Corrosion behaviour of amorphous Ni–Cr–Nb–P–B bulk alloys in 6M HCl solution

Amorphous Ni–Cr–Nb–16P–4B bulk alloys highly corrosion-resistant in a 6 M HCl solution have been obtained by copper mould casting. The as-cast Ni–Cr–Nb–16P–4B alloys (Cr, Nb=5 or 10 at.%) of 1 mm diameter are composed of an amorphous single phase, and they are spontaneously passive in the 6 M HCl solution at 303 K. Their polarization curves are almost the same as those of the corresponding rapidly quenched amorphous alloy ribbons. A larger Ni–10Cr–5Nb–16P–4B bulk specimen of about 20×20×2 mm in size has been also prepared by consolidation of gas-atomized amorphous alloy powders by sheath rolling at 708 K, at which the alloy is in the super-cooled liquid state. Although X-ray diffraction pattern of the specimen revealed a halo, typical of amorphous structure, its corrosion resistance in the 6 M HCl is not as high as that of the as-cast one. After immersion of the sheath-rolled specimen in the 6 M HCl solution, spherical pit-like attacks have been observed on the surface, suggesting the presence of nanocrystalline precipitates locally. Thus, for the Ni–Cr–Nb–P–B alloys the formation of single amorphous phase appears to be necessary to have high corrosion resistance in aggressive environments.