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 a lloys of 1 and 2 mm in diameter have been obtained by a copper mould castin g method. These amorphous alloys are spontaneously passive in 6 M HCl at 30 3 K, and the corrosion resistance of the bulk alloys of 1 nm diameter in 6M HCl at 303 Ii; is as high as that of the rapidly quenched alloy ribbons wi th. the same composition. However potentiodynamic polarization reveals that the corrosion rate of the Ni-10Cr-5Ta-16P-4B alloy of 2 nun diameter is ab out 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 diffractio n patterns. The further detailed structural analysis using high resolution transmission electron microscopy indicates the presence of fee nickel preci pitates, 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 p recipitates, about 20 nm rite, of fee nickel, showing about two orders of m agnitude higher anodic current density than the amorphous single phase allo y of 1 nun diameter Thus, the precipitation of a nanocrystalline nickel pha se, which actively dissolves in 6M HCl, is detrimental to the corrosion res istance of the amorphous Ni-Cr-(Mo or Ta)-P-B alloys, although the smaller size of the precipitates is less detrimental. (C) 2001 Elsevier Science B.V . All rights reserved.