Initial stages of multi-phased aluminium alloys anodizing by MAO: micro-arc conditions and electrochemical behaviour

International audienceThe electrochemical behaviour of AA1050 and AA2214 alloys was studied in a KOH/silicate in order to investigate the first stages of the coating formation during MAO process (by SEM and electrochemical measurements). Before the sparking initiation, occurring around 300 V for both alloys, a thin inner layer of aluminium oxide grows by a classical anodizing mechanism, and induces a great enhancement of the electrochemical resistance. Then, the micro-arc regime beyond 300 V leads to the formation of a dense, thicker but cracked external aluminium oxide layer. Afterwards, from 400 V, a sharp increase of current density named ``current wall'' and due to an increase of oxide conductivity, leads to a significant oxygen release and the occurrence of large and energetic sparks. Microscopic analyses reveal that the presence of copper-rich intermetallic phases in the multi-phased alloy (AA2214) induces the incorporation of copper nanoparticles in the gamma-alumina layer formed by the sparking phenomenon. These copper particles increase the reaction of water oxidation during the classical anodizing stage, and result in a delay of the sparking initiation during the galvanostatic anodizing and in a lower electrochemical resistance of the anodized layers

Micro-arc oxidation of AZ91 Mg alloy: An in-situ electrochemical study

International audienceThe growth of coatings by micro-arc oxidation is based on the formation of spark discharges appearing at the metal/electrolyte interface over the dielectric breakdown potential. On Mg and Mg alloys, this anodizing process is usually performed in alkaline electrolytic bathes in which fluorides or silicates are added. In concentrated KOH electrolytes, in-situ electrochemical impedance spectroscopy measurements are performed at high voltage (up to 80 V) and show that the dielectric breakdown occurs for a threshold capacitance at the electrochemical interface. The main positive role of fluorides and silicates in both the reinforcement of the resisting properties of the coating and its further growth under spark effect is highlighted

Electrochemical behavior of zinc in KOH media at high voltage: Micro-arc oxidation of zinc

International audienceElectrochemical behavior of pure zinc was studied in alkaline KOH-based media over a large range of voltage (0-300 V) in order to investigate the occurrence of the micro-arc phenomenon, responsible for the formation of ``ceramic-like'' oxide layer. According to electrolyte composition, three anodizing steps can be observed, first, a classical anodizing process at low voltage, a voltage range of micro-arc phenomenon with a more or less intense water oxidation, and a sharp current rise corresponding to an increase of electronic conductivity of ``metal/oxide/electrolyte'' interface. Both KOH concentration and presence of additives in the electrolytic bath control the micro-arc voltage range, and the composition of anodized layer. Nevertheless, the internal layer of the coating remains mainly constituted by crystallized ZnO, formed by a mechanism of solid-state diffusion of O2- under electric field. Despite the significant coating thickness (several microns), the electrochemical characterization of the corrosion behavior in NaCl electrolyte only shows a slight improvement of the performances of the zinc anodized in presence of additives in KOH electrolyte, compared to an untreated one

Corrosion behavior of AZ91 Mg alloy anodized by low-energy micro-arc oxidation: Effect of aluminates and silicates

International audienceAZ91 Mg alloy was anodized by micro-arc oxidation under a low constant current density (10 mA cm(-2)) in an electrolytic bath containing KOH 3M + KF 0.5M + Na3PO4 center dot 12H(2)O 0.25 M. The effects of the anodizing process duration and the presence of aluminate (NaAlO2 0.2 M) or silicate (Na2SiO3 center dot 9H(2)O 0.2 M) as additives were investigated. In terms of corrosion resistance, electrochemical methods (namely, potentiodynamic scans and chronoamperometric measurements) corroborate the results of salt spray test and show that the resistance to pitting corrosion of the treated pieces is not systematically improved by a thicker anodized layer. Actually, the composition of the protective coating is the key factor: the best resistance is obtained in the presence of silicate, which plays the role of self-healing agents in corrosive conditions, whereas the incorporation of aluminate in the oxide has a very weak effect on the corrosion resistance of the treated alloy

Electrochemical behaviour of titanium in KOH at high potential

International audienceElectrochemical behaviour of pure titanium was studied in KOH-based media over a large voltage range (0-700 V) in order to understand the micro-arc phenomenon occurrence, leading to the formation of a “ceramic-like” oxide layer. The influence of various KOH concentrations in the electrolytic bath (from 0.01 to 0.2 M) was investigated on titanium substrate by a potentiodynamic scan. Whatever the concentration, the electrochemical behaviour of titanium in KOH media can be described in three steps according to voltage: i) a “conventional anodizing” step controlled by the solid-state diffusion of O2− and Ti4+ ions under the electric field effect, ii) a voltage range during which the micro-arc phenomenon occurs, named “micro-arc region”, iii) a “sharp current rise” induced by the increase of oxide conductivity. After anodizing in galvanostatic conditions, XRD and EDS analyses combined with SEM observations show that all samples have similar morphology with a dense internal layer and a porous external one, constituted by titanium dioxide under anatase and rutile crystallized forms. The electrochemical behaviour of anodized layers was investigated by electrochemical impedance spectroscopy measurements. A Mott-Schottky analysis reveals a n-type semi-conducting behaviour of the oxide layers with a very high donor concentration between 1022 and 1023 cm−3

Initial stages of AZ91 Mg alloy micro-arc anodizing: Growth mechanisms and effect on the corrosion resistance

10th International Symposium on Electrochemical Methods in Corrosion Research (EMCR), Maragogi, BRAZIL, NOV 18-23, 2012International audienceIn the framework of the new ecological regulations, micro-arc oxidation (MAO) appears as an alternative to usual processes in the field of corrosion protection of Mg alloys. In this work, the initial stages of anodic layer growth in KOH-based electrolytes are studied up to and beyond the initiation of the micro-arc regime. The properties of the first anodized film preceding the occurrence of the dielectric breakdown (corresponding to the start of the micro-arc regime) are mainly determined by the incorporation of additives (fluorides or silicates) in the film, as shown by in situ electrochemical measurements. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy reveal both the change of morphology and chemical state of silicate and fluoride in the anodized layer before and after the micro-arc regime. In terms of electrochemical behaviour, investigated by stationary methods and electrochemical impedance spectroscopy (EIS) in reference corrosive water, the anodic film grown in the silicate medium provides the best corrosion resistance thanks to a thick layer containing Mg2SiO4, whose degradation products seal the porosities of the coating

New zinc-rare earth alloys: Influence of intermetallic compounds on the corrosion resistance

International audienceNew ZnRE 1-2 wt.% alloys (RE = Ce, La and Mischmetal) were synthesized. Microstructural analyses reveal that rare-earth metals are exclusively present in intermetallic phases contained within the zinc matrix: CeZn11 or LaZn13 in binary alloys; Ce1-xLaxZn11 and CeyLa1-yZn13 in Zn-Mischmetal alloys. This phase equilibrium is confirmed by a thermodynamic description of the Zn-rich corner of Zn-Ce-La system. Both intermetallic phases CeZn11 and LaZn13 induce a cathodic inhibition of the corrosion of the ZnRE alloys in comparison with pure zinc. In the specific case of cerium addition, a significant anodic inhibition is also observed

Oxide Growth Mechanism on Mg AZ91 Alloy by Anodizing: Combination of Electrochemical and Ellipsometric In-Situ Measurements

International audienceThe sparking initiation during the anodizing of metals in passivating electrolytic media is highly dependent on the physico-chemical properties of the evolving dielectric interface. The present paper focuses on magnesium alloy AZ91 and describes its anodizing in KOH 3 M over the potential range 0 to 40 V by combining in situ ellipsometric and electrochemical methods. In a first step, a thin and compact MgO anodic film is formed by ionic migration with a rate of 0.4 nm V−1. At 4 V to 5 V, the growth stresses lead to a sharp cracking of this MgO layer and the precipitation of a porous external Mg(OH)2 layer. Under this overlayer acting as a membrane, a defective MgO film grows with a higher growth rate (1.1 nm V−1), inducing a new passivation state. This inner MgO layer is characterized by a relatively high dielectric constant and a large thickness (100 nm at 40 V) and is probably responsible for the low dielectric breakdown voltage observed during the anodizing process of magnesium alloy

In situ spectroelectrochemical ellipsometry using super continuum white laser: Study of the anodization of magnesium alloy

This paper is part of the Conference Collection: 8th International Conference on Spectroscopic Ellipsometry 2019, ICSEInternational audienceThis work shows the interest to use a real time white laser-based ellipsometer to characterize complex electrolyte | electrode interface during electrochemical process in aqueous-based medium. This method is proposed to probe electrochemical interfaces which are usually not suitable to the full extent application of ellipsometry due to great disturbance of the reflected light flux provoked by gas evolution or roughness. In situ spectroelectrochemical ellipsometry combining such a visible super continuum fiber laser-band source was not previously reported to the best of the authors' knowledge. The setup was employed to monitor an electrochemical process whose the mechanism was previously incompletely described: the pre-spark anodization regime of plasma electrolytic oxidation process of Mg alloy AZ91D in 3 M KOH electrolyte. Above the anodization voltage of 4 V, the side water oxidation reaction induced light diffusion that reduces reflected light beam intensity. The process is here monitored in an extended This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS 2 voltage range from 4 to 40 V and in an extended spectral range (495-800 nm). In the presented case, the use of a visible super continuum fiber laser-band source enhanced the signal-to-noise ratio giving access to a more deeper picture of the triplex layer structure during surface repassivation by monitoring the evolution of the outer, inner and interfacial layers

Conversion of steel by polyphenolic model molecules: Corrosion inhibition mechanism by rutin, esculin, esculetol

International audienceThis paper focuses on the potential corrosion protection of steel in presence of three polyphenolic model molecules (rutin, esculin, esculetol) in a weak acidic medium (pH = 4). From an electrochemical point of view, the cathodic inhibition was attributed to the oxygen scavenger role of the considered antioxidant compounds. SEM observations, Raman spectroscopy and Synchrotron X-ray absorption spectroscopy revealed furthermore the formation of a surface film by precipitation of iron polyphenolate conversion products, in which iron III is likely coordinated with free-catechol groups. Glycosylated catechol groups are rather adsorbed on a FeOOH layer