Pilling–Bedworth ratio of thick anodic aluminium porous films prepared at high voltages in H2SO4 based electrolyte

Thick porous anodic films have been prepared using high voltages in a sulphuric acid based electrolyte. The use of H2SO4 low concentration, low bath temperature and the boric acid as modifier allows the anodic porous film to be significantly thickened preventing its chemical dissolution. A new relation including the Pilling–Bedworth ratio, especially of the thick anodic porous films, is proposed here to take into account the nanoporosity and the anodic current efficiency

Chemical analysis of a single basic cell of porous anodic aluminium oxide templates

We prepared anodic aluminium oxide (AAO) templates with “honeycomb” geometry, i.e. hexagonally ordered circular pores. The structures were extensively studied and characterized by EPMA coupled with FEG-SEM and FEG-TEM coupled with EDX at meso and nanoscopic scales, in other words, at the scale of a single basic cell making up the highly ordered porous anodic film. The analyses allowed the identification of the chemical compounds present and the evaluation of their levels in the different parts of each cell. Of note was the absence of phosphates inside the “skeleton” and their high content in the “internal part”. Various models of porous anodic film growth are discussed on the basis of the results, contributing to a better understanding of AAO template preparation and selfnanostructuring phenomena

Chemical behavior of tungstate solutions. Part 1. A spectroscopic survey of the species involved

This study is focused on the composition and the evolution of tungstate ions solutions as a function of pH and increasing concentrations. The Raman analysis showed that, during the titration of the tungstate solutions, WO4 2−, HWO4 − ions and probably W2O7 2−, HW2O7 2− and H2W2O7 solvated species could exist in aqueous solutions. For diluted solutions, additions of a strong acid does not cause any precipitation, whereas the formation of the unstable solid tungstic acid (H2WO4 or WO3·H2O) could occur in concentrated solutions

Impact of the type of anodic film formed and deposition time on the characteristics of porous anodic aluminium oxide films containing Ni metal

Porous anodic films containing nickel were prepared by AC electro-deposition. The porosity of the films was controlled by using different working conditions (anodisation electrolyte, voltage, and time). Then nickel was electro-deposited using an alternating voltage. The impact of the anodic film on the current density waveforms and the metal content can largely be explained by the porosity differences, while changing the deposition time caused changes due to over-oxidation of the aluminium substrate, experimentally proved by TEM. Finally, the impact of deposition time on the deposited metal was successfully fitted using an Elovich type law over a large time-span (up to 1800 s), showing the ability to achieve precise control of the metal content

Influence of the anion specificity on the anodic polarization of titanium

The anodic polarization of titanium in the presence of selected electrolytes at 0.1 mol/L concentration has been investigated. The results were interpreted according to the categorization of anions based on their cosmotrope/chaotrope character. The aggressiveness of the anions is found to be related for a part to their proneness to loose their hydration shell during the penetration of the passive film, provided they can access sufficiently to the passive surface

Black anodic coatings for space applications: study of the process parameters, characteristics and mechanical properties

Black inorganic anodized aluminium alloys are used for managing passive thermal control on spacecraft and for avoiding stray light in optical equipment. Spalling of these coatings has sometimes been observed after thermal cycling on 2XXX and 7XXX aluminium alloys. This phenomenon could generate particulate contamination in satellites and may affect mission lifetime. In this work, the influences of the four main steps of the process (pretreatments, sulphuric anodizing, colouring and sealing) on the coating characteristics have been studied for a 7175 T7351 aluminium alloy. The chemical heterogeneity of the coating has been underlined, and its mechanical behaviour observed through crazing. Scratch-testing, used to evaluate coating adhesion to its substrate, revealed the negative impact of thermal cycling

Electrical behaviour, characteristics and properties of anodic aluminium oxide films coloured by nickel electrodeposition

Porous anodic films on 1050 aluminium substrate were coloured by AC electrodeposition of nickel. Several experiments were performed at different deposition voltages and nickel concentrations in the electrolyte in order to correlate the applied electrical power to the electrical behaviour, as well as the characteristics and properties of the coatings. The content of nickel inside the coatings reached 1.67 g/m2, depending on the experimental conditions. According to the applied AC voltage in comparison with the threshold voltage Ut, the coating either acted only as a capacitor when U\Ut and, when U[Ut, the behaviour during the anodic and cathodic parts of the power sine wave was different. In particular, due to the semi-conducting characteristics of the barrier layer, additional oxidation of the aluminium substrate occurred during the anodic part of the electrical signal, whilst metal deposition (and solvent reduction) occurred during the cathodic part; these mechanisms correspond to the blocked and pass directions of the barrier layer/electrolyte junction, respectively

Influence of the anion specificity on the electrochemical corrosion of anodized aluminum substrates

The electrochemical corrosion of anodized aluminum substrates in the presence of various electrolyte solutions at 0.01 mol L−1 concentration has been investigated. The comparative results were found to exhibit a correlation between the aggressiveness of the anions and their cosmotrope/chaotrope nature. The origin of the observed behavior was assumed to result from a variable resistance against dehydration during the pitting process

Mechanical behavior of black anodic films on 7175 aluminium alloy for space applications

Because of their low outgassing and their thermo-optical properties, black anodized aluminium parts are often used near optical instruments to manage thermal control in space applications. However, critical cases of flaking of the film were observed after simulated thermal ageing. To understand the mechanisms leading to flaking, the influence of the initial porosity of the film on its mechanical behavior during and after the black anodizing process has been investigated. The decrease of limit tensile stress with the porosity, the coloring and the sealing combined to thermal stresses due to the difference of thermal expansion coefficients between film and substrate have been shown to cause crazing in articular conditions. For high initial porosity films, thermal cycling ageing has a detrimental influence on the adhesion measured by scratchtesting. Numerical simulation has been used to simulate the combined effects of thermal stresses and film cracking on the stress field at the interface

Investigations into the coefficient of thermal expansion of porous films prepared on AA7175 T7351 by anodizing in sulphuric acid electrolyte

The aim of this study was to investigate the Coefficient of Thermal Expansion (CTE) of anodic films on 7175 T7351 aluminium alloy and to evaluate the influence of the film characteristics on this value. In particular, effects of porosity and post-treatments, such as coloring and sealing, were studied. Beam bending analysis was used as the experimental method and a numerical finite element model was developed to verify theoretical relationships hypotheses. The values determined and the choice of experimental method were then validated by comparing the experimental cracking temperature of anodic films with a theoretical value directly depending on the previously determined CTE