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
The aluminum + water reaction proceeds by dissolution of the outer surface of the amorphous oxide and precipitation of a porous hydroxide. The reaction is accelerated by the hydroxide layer, as long as it provides sites for deposition of soluble close to the surface. Inhibitors prevent hydroxide formation and eliminate such deposition sites so that soluble species must diffuse into the bulk of the solution, a much slower process. Strong inhibitors include phosphate, silicate, arsenate, periodate, and tungstate. Inhibitor strength correlates well with the acidity and structure of the ion.A preceding paper presented the results of a study of the aluminum + water reaction. The essential steps were formation of amorphous oxide, dissolution of the oxide, and precipitation of aluminum hydroxide. The rate of dissolution of the amorphous oxide is controlled below pH 10 by hydrolysis of surface Al-0 bonds, and the rate is independent of pH. Above pH 10 the rate is limited by hydroxyl ion diffusion to the surface. Between pH 3 and 11 a porous hydroxide layer forms by nucleation and growth from soluble aluminium species and is composed of pseudoboehmite.The overall rate of the reation is controlled by the disposition of soluble aluminium species. The presence of the porous aluminium hydroxide layer facilitates the reaction by providing deposition sites close to the surface, while in the absence of hydroxide the soluble species have to diffuse much farther into the bulk of the solution. Ultimately the growth and densification of the hydroxide layer retard the reaction, but even then it still proceeds much faster than it would if no hydroxide layer were present.Certain impurities, notably silicate and ph~sphate,~. greatly reduce the reaction rate ; many other substances such as Cl-, NO;, SO:-, HCOT, MoOj-, and acetate have little influence ; and citrate accelerates the a t t a ~k . ~ This paper describes a study of the inhibition of the aluminum + water reaction.We have found that strong inhibitors form surface compounds containing the inhibitor. Inhibition results mainly because the inhibitor prevents the formation of the porous aluminum hydroxide layer.
Pleochroism of the infrared absorption of muscovite has been measured under moderate resolution. The results are discussed in terms of the idealized mica structure. From the pleochroism of the OH stretching band, orientation of the vibrational transition moments can be determined. These moments are tilted out of the cleavage plane by about 16°. Their projection onto the cleavage plane makes an angle of about 32° with the b axis. These directions furnish approximations to the orientation of the OH ions, but the danger of equating the two is stressed. Vibrational coupling is small. This result has been used to decide rather conclusively between two models of muscovite which differ in the orientation of the OH ions. The librational motion has been studied directly, as well as in combination bands with the OH fundamental. It is shown that the SiO lattice also gives rise to combination bands with νOH.
No. 49tsFebruary l, 1964 NATURE 495 branches of the experimental curves correspond to a mean chain lifetime of about 200 msec. When assuming a purely bimolecular disappearance, we obtain k 2 /a = 3•5 x IO" M-1 , independently of dose-rate in the range studied. As distortion of tho curves due to the electronic circuit could bo excluded, a discrepancy arises between both tho rising portions of the curves and the results of the stationary measurements on one hand, and the d ecay curves on tho other. It appears as though thoro would be both first and second order reactions during irradiation, and only second order reactions in the absence of irradiation. Thus, the simple mechanism wo proposed as a working hypothesis needs to be extended.
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