Evolution of Microscopic Surface Topography during Passivation of Aluminum

The time evolution of microscopic topography on corroding aluminum surfaces during oxide film passivation was characterized. Passivation was studied after galvanostatic etching in 1N HCl at 65 C, in both aluminum etch tunnels (by scanning electron microscopy) and micron-size cubic etch pits (by atomic force microscopy). Step reductions of applied current initiated passivation. At times of 1 to 300 ms after current steps, the corroding surface was microscopically heterogeneous, consisting of a number of small corroding patches 0.1 to 1 [mu]m in width, which were surrounding by passive surface. As some patches grew by dissolution, others were passivated, until eventually only one patch remained on the pit or tunnel surface. The topography of the corroding surface was controlled by the potential: the surface dissolved uniformly at the repassivation potential E[sub R], while partial passivation to produce patches occurred at potentials more cathodic than E[sub R]. Patches were unstable at potentials below E[sub R] and would ultimately passivate. Topographic evolution during passivation was very similar for pits as for tunnels, except that the time scale of the process is much longer for tunnels than for pits (200--300 ms vs. 11--20 ms). The difference of time scales was due to the differentmore » corroding surface areas of pits and tunnels. Patches are probably defined by a surface layer which inhibits passivation.« less