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
This paper reports studies of the rates of dissolution of natural Brucite, optical grade normalMgO , and commercial normalMgfalse(OH)2 in aqueous solutions. Natural Brucite dissolves by means of a surface reaction requiring protons, and while some protonated ions accelerate the dissolution no inhibitors were found. Dissolution of less perfect normalMgfalse(OH)2 is normally diffusion limited; several substances which reduced the rate of dissolution of such normalMgfalse(OH)2 were found. normalMgO first reacts with water to form an normalMgfalse(OH)2 layer and the rate is controlled by dissolution of the normalMgfalse(OH)2 .
Stresses in anodic films produced on several metals were measured by clamping the upper end of a thin foil of the metal and observing the motion of the lower end through a telescope. 1 Foils were about 10 cm long, and one side was coated with stopcock grease to prevent anodization. (Grease is preferable to lacquer which shrinks on drying and can support some stress.) Deflections were of the order of a millimeter in most experiments; the minimum measurable deflection was about 0.005 cm. No twisting of the foil was observed. The stress measured on the
The abnormally long jump distance X which characterizes the ionic conduction of glasses as a function of electric field,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.