The Nature of Metals and Alloys

About eighty of the more than one hundred elementary substances are metals. A metal may be defined as a substance that has large conductivity of electricity and of heat, has a characteristic luster, called metallic luster, and can be hammered into sheets (is malleable) and drawn into wire (is ductile) in addition, the electric conductivity increases with decrease in temperature.  

The metals themselves and their alloys are of great usefulness to man, because of the properties characteristic of metals. Our modern civilization is based upon iron and steel and valuable alloy steels are made that involve the incorporation with iron of vanadium, chromium, manganese, cobalt, nickel, molybdenum, tungsten, and other metals. The importance of these alloys is due primarily to their hardness and strength. These 

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The study of corrosion is essentially the study of the nature of the metal reaction products (corrosion products) and of their influence on the reaction rate. It is evident that the behaviour of metals and alloys in most practical environments is highly dependent on the solubility, structure, thickness, adhesion, etc. of the solid metal compounds that form during a corrosion reaction. These may be formed naturally by reaction with their environment (during processing of the metal and/or during subsequent exposure) or as a result of some deliberate pretreatment process that is used to produce thicker films or to modify the nature of existing films. The importance of these solid reaction products is due to the fact that they frequently form a kinetic barrier that isolates the metal from its environment and thus controls the rate of the reaction the protection afforded to the metal will, of course, depend on the physical and chemical properties outlined above. 

The environment has negative effects on most metals thus, when metallic archaeological objects are eventually found, they are generally in an advanced state of decay. The decay of metals and alloys caused by the chemical action of gases and/or liquids in the environment is known as corrosion. Corrosion processes are natural destructive processes that result in the waste of most metals and alloys. The ultimate result of all corrosion processes is the reversion of most metals from the metallic condition in which they are used, to the chemically combined form in which they naturally occur in the crust of the earth. Rust, the reddish-brown corrosion product that forms on... 

Outdoor tests aim at assessing the behaviour of metals and alloys in natural environments (as compared to artificial media used in laboratory tests). These are generally long-term tests, which can last for several years. It is not uncommon to find reports on test results carried out over 20 years and longer in outdoor exposure stations. 

Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-ceU reactions which control the rate of corrosion. 

Fig. 21.1 Practical galvanic series of metals and alloys showing potentials on the hydrogen scale. (Note that the potentials shown are typical values that will vary according to the nature of the solution.) (after Butler, G. and Ison, H. C. K., Corrosion and its Prevention in Water, Leonard Hill, London (1966))... 

Finally, the use of simple valence bond theory has led recently to a significant discovery concerning the nature of metals. Many years ago one of us noticed, based on an analysis of the experimental values of the saturation ferromagnetic moment per atom of the metals of the iron group and their alloys, that for a substance to have metallic properties, 0.72 orbital per atom, the metallic orbital, must be available to permit the unsynchronized resonance that confers metallic properties on a substance.34 38 Using lithium as an example, unsynchronized resonance refers to such structures as follows. 

The Natural Passivation and Corrosion of Metals and Alloys XPS studies of the air-formed natural passive layer on aluminium surfaces have identified a number of hydroxides as well as alumina (Barr, 1977). The oxidation of pure iron and of stainless steels and other iron alloys have also been extensively... 

FIGURE 39 Corrosion. Corrosion is the process of gradual deterioration of metals and alloys as a result of their interaction with the environment. The corrosion process is a reversal of metallurgical processes, whereby metals are recovered from the minerals in which they occur in nature (a). It is an electrolytic process, brought about by the passage of electric currents. Any metal or alloy contains sites in which there are slight local compositional differences. When such compositional differences are exposed to a humid or wet environment, extremely small electrolytic cells as the one shown in (b) are created in each cell, an electric current drives the otherwise nonspontaneous corrosion reactions. In a surface undergoing corrosion there are millions of electrolytic cells. 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

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