IN-SITU OBSERVATIONS OF GROWING PITS DURING TUNNEL ETCHING OF ALUMINUM

A new technique for studying the mechanisms of tunnel etching is descr ibed. During the electrochemical etching of an aluminum foil in a chlo ride solution, the propagation of initiated pits to [100] crystallogra phic directions on the specimen surface was observed with an optical m icroscope and dynamic TV images of the pit growth were recorded on a v ideo tape. In situ observations revealed that tunnel pits grow continu ously at a constant rate without hydrogen gas evolution and then stop growing abruptly at a limited length. Frequent changes in the growth d irections through right angles were also detected clearly. At the mome nt of growth cessation, it was observed frequently that the electrolyt e inside the pits convected out rapidly from the tunnel tip toward the base and a gas phase was formed.