Nickel reaction layer from

The organic layer obtained from the phosphide reaction still under argon is rapidly transferred by cannula into the stirred-nickel(II) solution. A yellow-brown precipitate forms immediately. The aqueous layer from the phosphide reaction, still in the reaction flask under argon, is extracted with three successive 50-mL portions of diethyl ether added via syringe, and the diethyl ether extracts are transferred by cannula into the nickel(II) mixture. The stirring of the mixture is continued for about 5 min and then it is filtered through a 170-mL glass filter with 25- to 50-// porosity and washed sequentially with two 60-mL portions of 95% ethanol and three 60-mL portions of pentane. The air-stable yellow-brown precipitate is then sucked dry on the frit. 

Recent investigations (9, 11, 83, 92, 178, 272) of regeneration by 02 have resulted in modest success. Table XXII lists studies made of Ni (11, 178), Cu (92), Pt (83), and Mo (272). These studies indicate that sulfur can be removed as S02 by low-pressure (PQl = 3 x 10 9 7 x 10 kPa) oxidation at temperatures ranging from ambient to nearly 1600 K. Significant differences in the kinetics of sulfur removal observed in these studies are attributed to different reaction mechanisms being predominant at different temperatures. On the other hand, attempts to remove sulfur from polycrystalline Ni surface by atmospheric pressure oxidation (P0l = 10-760 Torr) at temperatures from 300 to 800 K were unsuccessful (9). Instead, nickel oxide layers were formed on top of the sulfur layer. Treatment of the oxidized catalyst with H2 at 700 K for 2 hr reduced the Ni oxide completely to the metal at the same time it caused the return of the sulfur layer to the surface. 

FIGURE 5.7 Optical micrographs of the reaction products between Ni and Al. (a) Duplex phases of nickel aluminides formed after a treatment of 1000°C for 1 hour in vacuum (b) a cracked nickel aluminide layer formed after being treated at 640°C for 1 hour in vacuum and (c) a cross-section of a nickel and zirconia joint which was joined together through a nickel aluminide layer at a temperature of 680°C in vacuum. N nickel, R nickel aluminide, A aluminium, and C zirconia. (Reprinted from Mei, J. and Xiao, R, Joining metals to zirconia for high temperature applications, Scripta Materialia 40 (1999) 587-594, with permission from Elsevier Science.)... 

When the vapor-gas mixture is fed to the liquid shell surface, there arises a catalytic reaction of decomposition of a carbon-containing organic compound with dissolution of carbon in the nickel liquid phase. A saturation of the nickel liquid layer with carbon increases its thickness and decreases the surface curvature. This disturbs the system from equilibrium (Fig. 25 (b)) as the liquid-layer thickness increases, the lines of equilibrium between the liquid layer and the crystalline core shift towards higher temperatures (see relationship (10)). 

The question one has to put is that, if this is the case, would copper then continue to deposit and nickel completely dissolve from the deposit However, snbseqnent analysis showed that copper emichment at the surface slowed down nntil the copper content reached about 80%, after which the reaction virtually stopped (Roy and Landolt, 1995 Bradley e/a/., 1996). The galvanic corrosion is controlled by copper mass transfer through the hquid phase at the beginning, showing a linear growth of the Cu-rich layer (Roy and Landolt, 1995). Since the reaction stops due to surface emichment of copper, the growth is inhibited quite suddenly when the thickness of the layer reaches about lOiun (Roy and Landolt, 1995 Bradley et al, 1996). 

The oxidation of nickel-copper alloys provides an example of die dependence of the composition of the oxide layer on the composition of the alloy. Nickel-copper alloys depart from Raoult s law, but as a first approximation can be taken as ideal. The Gibbs energy change for the reaction... 

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

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