Preparation of corrosion-resistant amorphous Ni–Cr–P–B bulk alloys containing molybdenum and tantalum

Cylindrical amorphous Ni–15Cr–10Mo–16P–4B and Ni–(10 and 15)Cr–5Ta–16P–4B alloys of 1 and 2 mm in diameter have been obtained by a copper mould casting method. These amorphous alloys are spontaneously passive in 6 M HCl at 303 K, and the corrosion resistance of the bulk alloys of 1 mm diameter in 6 M HCl at 303 K is as high as that of the rapidly quenched alloy ribbons with the same composition. However, potentiodynamic polarization reveals that the corrosion rate of the Ni–10Cr–5Ta–16P–4B alloy of 2 mm diameter is about three times higher than that of the alloy of 1 mm diameter, though both sizes of the specimens are identified to be amorphous from X-ray diffraction patterns. The further detailed structural analysis using high resolution transmission electron microscopy indicates the presence of fcc nickel precipitates, 2–3 nm size, in the amorphous matrix only for the specimen of 2 mm diameter. The Ni–15Cr–10Mo–16P–4B alloy of 2 mm diameter contains larger precipitates, about 20 nm size, of fcc nickel, showing about two orders of magnitude higher anodic current density than the amorphous single phase alloy of 1 mm diameter. Thus, the precipitation of a nanocrystalline nickel phase, which actively dissolves in 6 M HCl, is detrimental to the corrosion resistance of the amorphous Ni–Cr–(Mo or Ta)–P–B alloys, although the smaller size of the precipitates is less detrimental.