Aluminum oxide formation

It is welt known that aluminum corrodes in both acid and alkaline solutions, but not in neutral ones (pH = 4 to 8) because of aluminum oxide formation whkh serves as a passivation film Ideally the pH of the water extracted solution of encapsulation material is controlled in this region. But when halide ions like Q and Br exist in... 

Formamide decomposes thermally either to ammonia and carbon monoxide or to hydrocyanic acid and water. Temperatures around 100°C are critical for formamide, in order to maintain the quaUty requited. The lowest temperature range at which appreciable decomposition occurs is 180—190°C. Boiling formamide decomposes at atmospheric pressure at a rate of about 0.5%/min. In the absence of catalysts the reaction forming NH and CO predominates, whereas hydrocyanic acid formation is favored in the presence of suitable catalysts, eg, aluminum oxides, with yields in excess of 90% at temperatures between 400 and 600°C. 

Scrap that is unsuitable for recycling into products by the primary aluminum producers is used in the secondary aluminum industry for castings that have modest property requirements. Oxide formation and dross buildup are encountered in the secondary aluminum industry, and fluxes are employed to assist in the collection of dross and removal of inclusions and gas. Such fluxes are usually mixtures of sodium and potassium chlorides. Fumes and residues from these fluxes and treatment of dross are problems of environmental and economic importance, and efforts are made to reclaim both flux and metal values in the dross. 

In the presence of aluminum, oxidative degradation or dimerization supply HCl for the formation of aluminum chloride, which catalyzes further dimerization to hexachlorobutene. The latter is decomposed by heat to give more HCl. The result is a self-sustaining pathway to solvent decomposition. Sufficient quantities of aluminum can cause violent decomposition, which can lead to mnaway reactions (1,2). Commercial grades of trichloroethylene are stabilized to prevent these reactions in normal storage and use conditions. 

In general, the stability of titanium oxide surfaces in moist environments is less of a concern than it is for aluminum oxide surfaces. For example, an FPL or PAA oxide on aluminum would be completely converted to hydroxide in less than 5 min after exposure to boiling water, whereas even after 24 h only slight changes such as crystallite formation and reduction in density of the cell structure occur for... 

Increasing the temperature increases the reaction rate, but decreases the equilibrium (K 500°C = 0.08). According to LeChatlier s principle, the equilibrium is favored at high pressures and at lower temperatures. Much of Haber s research was to find a catalyst that favored the formation of ammonia at a reasonable rate at lower temperatures. Iron oxide promoted with other oxides such as potassium and aluminum oxides is currently used to produce ammonia in good yield at relatively low temperatures. 

The catalytic system used in the Pacol process is either platinum or platinum/ rhenium-doped aluminum oxide which is partially poisoned with tin or sulfur and alkalinized with an alkali base. The latter modification of the catalyst system hinders the formation of large quantities of diolefins and aromatics. The activities of the UOP in the area of catalyst development led to the documentation of 29 patents between 1970 and 1987 (Table 6). Contact DeH-5, used between 1970 and 1982, already produced good results. The reaction product consisted of about 90% /z-monoolefins. On account of the not inconsiderable content of byproducts (4% diolefins and 3% aromatics) and the relatively short lifetime, the economics of the contact had to be improved. Each diolefin molecule binds in the alkylation two benzene molecules to form di-phenylalkanes or rearranges with the benzene to indane and tetralin derivatives the aromatics, formed during the dehydrogenation, also rearrange to form undesirable byproducts. 

The anodic oxidation of sheet aluminum has been used for a long time to protect aluminum against corrosion by a well-adhering oxide layer. Porous oxide layers are formed if acid electrolytes are used that can redissolve the aluminum oxide (mostly sulfuric or phosphoric acid). A compact oxide layer is formed at the beginning of the electrolysis (Fig. 20.3). Simultaneously, the current decreases, due to the electric resistance of the oxide. Subsequently follows a process in which the oxide is redissolved by the acid, and the current increases until it reaches a steady state. The electrochemical oxidation continues to take place with formation of pores. At the end of a pore, where it has the largest curvature, the electric field has its largest gradient and the process of redisolution is fastest. 

Formation of mesoporous aluminum oxide by anodic oxidation of aluminum in acid electrolytes... 

Formation of aluminum oxide (alumina) upon contact of aluminum metal with pure water occurs because the reaction... 

This phenomenon enables some aluminum ions to cross the O/S interface and go into the solution. If the efficiency of oxide formation, r)ox, is defined as the ratio of the amount of solid oxide actually formed to the amount which would be formed if no aluminum went into the solution, such a solubilization reduces this efficiency below 1. 

Active anodic dissolution occurs when all the electrochemically oxidized aluminum passes into the aqueous phase and the oxide layer does not grow, i.e., the current efficiency of oxide formation... 

W. A. Roth, U. Wolf, and O. Fritz, The Heat of Formation of Aluminum Oxide (Corundum) and of Lanthanum Oxide, Zeit. Elektro. u. Angew. Physik. Chemie, 46,42 (1940). 

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