Exchange studies have been made with electropolished singIe-crystal specimens of silver and zinc immersed in dilute solutions of their salts, using Ag110 and Zn 65 ions as tracers. In 0.02M solutions of the salts, exchange in the surface layer of the metals is rapid and complete, and continued tracer pickup is due to diffusion within the crystals. At lower salt concentrations the tracer pickup is slower, apparently because complete exchange is not maintained in the surface layer of atoms.In most previous studies of exchange between silver and zinc and their ions in solution, polycrystalline specimens have been used (1-3). This has the disadvantage that both grain-boundary and 1at-rice diffusion occur within the metal. The rate of radiotracer pickup depends on the grain size and grain-boundary area, and only the ratio of the two diffusion coefficients is significant. One run with a silver single crystal was carried out by Gerischer and Tischer (4), who concluded that the behavior was not essentially different from that of polycrystalline specimens. Zinc single crystals were used by Bushmanov and Vozdvizhenskii (5), who found somewhat different exchange rates on base and prism facets. Zinc bicrystals were investigated by Sandor (3), who found that the metal specimens corroded promptly even in well-deaerated solutions.The purpose of the present investigation was to find whether single-crystal specimens, prepared with minimum surface roughness, would show a consistent and understandable pattern of exchange with their ions in aqueous solution. We find that silver specimens do so, with no complication from corrosion or surface films. On the other hand, zinc is subject to corrosion by water in the absence of oxygen Zn ~-H20-~ ZnO -P Hf; AG ~ : --19 kcal A similar free energy value applies if Zn(OH)2 is the product. A polished specimen does not acquire a visible film for a long time in deaerated water; in salt solutions the initiation of visible film formation appears to be accidental and in general occurs sooner, the more concentrated the salt.
ExperimentalSingle-crystal rods of silver and zinc 2 about 2 cm in diameter were cut into disks 2 mm thick, and a 1 mm hole was drilled near the edge for suspending. After smoothing the surfaces the disks were annealed (silver at 400 ~ zinc at 120~ and cooled slowly. Before use each disk was electropolished to remove distorted material and to give a smooth, bright surface (silver in AgCN-NaCN, zinc in phosphoric acid-ethanol).active solutions the disks were electropolished to remove the active material.In electropolishing, the disks were suspended on a Pt wire, midway between 2 stainless steel cathodes. Disks were suspended in the radioactive solutions either on glass hooks or on wire hooks coated with insulating lacquer. Solutions of AgNO8 were not deaerated except in a few tests; Zn (C104)2 solutions were deaerated with nitrogen or hydrogen which was passed over hot copper, then through two wash bottles containing solution of the same concentration as the test solution. Immersi...
A study has been made of the behavior of pure iron, immersed in solutions of ferrous and ferric perchlorates, using radioactive iron ions as tracer. The results indicate that exchange in depth is negligible at room temperature and that most, if not all, of the activity acquired by the metal is incorporated in solid corrosion products. Dichromate reduces the rate of dissolution of the metal in Fe(C104)3, but activity is picked up in a corrosion film. Cathodic polarization also inhibits dissolution in Fe( CICWs but deposits a non-adherent film which contains active ions. The results are in accord with the very low rate of self-diffusion in iron, the low exchange current, and the great susceptibility of the metal to local-cell corrosion, This investigation was designed to find whether radiotracer studies could give any useful information concerning the nature of an iron surface in contact with solutions of its own salts, especially under conditions which minimize corrosion. The rate of self-diffusion in iron has been measured at high temperatures1 and extrapolation to room temperature appears to give a negligible value. Consequently internal diffusion should not cause continued tracer pickup unless grain-boundary diffusion should prove to be important. The exchange current is small; values of 2 X 10-8 and 2.5 X 10-11 amp./cm.2 have been reported in various solutions of FeS04 at room temperature.2•3 Radiotracter pickup, even in a surface monolayer, could not occur faster than the equilibrium exchange rate.
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