Spontaneous passivity of amorphous bulk Ni–Cr–Ta–Mo–Nb–P alloys in concentrated hydrochloric acids

Abstract

Rod-shaped amorphous bulk Ni–Cr–Mo-22 at.%Ta-14 at.%Nb–P alloys resistant to concentrated hydrochloric acids were prepared by copper-mold casting. Alloys of amorphous single phase and mixture of nanocrystalline phases in the amorphous matrix were all spontaneously passive in 6 and 12 M HCl and were immune to corrosion in 6 M HCl, although the corrosion weight loss was detected for heterogeneous alloys in 12 M HCl. Spontaneous passivation is due to presence of stable air-formed films in which chromium was particularly concentrated in addition to enrichment of tantalum and niobium. The angle resolved X-ray photoelectron spectroscopy revealed that chromium and molybdenum are rich in the inner part of the film. The major molybdenum species is in the tetravalent state, although penta- and hexavalent state molybdenum is also included. The high corrosion resistance was interpreted in terms of the high stability of the outer triple oxyhydroxide, Cr1−x−yTaxNbyOz(OH)3+2x+2y−2z, and the effective diffusion barrier of the inner Mo4+ and Cr3+ oxide layer.