Metastable Alloys

Abstract

A. Structural characteristics B. Corrosion-resistant alloys in aqueous solutions C. Factors determining the high corrosion resistance of amorphous alloys C1. Passive films rich in cations of alloying elements with high passivating ability C2. Homogeneous nature of amorphous alloys C3. High activity of amorphous alloys D. Alloys resistant to aqueous corrosion D1. Aluminum corrosion-resistant metal alloys D2. Chromium-refractory metal alloys D3. Molybdenum corrosion-resistant metal alloys D4. Tungsten corrosion-resistant metal alloys E. Alloys resistant to sulfidizing/oxidizing environments at high temperatures F. Summary References A. STRUCTURAL CHARACTERISTICSAmorphous alloys consist of at least two components and have no long-range atomic order. They are produced by a variety of methods based on rapid solidification of the alloy constituents from the gas, liquid, and aqueous phases. Mechanical alloying, that is, solid-state mixing, is also effective for preparation of amorphous alloy powders. Vitrification of metal surfaces is also made by destruction of the long-range atomic order in the surfaces of solid metals.The formation of the structure with no long-range atomic order is based on the prevention of solid-state diffusion during solidification, and hence the alloys are free of compositional fluctuations formed by solid-state diffusion, such as second phases, precipitates, and segregates. The amorphous alloys are therefore regarded as ideal, chemically homogeneous alloys composed of thermodynamically metastable single-phase solid solutions supersaturated with alloy constituents. This characteristic is particularly suitable in producing new alloys possessing specific properties. Even if amorphous single-phase alloys are not formed, alloys prepared by amorphization methods are often composed of nanocrystalline phases supersaturated with alloying elements. From a corrosion point of view they can be considered as homogeneous alloys. B. CORROSION-RESISTANT ALLOYS IN AQUEOUS SOLUTIONSThe corrosion behavior of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous Fe-Cr-metalloid alloys was reported in 1974 [1]. The preparation of amorphous iron-based alloys by rapid quenching from the liquid state using melt spinning generally requires the alloys to contain large amounts of metalloids, which are mostly close to the eutectic compositions.The addition of chromium to amorphous Fe-metalloid alloys is particularly effective in enhancing corrosion resistance. For instance, amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 M HCI at ambient temperature [2]. (The number denoting the concentration of an alloy element in amorphous alloy formulas is expressed as an atomic percent unless otherwise stated.)Uhlig's Corrosion Handbook, Third Edition, Edited by R. Winston Revie