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Articles:

Martin, J., Leone, P., Nominé, A., Veys-Renaux, D., Henrion, G. and Belmonte, T.
Surface and Coatings Technology, In Press - available on line
2014

Resume: The chemical stability of the electrolyte in Plasma Electrolytic Oxidation (PEO) has been the subject of much systematic investigation. The present study reports on experimental results on the ageing of a usual PEO electrolyte and its effect on the oxidation of the Al2214 aluminium alloy. A solution of potassium hydroxide ([KOH] = 1 g/L) and sodium silicate ([Na2SiO3] = 1.65 g/L) diluted in deionized water was either aged by running a process during 8 h at most or not. Comparison between materials treated in either solution was made next. Cross-checked experimental results from analytical chemistry, plasma diagnostics and materials science clearly evidence the effect of ageing of the electrolyte on the PEO process behaviour. The transition from arc to "soft" regime occurs earlier (from 18 min to 13 min processing time) as the electrolyte gets older. The concomitant decrease in the electrolyte electrical conductivity (from 7.5 mS.cm-1 to 6.8 mS.cm-1) is correlated with changes in the ionic species content in the electrolyte. Fast video-imaging (125 kfr./s) reveals that the micro-discharge characteristics are also sensitive since they become less numerous per area and unit time (from 220.103 cm-2s-1 to 120.103 cm-2s-1), exhibit shorter lifetime and have smaller sizes as the electrolyte ages. SEM observations of the as-grown coatings combined with roughness measures indicate a decrease in coating thickness ( ~ 20 %) and roughness ( ~ 30 %) while species from the electrolyte accumulate in the outer porous layer simultaneously with a gradual enrichment in a-Al2O3 phase in the inner compact layer (from 36 % to 43 %). Finally, within the PEO conditions that were used, it is shown that the ageing phenomenon of the PEO electrolyte starts to influence the process after 2 hours aged.

Equipe: Département CP2S : Expériences et Simulations des Plasmas Réactifs - Interaction plasma-surface et Traitement des Surfaces ESPRITS

Veys-Renaux, D., ROCCA, E., Martin, J. and Henrion, G.
Electrochimica Acta, 124:30-45
2014

Resume: In the framework of the new ecological regulations, micro-arc oxidation (MAO) appears as an alternativeto usual processes in the field of corrosion protection of Mg alloys. In this work, the initial stagesof anodic layer growth in KOH-based electrolytes are studied up to and beyond the initiation of themicro-arc regime.The properties of the first anodized film preceding the occurrence of the dielectric breakdown (corre-sponding 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 electronmicroscopy (SEM), X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy reveal boththe change of morphology and chemical state of silicate and fluoride in the anodized layer before andafter the micro-arc regime. In terms of electrochemical behaviour, investigated by stationary methodsand electrochemical impedance spectroscopy (EIS) in reference corrosive water, the anodic film grownin the silicate medium provides the best corrosion resistance thanks to a thick layer containing Mg2SiO4,whose degradation products seal the porosities of the coating.

Equipe: Département CP2S : Expériences et Simulations des Plasmas Réactifs - Interaction plasma-surface et Traitement des Surfaces ESPRITS

Belmahi, M., Bulou, S., Thouvenin, A., de Poucques, L., Hugon, R., Le Brizoual, L., Miska, P., Genève, D., Vasseur, J.-L. and Bougdira, J.
Plasma Processes and Polymers, 11:551-558
2014

Equipe: Département CP2S : Expériences et Simulations des Plasmas Réactifs - Interaction plasma-surface et Traitement des Surfaces ESPRITS

Nominé, A., Martin, J., Noel, C., V. Bardin, I., L. Kovalev, V., Belmonte, T., G. Rakoch, A. and Henrion, G.
Applied Physics Letters, 104:081603
2014

Resume: Plasma electrolytic oxidation (PEO) processing of EV31 magnesium alloy has been carried out in fluoride containing electrolyte under bipolar pulse current regime. Unusual PEO cathodic microdischarges have been observed and investigated. It is shown that the cathodic micro-discharges exhibit a collective intermittent behavior, which is discussed in terms of charge accumulations at the layer/electrolyte and layer/metal interfaces. Optical emission spectroscopy is used to determine the electron density (typ. 1015cm_3) and the electron temperature (typ. 7500K) while the role of F_ anions on the appearance of cathodic micro-discharges is pointed out.

Equipe: Département CP2S : Expériences et Simulations des Plasmas Réactifs - Interaction plasma-surface et Traitement des Surfaces ESPRITS

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