Role of Phase Composition of PEO Coatings on AA2024 for In-Situ LDH Growth

Plasma electrolytic oxidation (PEO) is an environmentally friendly anodizing technique leading to the formation of a ceramic-like coatings under high-voltage discharges. Layered double hydroxides (LDHs) were grown directly on γ, α, and amorphous Al2O3 powders, respectively, in order to investigate the phase responsible for in-situ LDH growth on PEO coating. Furthermore, it is shown that LDH growth is limited by the high tortuosity of the PEO layer and the accessibility of Al(OH) − 4 anions from the substrate covered with thin amorphous aluminum oxide, through the pores

Comparison of 2D and 3D Plasma Electrolytic Oxidation (PEO)-Based Coating Porosity Data Obtained by X-ray Tomography Rendering and a Classical Metallographic Approach

In this work, the porosity of plasma electrolytic oxidation (PEO)-based coatings on Al- and Mg-based substrates was studied by two imaging techniques-namely, SEM and computer microtomography. Two approaches for porosity determination were chosen; relatively simple and fast SEM surface and cross-sectional imaging was compared with X-ray micro computed tomography (microCT) rendering. Differences between 2D and 3D porosity were demonstrated and explained. A more compact PEO coating was found on the Al substrate, with a lower porosity compared to Mg substrates under the same processing parameters. Furthermore, huge pore clusters were detected with microCT. Overall, 2D surface porosity calculations did not show sufficient accuracy for them to become the recommended method for the exact evaluation of the porosity of PEO coatings; microCT is a more appropriate method for porosity evaluation compared to SEM imaging. Moreover, the advantage of 3D microCT images clearly lies in the detection of closed and open porosity, which are important for coating properties

The Role of Cu-Based Intermetallic on the Direct Growth of a ZnAl LDH Film on AA2024

The direct ZnAl layered double hydroxide growth on AA2024 is a fast-occurring reaction, yet is characterized by an inhomogeneous film thickness. It has been shown that at the periphery of Cu-rich intermetallic, the flakes tend to be larger and denser. A combination of in situ and ex situ measurements were used to monitor the changes in the layered double hydroxide film grown on the regions of intermetallics. Immediately after immersion, an activation of the intermetallic phases is observed due to the dealloying process with an almost immediate film growth. Dealloying is followed by trenching of the adjacent Al matrix leading to an excessive production of large and dense layered double hydroxide flakes at the periphery of the intermetallic. However, the scanning electron microscopy cross-section images revealed that the trenching process leads to defects in the area surrounding the intermetallic. This could weaken the corrosion resistance performance of the layered double hydroxide conversion coating and lead to adhesion failure of consecutive polymer coatings. Nevertheless, this work highlights a few advantages and drawbacks of the layered double hydroxide conversion coatings and pathways to its potential optimization and improvement

Small-angle neutron scattering and magnetically heterogeneous state in Sr2FeMoO6–δ

Single-phase strontium ferromolybdate (Sr2FeMoO6–δ) samples with different degrees of the superstructural ordering of the Fe/Mo cations were obtained from partially reduced SrFeO3–х, SrMoO4 precursors by the solid-state technology. The study of the temperature dependences of the magnetization measured in the field-cooling and zero-field-cooling regimes indicated an inhomogeneous magnetic state of the samples. The presence of magnetic regions of different nature has also been revealed by the small-angle neutron scattering. For the Sr2FeMoO6–δ samples with different superstructural ordering of the Fe/Mo cations and for all values of the magnetic field induction in the range up to 1.5 T and of the scattering vector in the interval 0.1 >q >0.005 Å–1, the analytical dependence I ~ q–α obeys the Porod law (α ≈ 4), which corresponds to an object with a smooth and well-marked surface and polydisperse grain size. Deviations from the Porod law in the q > 0.1 Å–1 region and a weakening of the neutron scattering in applied magnetic fields may be ascribed to magnetic inhomogeneities with diameters D <6 nm, which are partially destroyed /oriented by magnetic fields В ≥1.5 T.publishe

Computational modelling of plasma electrolytic oxidation process induced damage in extruded Mg material

Magnesium (Mg) alloys are an attractive constructive material due to their light weight and high mechanical strength. Plasma electrolyte oxidation (PEO) treatment of Mg alloys creates a thin ceramic coating with protective effects against mechanical wear and corrosion. The coating properties like its porosity and thickness can be adjusted by PEO process parameters and at the same time affects the material behaviour under tensile strength. In this work, dedicated slow-strain rate experiments of differently PEO coated Mg alloy dog-bone shaped specimen were conducted and the coating porosity, thickness and crack spacing were analyzed in order to deduce a predictive Finite Element Method (FEM) damage model. The results indicate that the thicker, more porous coatings lead to material failure at smaller strains in plastic regions. The effect can be implemented via partial differential equation into the FEM model

The Influence of PSA Pre-Anodization of AA2024 on PEO Coating Formation: Composition, Microstructure, Corrosion, and Wear Behaviors

In the frame of the current work, it was shown that plasma electrolytic oxidation (PEO) treatment can be applied on top of phosphoric sulfuric acid (PSA) anodized aluminum alloy AA2024. Being hard and well-adherent to the substrate, PEO layers improve both corrosion and wear resistance of the material. To facilitate PEO formation and achieve a dense layer, the systematic analysis of PEO layer formation on the preliminary PSA anodized layer was performed in this work. The microstructure, morphology, and composition of formed PEO coatings were investigated using scanning electron microscopy (SEM), x-ray diffraction (XRD), and glow-discharge optical emission spectroscopy (GDOES). It was shown that under constant current treatment conditions, the PSA layer survived under the applied voltage of 350 V, whilst 400 V was an intermediate stage; and under 450 V, the PSA layer was fully converted after 5 min of the treatment. The comparison test with PEO formation on the bare material was performed. It was confirmed that during the "sparking" mode (400 V) of PEO formation, the PEO coatings, formed on PSA treated AA2024, were more wear resistant than the same PEO coatings on bare AA2024

Magnetic Properties of La0.9A0.1MnO3 (A: Li, Na, K) Nanopowders and Nanoceramics

Nanocrystalline La0.9A0.1MnO3 (where A is Li, Na, K) powders were synthesized by a combustion method. The powders used to prepare nanoceramics were fabricated via a high-temperature sintering method. The structure and morphology of all compounds were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). It was found that the size of the crystallites depended on the type of alkali ions used. The high-pressure sintering method kept the nanosized character of the grains in the ceramics, which had a significant impact on their physical properties. Magnetization studies were performed for both powder and ceramic samples in order to check the impact of the alkali ion dopants as well as the sintering pressure on the magnetization of the compounds. It was found that, by using different dopants, it was possible to strongly change the magnetic characteristics of the manganites

Le contrôle de réactivité d'aluminium en peinture anti-corrosion résistant à la haute température

The aim of this work is to understand Al reactivity at a fundamental level and to use this knowledge for the development of Cr(VI)-free sacrificial paints for high temperature aeronautic applications.Pure Al, Al intermetallics and alloys are studied. The atomic emission spectroelectrochemistry is used to isolate the individual phenomena during paint degradation. A linear relationship between cathodic current and Al dissolution is shown and interpreted with a simple model where OH- generation, Al(OH)3 formation/dissolution and Al(OH)4− diffusion are kinetically coupled. For early formulations significant binder dissolution is measured under cathodic polarization, while Al reactivity is suppressed. Similar Al behavior is observed for Al-Mg intermetallics. In terms of the cathodically generated hydroxide mechanism these results are interpreted as the reaction of either Si or Mg with OH-. These results confirm the cathodic Al reactivity model. The loss of anodic activity is explained by the loss of electrical contact at the metal/oxide/polymer/substrate interfaces and the oxide layer modifications are studied to circumvent this problem. It is found that Mg2+ ions retard Al passivation and improve conductivity due to the formation of semiconducting spinel. Finally, two major factors are found to be critical for the Al reactivity control: solution pH and oxide properties. Using additives to control them, the new coating formulation stable up to 550°C and protective more than 1000H in salt spray test is developedL'objectif de ce travail est de comprendre le mécanisme fondamentales de la réactivité de l'Al et d'utiliser ces connaissances pour développer une peinture sacrificielle sans Cr(VI). L'Al pur, ses alliages et ses intermétalliques sont étudiés. La spectroélectrochimie atomique à émission de plasma est utilisée pour isoler les phénomènes individuels au cours de la dégradation. La relation linéaire entre le courent cathodique et la vitesse de dissolution de l'Al est démontrée et interprétée par un modèle dans lequel la génération d'OH-, la formation/dissolution d'Al(OH)3 et la diffusion d'Al(OH)4- sont cinétiquement couplées. La dissolution significative du liant de première formulation a été accompagnée de la passivation d'Al pendant la polarisation cathodique. Un comportement similaire est observé pour des intermétalliques Al-Mg. Ceci est interprété comme la réaction des composants (du Mg ou du Si) avec OH-. Ces résultats confirment le modèle de réactivité cathodique de l'Al. La perte d'activité anodique est expliquée par la perte de contact électrique au niveau des interfaces métal/oxyde/polymère/substrat. Les modes de transformation de la couche d'oxyde sont étudiées. Les ions de Mg2+ retardent la passivation d'Al par la formation de spinelle semi-conducteur qui est responsable de l'amélioration de la conductivité.Finalement, deux facteurs principaux sont jugés essentiels pour la réactivité de l'Al: le pH de la solution et la conductivité de l'oxyde. En utilisant des additifs pour contrôler ces facteurs, on a proposé la formulation d'une nouvelle peinture, stable jusqu'à 550°C et sacrificielle plus de 1000 H au test de brouillard salin

The control of Al reactivity for high temperature anticorrosion paint formulation

The aim of this work is to understand Al reactivity at a fundamental level and to use this knowledge for the development of Cr(VI)-free sacrificial paints for high temperature aeronautic applications.Pure Al, Al intermetallics and alloys are studied. The atomic emission spectroelectrochemistry is used to isolate the individual phenomena during paint degradation. A linear relationship between cathodic current and Al dissolution is shown and interpreted with a simple model where OH- generation, Al(OH)3 formation/dissolution and Al(OH)4 diffusion are kinetically coupled. For early formulations significant binder dissolution is measured under cathodic polarization, while Al reactivity is suppressed. Similar Al behavior is observed for Al-Mg intermetallics. In terms of the cathodically generated hydroxide mechanism these results are interpreted as the reaction of either Si or Mg with OH-. These results confirm the cathodic Al reactivity model. The loss of anodic activity is explained by the loss of electrical contact at the metal/oxide/polymer/substrate interfaces and the oxide layer modifications are studied to circumvent this problem. It is found that Mg2+ ions retard Al passivation and improve conductivity due to the formation of semiconducting spinel. Finally, two major factors are found to be critical for the Al reactivity control: solution pH and oxide properties. Using additives to control them, the new coating formulation stable up to 550C and protective more than 1000H in salt spray test is developedL objectif de ce travail est de comprendre le mécanisme fondamentales de la réactivité de l Al et d utiliser ces connaissances pour développer une peinture sacrificielle sans Cr(VI). L Al pur, ses alliages et ses intermétalliques sont étudiés. La spectroélectrochimie atomique à émission de plasma est utilisée pour isoler les phénomènes individuels au cours de la dégradation. La relation linéaire entre le courent cathodique et la vitesse de dissolution de l Al est démontrée et interprétée par un modèle dans lequel la génération d OH-, la formation/dissolution d Al(OH)3 et la diffusion d Al(OH)4- sont cinétiquement couplées. La dissolution significative du liant de première formulation a été accompagnée de la passivation d Al pendant la polarisation cathodique. Un comportement similaire est observé pour des intermétalliques Al-Mg. Ceci est interprété comme la réaction des composants (du Mg ou du Si) avec OH-. Ces résultats confirment le modèle de réactivité cathodique de l Al. La perte d'activité anodique est expliquée par la perte de contact électrique au niveau des interfaces métal/oxyde/polymère/substrat. Les modes de transformation de la couche d'oxyde sont étudiées. Les ions de Mg2+ retardent la passivation d Al par la formation de spinelle semi-conducteur qui est responsable de l'amélioration de la conductivité.Finalement, deux facteurs principaux sont jugés essentiels pour la réactivité de l Al: le pH de la solution et la conductivité de l oxyde. En utilisant des additifs pour contrôler ces facteurs, on a proposé la formulation d une nouvelle peinture, stable jusqu'à 550C et sacrificielle plus de 1000 H au test de brouillard salin.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Active self-healing coating for galvanically coupled multi-material assemblies

A new \"self-healing\" coating concept for active corrosion protection on galvanically coupled multi-material structures is reported in the present work. The novel approach is based on the combination of two types of nanocontainers with two different inhibitors in the same coating system. The nanocontainers confer a triggered release of both inhibitors which act in a synergistic way when an aluminum alloy is galvanically coupled with carbon fiber reinforced plastic. The layered double hydroxide and bentonite were used as functional nanocarriers for 1,2,3-benzotriazole and Ce3+ inhibitors respectively. Scanning vibrating electrode technique has been applied for the monitoring of galvanic corrosion activities and kinetics of self-healing processes in confined defects. The effective inhibition of electrochemical activity in the defects on coated galvanically coupled aluminum alloy with carbon fiber reinforced plastic was demonstrated for the first time. (C) 2014 Elsevier B.V. All rights reserved