Initial stages of the anodic etching of aluminum foils studied by atomic force microscopy

The modification of the surface morphology of two types of aluminum foils d uring galvanostatic etching in hot HCl/H2SO4 solution was studied as a func tion of etch time by Atomic Force Microscopy (AFM) and complementary electr ochemical measurements. Particular emphasis was placed on the nucleation an d growth of etch pits and tunnels. Three stages of the etch process with ch aracteristic differences in the surface morphology can be identified: for e tch times up to 5-10 s predominantly large etch pits with surface areas gre ater than or equal to 0.5 mu m(2) are formed (stage I), followed by the nuc leation of small rectangular pits and channels (0.01-0.5 mu m(2) area) for etch times up to 20 s (stage II), and finally by the formation and lateral growth of surface areas with a rough, strongly etched surface morphology fo r etch times > 20 s (stage III). The distribution of smaller etch pits and tunnels and the rough morphology in stage III differ strongly for the two d ifferent materials, which can be correlated with differences in the submicr ometer scale morphology of the foils prior to etching. The etch process is accompanied by the deposition and redissolution of characteristic hemispher ical hills. (C) 1998 Elsevier Science Ltd. All rights reserved.