Surface Charge at the Oxide/Electrolyte Interface: Toward Optimization of Electrolyte Composition for Treatment of Aluminum and Magnesium by Plasma Electrolytic Oxidation

Nominé, Alexandre; Martin, Julien; Noël, Cédric; Henrion, G.; Belmonte, T.; Bardin, I. V.; Lukeš, Petr

doi:10.1021/acs.langmuir.5b03873

Cited by 44 publications

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“…The presence of cathodic discharge on Aluminium surface at a pH significantly higher than the isoelectric point (IEP) may appear at first sight surprising and somehow contradictory with some published results [23]. On the other hand, the fact that the presence of cathodic discharges is triggered at high frequency (above 2 kHzsee fig.…”

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confidence: 70%

“…However, recent studies have shown the possibility to observe micro-discharges under cathodic polarization, both on Magnesium [20][21][22][23] and Aluminum [23,24] substrates either by using uncommon compositions of electrolytes [20,21] or by applying high-current density (several kA dm −2 ) [24]. A subsequent study in similar conditions to reference [20] has shown that cathodic discharges do not take part in the growth of an oxide coating and rather have detrimental effect on it [22].…”

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confidence: 99%

“…Conditions in which cathodic and anodic discharges in different substrate and electrolyte conditions have been set out by Nominé et al [23] and an explanation based on the formation of an electrical double layer (EDL) has been proposed. Results of this study tend to show that an EDL may form on the surface of PEO coatings and generate a local electric field that can either screen or amplify the externally-applied electric field.…”

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High-Frequency-Induced Cathodic Breakdown during Plasma Electrolytic Oxidation

et al. 2017

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The present communication shows the possibility of observing micro-discharges under cathodic polarization during Plasma Electrolytic Oxidation (PEO) at high frequency. Cathodic micro-discharges can ignite beyond a threshold frequency found close to 2 kHz. The presence (resp. absence) of an Electrical Double Layer (EDL) was put forward to explain how the applied voltage can be screened, which therefore prevents (resp. promotes) the ignition of a discharge. Interestingly, in the conditions of the present study, the EDL requires between 175 and 260 µs to form. This situates the expected threshold frequency between 1.92 and 2.86 kHz, which is in good agreement with the value obtained experimentally.Plasma Electrolytic Oxidation (PEO) is at a pivotal moment of its development. If this process has undeniable assets for surface treatments of light alloys (high growth rate, high ultimate coating thickness, ecofriendliness, wear and corrosion resistance, etc.), the high energy consumption and the lack of understanding of the fundamental processes underlying the breakdown and the oxidation mechanisms still limit a larger scale development.Improving the energetic efficiency of PEO is a major stake and several strategies can then be followed: dual treatments[1], addition of nanoparticles [2,3], electrolyte composition [4,5] and electric regimes [5][6][7][8]. Here, we focus on the influence of the electric regime as it has a direct and strong influence on both the space and time properties of micro-discharges (MD), which affect the coating growth. Interestingly, the best properties and the highest growth rates of coatings are obtained while using bipolar current waveform [5,9] even though MDs are only observed during the anodic (i.e. positive) half-period [10][11][12] in these conditions.To explain this observation, one should keep in mind that the growth of PEO coatings relies on a balance between the constructive effect of MDs (oxidation and coating growth) and their destructive effect (large craters and porosities). Several studies conducted either by high speed video imaging [5,6,[11][12][13][14][15][16][17][18] or by direct electrical measurements [10,19] have investigated this balance. They made it possible to correlate space and time parameters of MDs (number, life-time, size) and the properties of as-grown coatings (thickness, porosity). In short, large and long-lived discharges promote the growth of high temperature phases but create large porosities (several tens of µm), high roughness at limited growth rates (strong destructive effect of MDs). On the other hand, too small and short-lived MDs will create more homogeneous coating but with lower amount of high-temperature phases and also at limited growth rates (weak constructive effect).Nevertheless, all these studies support the same conclusion: adjusting the cathodic half period probably opens up the possibility to tune the behaviour of anodic MDs. Then, combining high-growth rates, low levels of large-scale porosity (bigger than 10 µm) and a significant proportio...

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High-Frequency-Induced Cathodic Breakdown during Plasma Electrolytic Oxidation

et al. 2017

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“…31 The vigorous plasma discharges further facilitated the incorporation of Ni(OH) 2.4 0.4À into the inorganic layer. Ni , and H 2 O to produce Ni(OH) 2 , as shown in eqn (6). The instantaneous high temperature and pressure around the discharge channels during the PEO process led to partial the oxidation of Ni(OH) 2 to form NiO, as described in eqn (8).…”

Section: 67mentioning

confidence: 99%

“…6 The characteristics of the inorganic layer formed by the PEO process are affected by the plasma characteristics (size, density, intensity, and duration), which can be controlled by the electrolyte conditions, e.g. its chemical composition, and the processing variables, such as electrical parameters, including current density, frequency, and current mode.…”

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Decoration of an inorganic layer with nickel (hydr)oxide via green plasma electrolysis

Fatimah

Khoerunnisa

2018

RSC Adv.

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In this study, we describe the green plasma electrolysis of a magnesium alloy in alkaline electrolyte to produce a hybrid inorganic layer with nickel (hydr)oxide incorporated in a matrix of magnesium oxide, and investigate the electrochemical and optical properties of this material. The addition of Ni(NO 3 ) 2 $6H 2 O to the electrolyte reduced the size of the micro-defects found in the inorganic layer after plasma electrolysis by inducing soft plasma discharges. As a result, through cyclic voltammetry and polarization tests, the corrosion stability of the sample containing nickel (hydr)oxide was significantly enhanced. Measurement of the optical properties reveals that the material possesses excellent energy efficiency as indicated by a high solar absorptivity of $0.92 and a low infrared emissivity of $0.13 which are presumably due to the inherent dark-brown colour of nickel (hydr)oxide. We expect that these results will have implications in the development of functional materials with excellent optical and corrosion properties by considering green processing utilizing alkaline electrolyte.

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