Some amorphous alloys possess extremely high corrosion resistance to aggressive environments such as concentrated HCl where conventional metallic materials cannot be used. Copper mold casting is the easiest and simplest method to form amorphous bulk alloy rods, and if copper-mold cast 1 mm diameter rods become amorphous, desirable shapes of amorphous bulk alloys can be processed in the super-cooled liquid state of the amorphous alloy powders. The present work applying copper-mold casting aimed to tailor amorphous Ni-Cr-Mo-Ta-Nb-P alloys resistant to concentrated hydrochloric acids. The roles of corrosion-resistant alloying elements were studied mostly by XPS.

Ni-(1-4)Cr-1Mo-22Ta-14Nb-7P and Ni-1Cr-(1-8)Mo-22Ta-14Nb-7P alloy rods of 1 mm diameter were identified as the amorphous single phase, while Ni-5Cr-1Mo-22Ta-16Nb-7P and Ni-1Cr-10Mo-22Ta-16Nb-7P alloy rods in addition to Ni-0Cr-1Mo-22Ta-16Nb-7P and Ni-1Cr-0Mo-22Ta-16Nb-7P alloys were composed of major amorphous and minor nanocrystalline phases. The single amorphous phase alloys were immune to corrosion in 12 M HCl at 30°C but the alloys with a nanocrystalline phase showed the detectable corrosion loss. All alloys including those of phase mixtures were immune to corrosion in 6 M HCl at 30°C. They were all spontaneously passive. The presence of molybdenum was prerequisite for immunity to corrosion but the excess increase in molybdenum resulted in the increase in the corrosion loss.

According to XPS the air-formed film contained Mo⁴⁺, Mo⁵⁺ and Mo⁶⁺ in addition to concentrated Ta⁵⁺, Cr³⁺ and Nb⁵⁺, consisting of the outer triple oxyhydroxide of Ta⁵⁺, Cr³⁺ and Nb⁵⁺ and the inner Mo⁴⁺ oxide. The high stability of the alloys with these air-formed films were responsible for spontaneous passivity in concentrated hydrochloric acids. The outer triple oxide seemed to be stable in the concentrated hydrochloric acids and the inner Mo⁴⁺ oxide was assumed to act as the barrier for diffusion of matters through the film.

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