ON THE UNUSUAL MORPHOLOGY OF PITTING CORROSION OF AMORPHOUS NI-ZR ALLOYS

The pitting corrosion behaviour of the melt-spun amorphous Ni-Zr alloys was studied in 1 N HCl solution at room temperature. It was found that amorphous Ni-Zr alloys were pitted by anodic polarization at about -0.1 V(SCE), and their pitting potentials were lower than that of pure zirconium in the solution studied although the cationic fraction of Zr4+ in the surface film on amorphous Ni-Zr alloys was >0.97. In situ observation of the pitted surface of the specimen during the anodic polarization showed that the development of pits is accompanied by the evolution of hydrogen and cracks by hydrogen embrittlement. It is suggested that irregular surface morphology of the melt-spun alloys could be one of the main origins of their low pitting potentials. It is also pointed out that the internal tensile stress introduced during the rapid quenching can cause the unusual shape of the pits with cracks on the melt-spun amorphous Ni-Zr alloys.