Thermodynamics magnesium alloys

Cadmium-magnesium alloy (Cd5Mg5), calculation of thermodynamic quantities, 136, 142 Cadmium-tin alloy (Cd5Sn5), calculation of thermodynamic quantities, 136 Carbanion, 180 Carbon bound, 367... 

Application of SPD techniques to metal hydrides is a new field of research. Skripnyuk and Rabin were the first to use ECAP to improve the hydrogen storage properties of Mg-based alloys [254, 255]. The first study on ECAP-processed magnesium alloy ZK90 showed an improvement in sorption kinetics without loss of hydrogen capacity or change in thermodynamic parameters [254]. figure 4.12 shows comparable results for the Mg-Ni eutectic alloy [255]. 

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

The thermodynamic data of the compounds formed by magnesium alloyed with rare earths and thorium are reported in table 3. There appear to be serious discrepancies between the values of the Gibbs energies of formation of the RMg compounds obtained by Ogren et al. (1967) and later by Pahlman and Smith (1972) (fig. 16). 

Most barium compounds are not as thermodynamically stable as the corresponding compounds of magnesium and calcium and therefore can be reduced by these metals. However, rather than producing pure barium, barium alloys are formed. Barium combines with most metals, forming a wide range of alloys and intermetaUic compounds. Among the phase systems that have been better characterized are those with Ag, Al, Bi, Hg, Pb, Sn, Zn, and the other Group 2 metals (12). 

B. Bogdanovic, K. Bohmhammel, B. Christ, A. Reiser, K. Schlichte, R. Vehlen, U. Wolf, Thermodynamic investigation of the magnesium-hydrogen system, J. Alloys Compd. 282 (1999) 84-92. 

Z. Dehouche, R. Djaozandry, 1. Huot, S. Body, 1. Goyette, T.K. Bose, R. Schulz, Influence of cycling on the thermodynamic and structure properties of nanocrystalhne magnesium based hydride, J. Alloys Compd. 305 (2000) 264-271. 

N.E. Tran, S.G. Lambrakos, M.A. Imam, Analyses of hydrogen sorption kinetics and thermodynamics of magnesium-misch metal alloy, J. Alloys Compd. 407 (2006) 240-248. 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

Bogdanovic, B., Bohmhammel, K., Christ, B., Reiser, A., Schlichte, K., Vehlen, R. and Wolf, U. (1999) Thermodynamic investigation of the magnesium-hydrogen system. Journal of Alloys and Compounds, 282, 84—92. 

Influence of cycling on the thermodynamic and structure properties of nanocrystalline magnesium based hydride, Journal of Alloys and Compounds, 305, 264-271. 

The most common matrices are the low-density metals, such as aluminum and aluminum alloys, and magnesium and its alloys. Some work has been carried out on lead alloys, mainly for bearing applications, and there is interest in the reinforcement, for example, of titanium-, nickel- and iron-base alloys for higher-temperature performance. However, the problems encountered in achieving the thermodynamic stability of fibers in intimate contact with metals become more severe as the potential service temperature is raised, and the bulk of development work at present rests with the light alloys. 

For nearly all metals, oxidation is a thermodynamically favorable process in air at room temperature. When the oxidation process is not inhibited in some way, it can be very destructive. Oxidation can also form an insulating protective oxide layer, however, that prevents further reaction of the underlying metal. On the basis of the standard reduction potential for Al, for example, we would expect aluminum metal to be very readily oxidized. The many aluminum soft-drink and beer cans that litter the environment are ample evidence, however, that aluminum undergoes only very slow chemical corrosion. The exceptional stability of this achve metal in air is due to the formation of a thin protective coat of oxide—a hydrated form of AI2O3—on the surface of the metal. The oxide coat is impermeable to O2 or H2O and so protects the underlying metal from further corrosion. Magnesium metal is similarly protected. Some metal alloys, such as stainless steel, likewise form protective impervious oxide coats. 

This chapter presents electrochemical reactions and corrosion processes of Mg and its alloys. First, an analysis of the thermodynamics of magnesium and possible electrochemical reactions associated with Mg are presented. After that an illustration of the nature of surface films formed on Mg and its alloys follows. To comprehensively understand the corrosion of Mg and its alloys, the anodic and cathodic processes are analyzed separately. Having understood the electrochemistry of Mg and its alloys, the corrosion characteristics and behavior of Mg and its alloys are discussed, including self-corrosion reaction, hydrogen evolution, the alkalization effect, corrosion potential, macro-galvanic corrosion, the micro-galvanic effect, impurity tolerance, influence of the chemical composition of the matrix phase, role of the secondary and other phases, localized corrosion and overall corrosivity of alloys. 

Phil] Phillips, H.W.L., The Constitution of Alloys of Aluminium with Magnesium, Silicon and Iron , J. Inst. Met., 72, 151-227 (1946) (Phase Diagram, Experimental,, , 86) [1946Phi2] Phillips, H. W.L., The Application of Some Thermodynamic Principles to flic Liquidus Sur-... 

Cao] Cao, R., Li, G., Wu, X., Some Thermodynamic Properties in Process of Thermal Reduction of Magnesium with High Aluminium Alloy (in Chinese), Acta Metall. Sin., 21, A471-A476... 

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