Metal surface layer

The capacity of the metal phase (CM) and the potential drop in the thin metal surface layer have been discussed by Amokrane and Badiali,122,348 as well as by Damaskin et a/.349 353 The value of was found to increase in the order Ga < In(Ga) < Tl(Ga) Hg if it was assumed that the capacity of a solvent monolayer C = const. The negative value of the surface charge density <7, at which the Cs,ff curve has a maximum, decreases in the order Ga > In(Ga) > Hg, i.e., as the hydrophilicity of the electrode decreases. 

In the anodic polarization of metals, surface layers of adsorbed oxygen are almost always formed by reactions of the type of (10.18) occurring in parallel with anodic dissolution, and sometimes, phase layers (films) of tfie metal s oxides or salts are also formed. Oxygen-containing layers often simply are produced upon contact of the metal with the solution (without anodic polarization) or with air (the air-oxidized surface state). 

It may be noted that the statement made above—that the surface potential in the electrolyte phase does not depend on the orientation of the crystal face—is necessarily an assumption, as is the neglect of S s1- It is another example of separation of metal and electrolyte contributions to a property of the interface, which can only be done theoretically. In fact, a recent article29 has discussed the influence of the atomic structure of the metal surface for solid metals on the water dipoles of the compact layer. Different crystal faces can allow different degrees of interpenetration of species of the electrolyte and the metal surface layer. Nonuniformities in the directions parallel to the surface may be reflected in the results of capacitance measurements, as well as optical measurements. 

The proximate placement of the tip over the surface allows measurements to probe the surface without impacting it, while ignoring the gas-phase environment. Therefore, the information obtained relates to the first few metal surface layers and any absorbed species. Jensen et al have developed this technique over the temperature range 25 to 400 °C and the pressure range 5 x 10 Torr to 1 atm, falling in the range of many operating conditions. ... 

Oxide Catalysts - Under appropriate conditions a bulk metal will form an oxide overlayer and this layer will be responsible for governing the catalytic behaviour. In a similar manner a bulk oxide can undergo reduction with a formation of a metallic surface layer. Such behaviour can be responsible for rate oscillations or hysteresis in reaction rate and is important when considering the catalytic behaviour of bulk oxides. 

The effect of metal structure and phase formation on the kinetics of catalytic oxidation reactions was treated in detail by Savchenko et al. (see, for example, refs. 83, 84, 117 and 118). In metal surface layers both reconstruction of the metal proper (faceting) and processes associated with the formation of surface oxides can take place. In this case the first to form can be chemisorption structures (without breaking the metal-metal bond) and then the formation of two-dimensional surface oxides is observed. Finally, three-dimensional subsurface oxides are produced. An important role is played by the temperature of disordering the adsorbed layer. 

Figure 15 Layer-resolved density of states (DOS) for hydrogen-covered metal slabs of palladium and rhodium. Top DOS on the hydrogen layer middle on the metal surface layer bottom on the metal subsurface layer. (From Ref. 82.)...

The addition of alloying elements can also indirectly affect the oxygen solubility gradient across the thin metallic surface layer by modifying the activity of an element that interacts with Al. For example, Fe forms very stable intermetallics with silicon, while the Fe-Al and Fe-Mg intermetallics are less stable. Additions of Fe to an Al-Si-Mg alloy, therefore, will decrease the Si activity, and thereby increase the Mg activity. This will lead to a lower oxygen potential gradient and a lower growth rate. 

In previous studies [3, 4], we had pointed out that the interphase formation mechanisms result from dissolution of the metallic surface layers, concomitantly with ion diffusion through the liquid prepolymer. In order to detect the dissolution phenomenon, pure amine (either DETA or IPDA) was previously applied to chemically etched metallic sheets (either A1 or Ti alloys were used, and had hydroxidic surfaces). After 3 h, the metallic surfaces were scraped with a PTFE spatula. The modified amine (i.e., the amine reacted with the metal) was analyzed. Whatever the natures of the amine and the metal were, metal ions were detected in the modified amines by ICP analysis and new peaks were detected by infrared spectroscopy [5]. To indicate hydroxide dissolution, a very thin layer of liquid amine was applied to chemically etched aluminum, and Infrared Reflection - Absorption Spectroscopy (IRRAS) spectra were recorded every 5 min (the hydroxide band intensity variation at ca. 3430 cm was followed). The OH group peak intensity decreased when the amine-metal contact time increased [5]. Conversely, if pure DGEBA monomer was apphed to the metal surfaces, even after 3 h in contact with the metallic surfaces, no metal ion was detected by ICP in the DGEBA recovered, and the infrared spectra remained identical before and after the contact with the metal. Finally, if pure amine monomer was applied to gold-coated substrates, no chemical reaction was observed (by either IGP or FTIR analyses). 

Wear resistance of the polyacetal-metal friction pair can be improved considerably by the introduction of higher fat acids or realizing their S3mthesis conditions in the friction zone. Passivation of metal surface layers by phos-phating formulations and epilamens may elevate wear resistance of friction bodies in which polyacetal, polyamide, fluoroplastics, and other substances rub against copper alloys, aluminum, chrome or titanium [108,117,118]. 

Fig. 9 The dependence of the effective thickness of the potential drop region in the metal surface layer, calculated by the Amokrane-Badiali model [19] (at constant capacitance of solvent monolayer, Cs), on the charge density for aqueous solution on different electrodes (a) 1, Sb(l 11) 2, Sb(OOl) 3, Bi(lll) 4, Hg 5, Bi(OOl) 6, Cd (0001) 7, Cd(1120) and 8, Ga and for Bi(OOl) in various solvents (b) 1, methanol 2, H2O 3, acetonitrile (updated from Ref [15, 15]).

In order to provide much more evidence for the chelate formation and metallic surface layer formed as a result of modification, ESCA spectra were measured. Figure 5 shows the typical ESCA spectra of Pb(II)-PTASH-Pb chelate and Ag-PTASH metallized films. Table IV summarizes the ESCA peaks together with those corresponding to pure metal state and metal oxide. From Figure 5(B), it can be noted that the Ag-PTASH film, which had been modified by aqueous NaBH4 solution, shows four peaks at 376.1,374.0,370.3 and 368.2 eV. 

These results coupled with the results of X-ray diffraction and SEM observations lead us to believe that the metallized films consist of a thin metallic surface layer supported on a thick chelate layer. 

Pores and macroscopic inclusions are three-dimensional crystal defects. From the standpoint of the reactivity of solids, pores can be very important. Consider, for instance, the formation of porous scales during oxidation (tarnishing) [11]. (For example, the decarburization of iron cannot occur if a non-porous oxide scale without grain boundaries is formed on its surface.) Or consider the direct reduction of ore [10] in which the reduction rate is greatly dependent upon the formation of porous metal surface layers. In many so-called solid state reactions, gaseous products are formed as well as solid reaction products as, for example, during the reaction of TiOa with BaCOs to produce BaTiOs with the formation of In such cases, just as in the case of ore reduction, the formation of a porous product surface layer is of decided importance for the progress of the reaction. 

In addition to oxide film or scale formation at elevated temperatures, titanium alloys can also experience oxygen enrichment of metal surfaces beneath the oxide scale. These hard, brittle metal surface layers, known as "alpha case," result from diffusion of oxygen or nitrogen interstitials, or both, into the metal. This time/temperature-dependent phenomenon becomes significant above 540-590°C depending on alloy type, and severely reduces metal formability, ductility, and fatigue life, particularly as metal section thickness is reduced. Typical examples of an alpha case in o/p and P titanium alloys are shown in Fig. 11. 

In [56], microprobe studies on the distribution of the elements in the surface layers of the pipes in the experiments [55] demonstrated areas of chloride and sulphate concentration at the metal/surface layer phase boundary. This gives the surface layers semi-conductor properties, leading to partial separation of regions at vhich anodic and cathodic part reactions proceed. As the concentration of copper ions increases, the cathodic part current is increased, so that pitting corrosion is intensified at the anodic regions. 

A surface-bulk phenomenon of technical importance is the diffusion of an evaporated metal surface layer into the bulk of the backing metal. Fig.9 shows how such a process can be followed in an electron spectrum as a function of time. The time evolution of the Au4f lines is observed as the evaporated gold diffuses into the various substrates, Z.n,... 

The product design was a three layer oriented propylene film with a treated copolymer metallizing surface layer and a heat sealable surface. The film was commercially manufactured and wound on 6 inch cores to a diameter of 20 inches. One roll was used for each treatment combination tested to insure uniformity of the process variables. 

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