Reactive elements alloys

Lead and its alloys are generally melted, handled, and refined in cast-iron, cast-steel, welded-steel, or spun-steel melting ketdes without fear of contamination by iron (qv). Normal melting procedures require no dux cover for lead. Special reactive metal alloys require special alloying elements, duxes, or covers to prevent dross formation and loss of the alloying elements. 

As copper is not an inherently reactive element, it is not surprising that the rate of corrosion, even if unhindered by films of insoluble corrosion products, is usually low. Nevertheless, although the breakdown of a protective oxide film on copper is not likely to lead to such rapid attack as with a more reactive metal such as, say, aluminium, in practice the good behaviour of copper (and more particularly of some of its alloys) often depends to a considerable extent on the maintenance of a protective film of oxide or other insoluble corrosion product. 

In the case of alloys having one constituent considerably more reactive to oxygen than the others, conditions of temperature, pressure and atmosphere may be selected in which the reactive element is preferentially oxidised. Price and Thomas used this technique to develop films of the oxides of beryllium, aluminium, etc. on silver-base alloys, and thereby to confer improved tarnish resistance on these alloys. If conditions are so selected that the inward diffusion of oxygen is faster than outward diffusion of the reactive element, the oxide will be formed as small dispersed particles beneath the surface of the alloy. The phenomenon is known as internal oxidation and is of quite common occurrence, usually in association with a continuous surface layer of oxides of the major constituents of the alloy. 

The trend of intermetallic reactivity and alloy stability of V, Nb and Ta with the different elements may be further discussed in terms of the melting points of the compounds as described in the following paragraphs. 

Much more efficient method of the synthesis of metal hydrides is a one-step method of reactive ball milling of elemental metal directly under hydrogen gas. The method of reactive mechanical alloying/milling of either elemental metals or preaUoyed... 

Between 1980 and about 2000 most of the studies on the electrodeposition in ionic liquids were performed in the first generation of ionic liquids, formerly called room-temperature molten salts or ambient temperature molten salts . These liquids are comparatively easy to synthesize from AICI3 and organic halides such as Tethyl-3-methylimidazolium chloride. Aluminum can be quite easily be electrode-posited in these liquids as well as many relatively noble elements such as silver, copper, palladium and others. Furthermore, technically important alloys such as Al-Mg, Al-Cr and others can be made by electrochemical means. The major disadvantage of these liquids is their extreme sensitivity to moisture which requires handling under a controlled inert gas atmosphere. Furthermore, A1 is relatively noble so that silicon, tantalum, lithium and other reactive elements cannot be deposited without A1 codeposition. Section 4.1 gives an introduction to electrodeposition in these first generation ionic liquids. 

The electrodeposition of reactive elements like Al, Si, Ge, Ta and a few others is possible. As discussed in Chapter 4.4 the successful electrodeposition of Ti, Mg, Mo and many others in relevant layer thicknesses has not yet been described, though attempts have been made in some cases. Apart from the availability of suitable precursors there is at least one other issue to consider ionic liquids can be reactive. It was found that magnesium and its alloys can form passivating films in ionic liquids with the bis(trifluoromethylsulfonyl)amide (Tf2N) anion, especially in the presence of water. It was found by two of our groups (Endres, MacFarlane) that, under certain circumstances, the Tf2N ion is subject to cathodic... 

Figure 6.28. a) Effect of small additions of an alloying element on the interfacial or liquid surface energies of a non-reactive binary alloy/ceramic system for very positive and very negative values of adsorption energy, b) A very negative value of the slope of 0 implies a negligible slope... 

The objective of surface modification by CVD is to develop additional corrosion protection for high-temperature alloys (e.g., Fe-25Cr-0.3Y) beyond that achieved by reactive-element additions which will be effective for a long period. Because CVD is a high temperature process, the following topics must be discussed before we can develop successful high-temperature corrosion-proteetion coatings. ... 

An important result of the present analytical study is the detection of S in Y-rich precipitates within the alloy. This observation supports the sulfur effect model", which proposes that reactive elements getter the S [16,17]. The presence of S reduces the surface energy of the metal and thereby promotes the formation of interfacial voids, as has been shown by Grabke ct al. [18]. Although no S-rich particles have been found in NiAl - 0.2 wt% Zr. it is possible that the Zr can lower the activity of S by solute - solute interaction. Further work is necessary in order to study the interaction between reactive elements and S. 

After oxidation at 1200 C a-A1203 was the only A1203 phase present in the oxide scale. Analytical studies revealed the segregation of reactive elements (Zr, Y) at the oxide grain boundaries as well as at the metal/oxide interface. Analytical studies on NiAl-0.1 Y further more indicate that Y can getter the S in the alloy. 

Oxide dispersions offer an excellent opportunity to improve both the mechanical properties and the oxidation resistance by the addition of a reactive element (RE) oxide such as Y203. (Commercial ODS alloys typically contain a mixed Y203-A1203... 

Table 3. Composition of iron-aluminium-chromium alloys investigated MM mischmetal, a mixture of reactive elements (mainly cerium and lanthanum) composition of alloy 800H nominal composition...

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