The specimen was first electro-polished, as shown in
Fig. 1(b). A pure Al sheet (99.999 %), 1 mm thick, was
degreased in acetone by ultrasonication. The specimen
was then rinsed several times in ethanol for at least
15 min and finally rinsed in deionized water. Next, the
specimen was electro-polished in a mixture of perchloric
acid and ethanol (HClO4:C2H5OH = 1:4 volumetric