AFM study of topographical changes on aluminum surfaces in sulfuric acid under low current anodic dissolution

A study of the anodic dissolution of polycrystalline aluminum utilizing in-situ atomic force microscopy (AFM) is reported. Terraced pyramidal walls with a constant characteristic width of 300–400 nm running relatively parallel to each other appear within a few minutes of dissolution. Upon further dissolution, these pyramidal walls are reduced to square terraced pyramids or ziggurats of constant width. AFM contour and profile plots reveal the extremely square and flat surfaces of the square plateaus on top of the ziggurats, all of them having the same size. Under the dissolution conditions utilized, nucleation occurs at dislocation sites with primarily layer-by-layer dissolution. A long-range interaction between approaching dissolution fronts decreases the dissolution rate leaving the terraced pyramidal walls. This interaction is interpreted to arise from band bending in the oxidic layer existing on the surface. These features suggest that a strong role in the dissolution kinetics is played by the non-local potentials caused by the electronic charge and band bending on the oxide layer. The terracing of the walls is believed to result from a short-range or chemical dissolution effect. Possible applications of these prepared surfaces are also discussed.