. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 1. Electropolished aluminium showing "whorl'" struc- ture, and a boundary between this and one of the interme- diate forms with "furrow" and "dotty"" structures. Shadow- cast formvar replica. Magnification 10,000. ture intermediate between these has also been ob- served. The above authors all employed aluminium oxide replicas obtained by anodisation of the specimen after polishing. It seemed possible that the structure could have been produced as well by the anodising as by
. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 1. Electropolished aluminium showing "whorl'" struc- ture, and a boundary between this and one of the interme- diate forms with "furrow" and "dotty"" structures. Shadow- cast formvar replica. Magnification 10,000. ture intermediate between these has also been ob- served. The above authors all employed aluminium oxide replicas obtained by anodisation of the specimen after polishing. It seemed possible that the structure could have been produced as well by the anodising as by the polishing process; consequently, the pol- ished surfaces were examined by plastic replicas in the present investigation to avoid any possible confusion. Since the structures still appeared, it was clear that they arose during the electropolishing process. The structure types may all appear on one speci- men, and it is not possible to predict which type may appear after the electropolishing. It seems likely (3, 6) that the type of structure is determined by the underlying crystal orientation. Sharp boundaries observed between one type of structure and another lend support to this supposition (fig. 1). When the size of the structure, as opposed to its type, is considered, however, it is found to be related in some way to the polishing conditions. Since, in a few cases, structureless surfaces were obtained, it was decided to attempt to define the conditions for obtaining structureless surfaces at will. The electrolyte used in this investigation was the per- chloric acid/alcohol mixture (4). This was preferred as being safer than the electrolyte containing acetic anhy- dride and used by Jacquet. The specimens were super purity aluminium ( °o) so as to avoid any metallo- graphic structures which might have confused the results. It was at first found very difficult to obtain reproducible results from the electropolishing process, and it was found
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