Reactions at the oxide-electrolyte interface of anodic oxide films on aluminum

Porous anodic alumina ͑PAA͒ films, formed by anodic oxidation in acidic solutions, contain hexagonal arrays of parallel cylindrical pores, with pore diameter and spacing between ten and several hundred nanometers. Simulations were developed for the electrical potential distribution in the film during steady-state PAA growth, and used to calculate the rates of metal-film and film-solution interface motion. In particular, a model using the assumption of no space charge ͑Laplace's equation͒ and one based on the current continuity equation, in each case coupled with high-field ionic conduction, were evaluated with respect to the requirement that the interface profiles are time invariant. Laplace's equation, on which prior simulations of PAA growth were based, yielded unrealistic behavior with highly nonuniform interface motion, suggesting the presence of significant space charge. In contrast, interface motion predicted by the current continuity equation was uniform, except near convex ridges on the metal-film interface between pores. To fully rationalize the steady-state PAA geometry, phenomena other than conduction should be considered, which are able to provide inhibition of the oxidation rate on these ridges.

Section: ͓18͔mentioning

confidence: 92%

“…20, indicate that is less than 0.1 V. Because the applied voltage V during PAA growth is greater than 10 V, the boundary condition = 0 was used at the film solution interface. The same interface boundary condition was used in Ref.…”

Section: Modelmentioning

confidence: 99%

Modeling the Potential Distribution in Porous Anodic Alumina Films during Steady-State Growth

Houser

Hebert

2006

“…30 The current through the film/solution interface is carried by iontransfer processes, whereby OH − or H 2 O in solution react to form O −2 ions in the film, and Al +3 ions in the film are ejected to form Al͑OH͒ 4 − ions in solution. 31 It is necessary to characterize these processes, because O −2 transfer leads to film growth or dissolution, which affects the conduction resistance. In view of the very low current densities in Fig.…”

Section: ͓13͔mentioning

confidence: 99%

Participation of Aluminum Hydride in the Anodic Dissolution of Aluminum in Alkaline Solutions

Adhikari

Hebert

2008

The mechanism of anodic alkaline dissolution of aluminum was investigated through the analysis of cyclic voltammetry (CV) and potential step experiments. Attention was focused on the role of aluminum hydride (AlH3) as a reaction intermediate, as suggested by the recent detection of AlH3 formation during open-circuit dissolution. Potential step experiments at pH 11.75 revealed that the potential at the metal–surface film interface was close to the Nernst potential of AlH3 oxidation. This finding suggested a reaction mechanism in which an interfacial AlH3 layer is formed continuously by reaction of cathodically formed H with Al, and is then oxidized to the dissolution product, aluminate [Al(OH)4−] ions. However, potential step experiments at pH 11 did not indicate the presence of interfacial AlH3 ; instead, the metal–film interface was close to the equilibrium potential of Al oxidation. Analysis of the CV indicated an abrupt transition in dissolution behavior between the two pH values, from a relatively rapid dissolution controlled by diffusion and film conduction in highly alkaline solutions, to a slow dissolution at a lower pH controlled by a highly resistive surface film. The formation of interfacial AlH3 occurs readily at the higher pH, but is suppressed as the pH approaches neutrality.

“…This may be related to the continuous aging of the oxide or may be triggered by the interfacial potential drop. 26,27 In this process the structure of the anodic oxide may change having as a result a different charge transport as previously observed during anodization of other valve metals. 25 In turn, this impacts the dissolution fractions as presented in Fig.…”

Section: Resultsmentioning

confidence: 84%

In-Situ Monitoring of Metal Dissolution during Anodization of Tantalum

Kollender

Mardare

Hassel

2017

The dissolution of Ta in acidic media during both potentiodynamic and potentiostatic anodic oxide formation was quantified in-situ by ICP-MS. Potentiodynamic polarization yields a constant dissolution rate of 4 pg s −1 cm −2 (5 fm s −1 ) while a continuous dissolution rate decay was observed during potentiostatic anodization. The rate of Ta species release increased with applied potential in good agreement to the measured current density transients. A higher percentage of the charge carriers was identified as responsible for generation of soluble Ta species in potentiostatic regime as compared to potentiodynamic oxide growth. The current efficiency of anodization was determined from the combined investigation to exceed 99.9%.