Preparation of Corrosion-Resistant Bulk Amorphous Alloys

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Corrosion-resistant bulk Ni-Cr-Nb-P-B, Ni-Cr-Ta-P-B and Ni-Cr-Me-P-B alloys with cylindrical shape of 1 and 2 mm diameter are prepared by a copper mold casting method. A good correlation between the glass forming ability and stability of super-cooled liquid state has been found for these alloy systems except the molybdenum-containing alloy system. The bulk amorphous alloys thus prepared are spontaneously passive even in very aggressive 6M and 12M HCl solutions open to air at 303 K.The bulk amorphous alloys reveal the same anodic and cathodic polarization curves as the corresponding melt-spun alloy ribbons, as far as amorphous single phase alloys are obtained. Precipitation of fcc Ni phase in the bulk amorphous alloys is detrimental to the corrosion resistance of the alloys. The influence of the precipitation is, however, strongly dependent upon the size of the precipitates. When fcc Ni with size of 20〜nm is formed in the amorphous matrix, the alloys show about two orders of magnitude higher passive current density in comparison with the amorphous single phase alloys. In contrast, precipitation of fcc Ni with only 2-3 nm size results in the increase only twice in the passive current density. Bulk amorphous Ni-10Cr-5Nb-16P-4B alloy plates of 2 mm thickness are also prepared by consolidation of gas-atomized amorphous alloy powders by sheath rolling at a super-cooled liquid state, that is, 708 K.The bulk alloy thus prepared is found to reveal similar extremely low corrosion rate in 6M HCl at 303 K to the corresponding melt-spun alloy. However, in 12M HCl solution, the sheath-rolled alloy shows preferential corrosion attack at boundaries of the gas-atomized powders, possibly due to the influence of the oxidation of the particle surface during heating for consolidation. Thus, further improvement of the preparation processes to obtain highly corrosion-resistant bulk amorphous alloys using this method.