The morphology of porous anodic oxide films formed on aluminium in phosphoric acid electrolytes at constant current density or voltage, and under changing electrical or electrolytic conditions, has been studied quantitatively by electron microscopy. Replicas from film sections and from both film interfaces have been prepared, as well as transmission micrographs of thin films, produced under accurately defined conditions. During formation at constant current density, pore initiation occurs by the merging of locally thickening oxide regions, which seem related to the substructure of the substrate, and the consequent concentration of current into the residual thin areas. The pores grow in diameter and change in number until the steady-state morphology is established. The film barrier layer thickness has been measured directly for the first time. The steady-state barrier-layer thickness, cell diameter and pore diameter are all observed to be directly proportional to the formation voltage. It becomes evident that the barrier-layer thickness, decided largely by an equilibrium established between oxide formation in the barrier-layer and field-assisted dissolution (probably thermally enhanced) at the pore bases, determines the cell and pore sizes by a simple geometrical mechanism. Anion incorporation into the film and its hydrogen-bonded structure play secondary roles to these factors in determining the actual film morphology, although not its subsequent properties. A consequence of the mechanism is that, at constant current density, relatively non-aggressive electrolytes give thicker barrier layers, larger cells and larger pores next to the barrier layer than aggressive media, although subsequent pore widening at the outer surface of the film by simple chemical dissolution is more severe in aggressive electrolytes.

The morphology and mechanism of formation of porous anodic films on aluminium

http://www.nuclear.lu.se/fileadmin/nuclear/Undervisning/TillSubFys/PIXE-Amsel-microanalysis.pdf

Microanalysis by the direct observation of nuclear reactions using a 2 MeV Van de Graaff

A theory of avalanche breakdown during anodic oxidation

Publisher Summary The anodizing and allied anodic processes on aluminum have been studied widely because of the great commercial exploitation of the metal, the requirement of protection against corrosion, and to gain further understanding of the mechanisms of the anodic behavior in the appropriate electrolytes. On one hand, fundamental investigators have been fascinated by many aspects of its behaviour, such as the apparently unique, relatively regular, and controlled morphology of its porous anodic films and, on the other hand, the mechanisms of pit initiation where aluminum is lost to solution as solvated ions. A review dealing with anodic film formation on aluminum should also take into account the behavior of the metal when immersed in solutions containing anions that are aggressive to aluminum surface. The anodic film formation implies thickening of the natural air-formed film on the macroscopic metal surface. However, in reality, flaws or weak spots are also present in the air-formed/passive film and in anodic films. Flaws, which can be of micro- or macro-dimensions, usually represent thinner regions of film or impure film or holes above some surface heterogeneity or cracks produced by a mechanical rupture of the film.

Anodic Films on Aluminium

Precision absolute thin film standard reference targets for nuclear reaction microanalysis of oxygen isotopes

The influence of the electrolyte on the composition of ‘anodic oxide films’ on tantalum

The use of the nuclear microanalysis of light elements in electrochemistry applications to anodic oxidation processes by direct determination of 16 O and by 18 O tracing

G. Feuillade

Papers

The influence of the electrolyte on the composition of ‘anodic oxide films’ on tantalum

The use of the nuclear microanalysis of light elements in electrochemistry applications to anodic oxidation processes by direct determination of 16 O and by 18 O tracing