Abstract
It is demonstrated that the presence of surface oxide films on aluminum, prior to anodizing in a pore‐forming electrolyte, can reduce the number of surface pores and the surface roughness compared to that normally obtained with fresh aluminum surfaces. Electron micrographs show that, in such a case, porous anodization proceeds by the branching of isolated surface pores and the development of pore colonies beneath the pre‐existing films. The colonies are shown to consist of uniform hemispherical arrays of anodization cells which grow by continual branching of the internal pore structure. Pore colonies can be produced at will by anodizing first in an ammonium tartrate solution and then in chromic acid solution. The colonies are not found to be distributed uniformly over the surface. With very pure metal, they are found to develop largely at those sites where stress cracking of the surface oxide may be expected.
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