performed in noncomplexing electrolytes indicated that passive layers on the alloy are easily formed and are stable in acidic solutions. They can be formed also in alkaline solutions, under conditions of larger current. In neutral solutions an effective protective layer cannot be obtained during anodization. In this pH range, selective oxidation of cobalt takes place, and amorphous tungsten is formed on the surface. The tungsten layer is loose and does not protect the base metal against further dissolution. Prolonged application of anodic current to the amorphous layers, at any pH, leads to the appearance of cracks on the surface. Also the behavior of amorphous and polycrystalline, homogeneous alloys were compared.