Aluminum + water reaction

Abstract

During the aluminum+water reaction, aluminum ions and electrons are removed in separate steps at different sites on the surface. Ions are removed nearly uniform over the entire surface, which is covered at all times with a thin amorphous oxide film. The outer surface of this oxide is first hydrolyzed and then dissolves to yield soluble species which either remain in solution or, at intermediate values of pH, precipitate as a porous hydroxide of extremely small particle size. The hydroxide appears to be identical to pseudoboehmite. The overall rate of the corrosion reaction is controlled by this dissolution of the film and by the disposition of the soluble products. The corrosion rate is nearly independent of specimen potential, solution pH below 10, and of the presence in solution of many salts at concentrations as high as 1 mole/l. The rate is strongly dependent on temperature, on the presence of specific inhibitive salts, and increases rapidly at high pH. In environments which preclude oxide dissolution, e.g., water + dioxane mixtures or water vapour, the corrosion rate is drastically reduced. The corrosion rate is constant when no solid hydroxide is formed, and is otherwise a strong function of the amount of reaction. At high temperatures the rate decreases with time as the precipitated hydroxide hinders transport. At lower temperatures the rate may first increase with time as nucleation of hydroxide provides sinks for soluble species close to the interface, and then at long times the rate decreases as the hydroxide layer thickens.Electrons are removed more readily at special sites, and in our specimens these sites were primarily at the grain boundaries. Electron removal results in increased hydroxyl ion concentration, and this in turn results in more rapid attack on the protective oxide. At such cathodes the oxide film is maintained at a small constant thickness at which there is a balance between the rates of its dissolution by the basic solution and growth because of the great a m i t y of aluminum for oxygen. At cathodic sites there is thus anodic activity as well, but electron removal predominates. The metal is corroded more rapidly at cathodes, producing pronounced grain boundary attack in our specimens. Application of anodic potentials eliminates grain boundary attack. STEPSIN THE A l + H 2 0 REACTION.-The essential steps in the reaction are (i) amorphous oxide formation ; (ii) dissolution of the amorphous oxide ; and (iii) precipitation of aluminum hydroxide. NATURE OF THE HYDROXIDE LAYER.-The hydroxide layer consists Of particles KINETICS-( 1) ROLE OF PHYSICAL STATE OF THE ENVIRONMENT.-Since dissolution O f

Changes Produced by Cathodic Polarization in the Electrical Conduction Behavior of Surface Films on Aluminum

“…For the low current Tafel region (anodic to -1.7 V), it is proposed that cathodic conduction, like anodic conduction in the same film, follows the high field conduction equation i = -ico exp (-~ ~lm-> ) [9] where Bc is the cathodic high-field conduction coefficient. When the current density in Eq.…”

Síntesis coloidal de materiales nanoestructurados de Al-ZrAl₃: Propiedades mecánicas mediante el ensayo miniatura de punzonado

“…Es bien conocido que el aluminio metálico está pasivado frente al medio ambiente por una delgada película de óxido/hidróxido que lo recubre [17][18][19] . Por tanto, teniendo en cuenta esta característica, en este trabajo se ha utilizado el aluminio metal como sustrato para provocar las reacciones necesarias, a través de un procesamiento coloidal, para obtener polvos nanocompuestos con los que, tras un tratamiento térmico en un horno convencional con atmósfera controlada o por prensado uniaxial en caliente, se obtienen materiales metálicos nanoestructurados con partículas intermetálicas de zirconio, extremadamente finas y homogéneamente distribuidas dentro de la matriz de aluminio.…”

Effects of solvation shells and cluster size on the reaction of aluminum clusters with water

“…[1][2][3][4][5][6][7] Although the reaction, 2Al + 6H 2 O → 2Al(OH) 3 + 3H 2 , is exothermic, formation of an aluminum-oxide layer on the aluminum surface prevents continuous reaction. 8 In order to overcome this bottleneck, continual removal of the oxide layer has been attempted using various promoters such as hydroxides, 5 oxides, 9 and salts.…”