The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-discharge intensity depending on pH of the electrolyte

Ракоч, А. Г.; Гладкова, А. А.; Linn, Zayar; Strekalina, D.

doi:10.1016/j.surfcoat.2015.02.026

Cited by 57 publications

(25 citation statements)

References 25 publications

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“…The outer layers contain irregular pores and discharge channels. Between the two layers elongated pore bands which are orientated parallel to the surface and normally observed when using phosphate based electrolyte and applying a DC mode on magnesium alloy substrates [10,19]. The amount seems to decrease over the electrode distance.…”

Section: Coating Morphologymentioning

confidence: 81%

“…Some papers [19,20] claimed that changing the current mode, like using pulsed bipolar current mode, can change the coating composition. Duration and ratio of different stages during PEO process can be altered by the applied electrical process parameters [10], and as a result they influence the number, intensity and energy of the micro-discharges [21] which can determine the thermal and chemical conditions during PEO process, and thus change the phase composition of the coating. Current density is the main source to furnish energy for micro-discharge.…”

Section: Phase Compositionmentioning

confidence: 99%

“…In stage four, less frequent discharges appear as relatively large and long lasting sparks due to the thicker coating causing more difficulty in the initiation of such discharges [9]. The coatings formed by PEO on magnesium alloys are typically thick (ranges from tens to hundreds of microns [10]), porous, hard and well-adherent to the substrate [11]. The frictional, corrosive, electrical and thermal properties of these coatings have generated interest for their use in mechanical, aerospace and engineering equipment components, as well as for biomedical devices [12,13].…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Investigation of electrode distance impact on PEO coating formation assisted by simulation

Zhang

Blawert

Höche

et al. 2016

Applied Surface Science

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To better understand the influence of electrode distance between anode and cathode during plasma electrolytic oxidation (PEO) process, present study investigates its impact on the coating properties through experiments and assisted simulation. Firstly a model was built to simulate the effect of electrode distance on anode current using COMSOL software package. Complementary, PEO coatings were fabricated on AM50 magnesium alloy in alkaline electrolyte with different electrode distances under constant voltage by a DC power supply. Coating morphology, phase composition and coating thickness were studied at each electrode distance. Through combining the simulation and experiment results, the interaction between electrode distance and coating features is explored. It is demonstrated that under constant voltage mode, PEO coating features on both front and back sides of magnesium samples are affected by electrode distances, which ascribes to the changed current distribution in the bath and related average current density on the surfaces induced by electrode distances.

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Section: Coating Morphologymentioning

confidence: 81%

Section: Phase Compositionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Investigation of electrode distance impact on PEO coating formation assisted by simulation

Zhang

Blawert

Höche

et al. 2016

Applied Surface Science

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“…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].…”

mentioning

confidence: 99%

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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“…The capacity-based PEO equipment, which was used to perform the PEO treatments was described in more details in [33].…”

Section: Methodsmentioning

confidence: 99%

Corrosion protection of magnesium alloy by PEO-coatings containing sodium oleate

2019

Int. J. Corros. Scale Inhib.

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The kinetics of plasma-electrolytic oxidation (PEO) coating growth on AZ31 alloy in an alkaline-phosphate-aluminate electrolyte was studied. The structure, elemental composition and corrosion resistance of obtained PEO coatings were studied as a function of thickness. It was established that formation of the coating layers takes place in the following sequence: anodic, external and internal layers. Their formation occurs due to: 1) dielectric layer formation due to anodic conversion of surface; 2) ignition of powerful microdischarges in the transversal pores of the initially formed coating, forming the main part of its outer layer in case of increasing duration of the PEO process; 3) etching of magnesium alloy as a result of electrolyte penetration through the transversal pores to the metal substrate, followed by its anodizing; 4) ignition of microdischarges under the coatings outer layer, leading to formation of an inner layer. Existence of the coating inner layer causes a significant increase in corrosion resistance; however, it is still insufficient for long-lasting standalone corrosion protection. It was shown that sodium oleate (SOl) is the best inhibitor for the AZ31 substrate, therefore it was selected for impregnating PEO coatings. Impregnation of the coatings in 10 mM SOl solution increases their protective ability. Corrosion tests of PEO coated AZ31 samples in a climate chamber showed that the effect of impregnating is most pronounced for thin PEO coatings (20 μm). Under more corrosive conditions of salt spray test, impregnating the PEO coating with SOl increased the time until the appearance of the first corrosion marks. It also significantly slowed down the development of the corrosion marks compared with the samples coated by PEO without impregnation.

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The evidence of cathodic micro-discharges during plasma electrolytic oxidation of light metallic alloys and micro-discharge intensity depending on pH of the electrolyte

Cited by 57 publications

References 25 publications

Investigation of electrode distance impact on PEO coating formation assisted by simulation

Investigation of electrode distance impact on PEO coating formation assisted by simulation

High-Frequency-Induced Cathodic Breakdown during Plasma Electrolytic Oxidation

Corrosion protection of magnesium alloy by PEO-coatings containing sodium oleate

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