The Mechanical Properties of Thin Anodic Films on Aluminum

While stress is thought to play an important role in the development of self-organized porous films, mechanisms of stress generation during anodizing are not yet understood. In order to reveal depth distributions of stress in anodic films, phase-shifting curvature interferometry was used to monitor force transients (in-plane stress integrated through the sample thickness) during formation of anodic oxides on aluminum in phosphoric acid, as well as subsequent open-circuit dissolution. The measurements were not influenced significantly by electrostatic stress, internal stress in the metal samples, thermal stress, or stress induced by open-circuit dissolution. At typical current densities, the force became more compressive during anodizing, while a net tensile force change was measured after anodizing followed by complete oxide dissolution. Thus, it was revealed that anodizing generates both compressive stress in the oxide and tensile stress near the metal-oxide interface. Analysis of the open-circuit stress change revealed separate contributions from diffusional stress relaxations, and removal of residual oxide stress by dissolution. Residual stress distributions in the oxide, at nanometer depth resolution, were determined from measurements of dissolution rate and stress at open circuit, and validated through variations of the open-circuit dissolution rate.

Section: Implications Of the Stress Measurements-mentioning

confidence: 76%

Measurement of Stress Changes during Growth and Dissolution of Anodic Oxide Films on Aluminum

Çapraz

Shrotriya

Hebert

2014

“…In some simulations, we in-corporated the effect of interfacial oxide plasticity, as inferred by Leach from observations of the relaxation of Al wires under loads during anodic oxidation at high current densities. 9,10 Our approach to simulate plasticity is based on models of the thermal oxidation of silicon, in which the viscosity of SiO 2 is dramatically decreased above a threshold stress. 31,32,34 Because the enhanced fluidity of the oxide would facilitate creep at the interface, we simply imposed a limiting critical stress at the metal-oxide interface, at which further increases in the creep rate would require negligible additional stress.…”

Section: ͓9͔mentioning

confidence: 99%

“…Leach and co-workers observed a current-dependent extension of loaded Al wires during anodizing, which they attributed to current-induced plasticity in the anodic film. 9, 10 Wüthrich showed that anodic alumina films deform without cracking during anodizing. 11 Zhou et al attributed the observations of growth and coalescence of oxygen bubbles during the passage of ionic current to the plasticity in the surrounding oxide.…”

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confidence: 99%

“…[14][15][16][17] Like the materials in these experiments, anodic alumina films are amorphous, and stresses large enough to drive significant creep ͑10-100 MPa͒ are found during the growth of both porous and planar anodic alumina. 9,11,[18][19][20][21] Stresses during anodizing may arise from volume constraints at the metaloxide interface, and from electrostatic forces in the oxide dielectric. [22][23][24] In this paper, we present a model for transport in planar anodic films by coupled electrical migration, plastic flow, and migration in the stress field.…”

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confidence: 99%

See 1 more Smart Citation

A Model for Coupled Electrical Migration and Stress-Driven Transport in Anodic Oxide Films

Hebert

Houser

2009

A model for transport in amorphous anodic oxide films was developed in which ion migration was driven by gradients of mechanical stress as well as electric potential and which included viscoelastic creep of the oxide. Simulations were presented for the galvanostatic growth of planar barrier-type anodic aluminum oxide films. It is assumed that stress originates at the metal-film interface due to the volume change upon oxidation. The average stress in the film decayed during growth and evolved from compressive to tensile with increasing applied current density. The model was fit to stress-thickness measurements using a viscosity of 1×1012Pas on the same order of magnitude as that of many other amorphous materials displaying viscous creep. The current density increased exponentially with electric field, in agreement with an empirical high field conduction behavior. The metal ion transport number was predicted based on the motion of markers in the film and increased with current density in quantitative agreement with experimental measurements. The model represents a unified quantitative interpretation of ionic conduction, transport numbers, and mechanical stress measurements in anodic films.

“…7 Owing to the plasticity of the amorphous alumina during the growth of the film, 8,9 and the high pressure of the contained oxygen gas, 7 the bubbles are able to grow within the film until the film eventually ruptures and the gas is released to the electrolyte. 7,10 The liberation of oxygen has been proposed to account for changes in the morphology of the porous oxide structure.…”

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confidence: 99%

Gravimetric Measurement of Oxygen Evolution during Anodizing of Aluminum Alloys

Torrescano-Alvarez

Curioni

Skeldon

2017

A gravimetric method based on changes in buoyancy force of a submerged gas-collecting container has been optimized to measure the evolution of oxygen during anodizing of an aluminum alloy. Previously, the gravimetric method has been used to measure hydrogen evolution from magnesium and aluminum during corrosion processes, either at the free corrosion potential or under relatively low polarization. However, during anodizing, the comparatively higher values of current applied and the heating effects associated with power dissipation might introduce artefacts in the gas measurement. Optimization of the experimental setup enabled reduction or elimination of such artefacts, so that a reliable measurement could be obtained. The results show that typically about 15 to 20% of the current applied to an AA 2024-T3 aluminum alloy during anodizing in sulfuric acid under the present conditions was used in generating oxygen. Anodic treatments are commonly used to produce protective oxide films and coatings on metals such as aluminum, magnesium, titanium and their alloys. During such processes, in addition to the electrochemical oxidation of the substrate that produces the desired protective layer, oxygen may be generated electrochemically as a side reaction.1-5 For aluminum, oxygen generation during anodizing can be significantly increased by the addition of certain alloying elements or by the presence of impurities in the substrate, such as, for example, copper and iron.6,7 The oxygen may be formed at locations of intermetallic particles, where the high concentration of alloying elements locally modifies the composition, morphology and electronic properties of the oxide.2 Oxygen can also form above the matrix regions, when elements such as iron or copper in solid solution are oxidized and their ions incorporated into the oxide. The resultant modifications of the oxide properties enable the reactionto proceed within the oxide resulting in the development of nanobubbles of oxygen gas within the anodic film. 7Owing to the plasticity of the amorphous alumina during the growth of the film, 8,9 and the high pressure of the contained oxygen gas, 7 the bubbles are able to grow within the film until the film eventually ruptures and the gas is released to the electrolyte.7,10 The liberation of oxygen has been proposed to account for changes in the morphology of the porous oxide structure.11 In particular, the process is believed to be responsible for the degeneration of the well-ordered porous morphology, typically observed on high purity aluminum, 12 into a globular or sponge-like morphology typically observed on copper containing alloys, 13,14 such as those of the 2xxx series 11,12 and 7xxx series.14 The morphology of the porous films on these alloys is of great technological interest, since anodizing is one of the most commonly applied corrosion-protection measures, 15 and anticorrosion performance and oxide morphology are closely related.16 As a consequence, the understanding of the interaction between the anodizing conditions, th...