Metal dissolution selective, alloys

The Raney metals discussed above are typical examples of production by selective removal. The aluminum of the alloy with the desired catalytically active metal is selectively dissolved by treatment with alkali. Because dissolution is performed at low temperatures, i. e. below ca 100 °C, sintering of the remaining metal particles does not occur. Despite the many papers on Raney nickel [3-12], less has been published on the preparation of other Raney metals. The production of the initial aluminum alloy and the thermal treatment of the alloy has not been extensively investigated. The phase diagram of the nickel-aluminum system is rather complicated. 

In this chapter, we review dealloyed core-shell nanoparticle catalysts, which were synthesized by selective dissolution of transition metals from the surface of a transition-metal-rich Pt alloys (e.g., PtMs). Figure 18.1 shows an illustration of the... 

Chapters 18-21 discuss core-shell and advanced Pt alloy catalysts (which also can be considered to have a core-shell structure). Chapter 18 studies the fundamentals of Pt core-shell catalysts synthesized by selective removal of transition metals from transition metal-rich Pt alloys. Chapter 19 outlines the advances of core-shell catalysts synthesized by both electrochemical and chemical methods. The performance, durability, and challenges of core-shell catalyst in fuel cell applications are also discussed. Chapter 20 reviews the recent analyses of the various aspects intrinsic to the core-shell structure including surface segregation, metal dissolution, and catalytic activity, using DFT, molecular dynamics, and kinetic Monte Carlo. Chapter 21 presents the recent understanding of activity dependences on specific sites and local strains in the surface of bulk and core-shell nanoparticle based on DFT calculation results. 

The slip dissolution model assumes that plastic deformation at the crack tip is responsible for the activation. But other mechanisms can have the same effect. Tensile stress at the crack tip could, for example, break a brittle tarnish film or passive oxide film, thereby exposing the base metal to the electrolyte. Selective dissolution of alloy components at the crack tip could locally weaken the metal matrix and thus permit... 

The electrochemical studies of the corrosion inhibition process of Al-Mg-Si alloy in seawater using three selected natural products as corrosion inhibitors show that the corrosion rate of the alloy significantly reduced upon the addition of studied inhibitors. PP measurement reveals that the studied inhibitors can be classified as mixed-type inhibitors without changing the anodic and cathodic reaction mechanisms. The inhibitors inhibit both anodic metal dissolution and also cathodic hydrogen evolution reactions. 

The analytical solution of the transient diffusion problem of selective anodic dissolution of a binary alloy in the potentiodynamic polarization mode allowed us to obtain the equations for the concentration profiles of an electronegative metal in an alloy, voltammograms as well as modified Randles-Sevcik expressions, taking into accoimt the mixed solid-liquid phase diffusion nature of the kinetic limitations of the process, equilibrium solid phase adsorption of the components, and surface roughness of an electrode. 

Thermodynamic data collection is the first step for the selected approach and it is explained in detail in [1], The first task of the project was to define the chemical system to be treated. The alloys under investigation contain many elements, the gas phase in a power plant/waste incinerator can contain a multitude of compounds, and the salt deposited on the heat exchangers may lead to dissolution of alloy components or even to solid-liquid equilibria among the salt phases. As a result, the whole data package has been split into metal, salt, oxide and sulphide subsystems, (Fig. 29.1). 

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

Parting selective dissolution of one metal (usually the most electro-reactive) from an alloy leaving a residue of the less reactive constituents. 

The electrochemical machining (ECM) of metals rests on the selective local anodic dissolution of metal. It is used to give metal parts the required shape and size, to drill holes, create hollows, cut shaped slots, and fashion parts of a complex pattern (e.g., the blades of gas turbines). It is an advantage of this method that it can also be used for hard metals (high-alloy steels and other alloys, metals in the quenched state, etc.). 

If a binary or multicomponent alloy is undergoing the liquid-metal attack, then its dissolution can be either selective or non-selective. In the former case, the more soluble component dissolves at a higher rate. Hence, the solid phase becomes depleted, while the liquid enriched in this... 

On the one hand, selective dissolution of transition-metal alloys in liquid aluminium might be expected in view of considerable differences in their solubilities in respective binary systems. On the other, however, in these alloys the atoms of different elements are connnected together by metallic bonds of nearly equal strength. Any of the elements can therefore scarcely be expected to leave the alloy lattice at a rate which significantly exceeds the rates of transition of other elements into liquid aluminium. 

The selective occupation of interstitial sites implies that the atoms in an alloy retain some individuality. This is a feature which has long been recognized as important in surface reactions on alloys, but it is a much more recent observation in respect of bulk properties. Certainly in the case of hydrogen dissolution in a wide range of alloys of Pd with transition or non-transition metal partners the H seems to be capable of differentiating between the metal atoms. 

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