The effect of structural heterogeneity on the pitting corrosion behavior of melt-spun amorphous NiZr alloys

The pitting behavior of amorphous NiZr alloys prepared by melt-spinning was thoroughly investigated by placing an emphasis on the effect of the microstructure and surface composition. It was found that the alloy ribbons showed different pitting corrosion behavior along the thickness direction from the dull side to the shiny side in de-aerated 0.5 M H2SO4 solution with 0.1 M NaCl. The dull side surface was most susceptible to pitting corrosion, but it became difficult to occur if the as-prepared dull side surface was removed by mechanical polishing. The shiny side surface of all the alloys passivated spontaneously and the pitting potential increased with an increase in alloy zirconium content. TEM observation indicated that all the shiny side surfaces were wholly amorphous, while some microcrystalline phases existed in the amorphous matrix in the dull side. The crystalline phases formed in the dull side were identified as Ni3Zr and NiZr2 for the Ni30Zr and Ni70Zr alloys, respectively. The presence of both the microcrystalline phases in the amorphous matrix and the irregular surface morphology of the dull side were responsible for the origin of the pitting corrosion of the dull side surface. The high corrosion resistance of the shiny side surface was attributed to the high protectiveness of the passive film and the structural homogeneity of the alloy.