Plasma Electrolytic Process Research Articles

Technological Methods and Equipment for Plasma-Electrolytic Processing of Metals and Alloys

The proposed review presents the results of the analysis of the equipment and technological methods for implementing the plasma electrolytic chemical-thermal treatment and plasma electrolytic polishing. A group of methods based on the immersion of workpieces in an electrolyzer was considered, as well as a method of a local treatment by the jet feed of an electrolyte to the area of the surface being treated. The influence of the design and characteristics of the electrolyzer, the power source and the system for feeding the work-piece into the electrolyte on the specifics of plasma electrolytic treatment methods is shown. Technological methods improving the quality and productivity of treatment are considered. Limitations in the applicability of the existing equipment were revealed, and the prospects for the improvement in the field of development of equipment and technologies for plasma electrolytic treatment were determined.

Simulation of voltage characteristics of vapor-gas shell during electrolyte-plasma treatment

Electrolytic plasma processing (EPT) is a method of surface treatment of materials based on the use of plasma and an electrolytic solution. This abstract discusses the principle of action, main applications and potential benefits of EPТ. The EPТ technique involves immersing the object being treated in an electrolytic solution, after which an electric current is applied, causing decomposition of the solution and the formation of a plasma cloud at the surface of the material being processed. Exposure to plasma and chemically active components of the solution makes it possible to modify the surface of the material, improving its properties, such as adhesion, strength and corrosion resistance. EPТ is widely used in a variety of industries, including metalworking, electronics, medical equipment and food processing. The advantages of the method include high efficiency, the ability to process complex shapes and materials, as well as environmental safety due to the absence of the use of chemically aggressive substances. Electrolytic plasma processing is a promising direction in the field of surface modification of materials with a wide range of potential applications. In this paper, theoretical studies of vapor-gas shell formation in the near-surface region of structural steels in the cathodic heating mode of electrolyte-plasma treatment were considered. The technology of electrolyte-plasma hardening, providing the required mechanical properties of products, which are often subjected to wear, temperature and force effects, was investigated. Based on the results of theoretical studies, mathematical calculations of voltage, current density, and dependence graphs were made to obtain a model of vapor-gas shell formation in the process of cathodic heating. Modeling of calculations of vapor-gas shell formation in the process of cathodic heating was carried out with the help of Maple program.

Plasma Electrolytically Surface-Engineered SS316L Bipolar Plates for PEM Fuel Cells

The proton exchange membrane fuel cell (PEMFC) technology is one of the clean energy technologies with advantages like, quick start-up, low working temperature (60-90 oC), high specific energy and specific power, and high reliability. Stainless steel bipolar plates for PEMFC offer many advantages over conventional graphite and graphite composites. Some of them have superior mechanical properties over graphite, the possibility to fabricate in thin cross sections, and easy formation of flow fields by stamping, which is more economical when compared with the grooving of the graphite counterparts. Despite these advantages, metallic bipolar plates are prone to corrosion in fuel cell working conditions which is often overcome by oxide layer formation on the surface. But the interfacial ohmic loss between the metallic bipolar plate and membrane electrode assembly due to oxide formation decreases the overall performance of PEMFC. For combating these problems, there are different surface engineering techniques like chemical vapor deposition, physical vapor deposition, sputtering, thermal nitridation, etc. Among all methods, plasma electrolytic processing has advantages like less processing time, use of environmentally friendly electrolytes, economical process, easy scale-up for mass production and etc.The 316L stainless steel was surface-engineered using the cathodic plasma electrolytic process in two different aqueous electrolytes containing urea and acetic acid, which acted as a nitrogen and carbon source, respectively. The processed samples were characterized using a scanning electron microscope to study surface morphology and for qualitative chemical analysis. It revealed the diffusion of nitrogen and carbon into the surface in respective electrolytes. The electrochemical performance of as-received/bare 316L and surface-engineered samples were evaluated in a simulated fuel cell environment (half-cell conditions). The electrolyte comprised 0.5 M H2SO4 with 2 ppm HF at 80 oC, and it was purged with N2 or air to simulate anodic and cathodic conditions, respectively. The potentiodynamic polarization studies revealed similar active-passive behavior for bare 316L and surface-engineered samples. Different electrochemical parameters like corrosion current density (icorr), corrosion potential (Ecorr), and passive current density (ipassive) were deduced. The corrosion potentials for the bare sample were observed to have -0.306 V and -0.316 V in cathodic and anodic conditions, respectively. After surface modification, corrosion potentials shifted to more noble values, i.e., -0.234 V, -0.250 V for samples processed in urea solution and -0.219 V, -0.198 V for samples processed in the acetic acid solution for anodic and cathodic conditions, respectively. But the surface-modified samples showed higher passive current densities for both anodic and cathodic conditions than the bare sample, suggesting inferior passivation ability.Also, long-term potentiostatic studies for 8 hours were conducted to understand the samples' behavior. The current densities at the end of the test were -28.7 µA cm-2 (anodic condition) and 7.5 µA cm-2 (cathodic condition) for bare samples. While the samples processed in urea solution showed 27.4 and 9.0 µA cm-2, and the samples processed in acetic acid solution had -115.5 and 89.8 µA cm-2 of current densities in anodic and cathodic conditions, respectively. At different loads, the interfacial contact resistance measurements between the samples and the gas diffusion layer for all samples were determined. At a load of 140 N cm-2, the bare SS316L and samples processed in urea and acetic acid solution showed values of 118.82, 26.78, and 10.97 mΩ cm2, respectively. These studies suggest plasma electrolytically processed SS316L in acetic acid solution offers the least interfacial contact resistance of 10.97 mΩ cm2 and almost satisfies the Department of Energy target of 10 mΩ cm2 for metallic bipolar plates. A scale-up of coating in a 4×4 cm area and its performance in a single-cell setup will be highlighted.

Characterization and Optimization of Plasma Electrolytic Process for Conversion of Molecular Nitrogen to Ammonia in Aqueous Water at Atmospheric Pressure

Ammonia (NH3) is widely used in the chemical and agricultural industries and is of growing interest as a potential energy carrier. Most of the current global NH3 production utilizes the Haber-Bosch process which has a large environmental and physical footprint. For this reason, there have been substantial efforts to develop an alternative process that can be sustainable and distributed. Recently, plasma-based electrolysis reactors have shown promise to convert molecular nitrogen to various forms of fixed nitrogen such as ammonia, nitrate, and nitrite in water at atmospheric pressure. However, selectivity and control over the products has not been adequately demonstrated. In this work, we studied a plasma formed in pure nitrogen gas or mixtures containing nitrogen gas at the surface of liquid water. Various products were characterized in the liquid phase including ammonium ions, nitrate ions, nitrite ions, and hydrogen peroxide as a function of pH, discharge current, and time. In addition, continuous direct current (DC) was compared to pulsed DC voltage plasma operation. We find that the selectivity to different nitrogen products depends on the solution conditions. These results could be used to inform optimization and scale up of similar plasma-liquid systems for sustainable nitrogen fixation.

Investigation of Plasma-Electrolytic Processing on EDMed Austenitic Steels.

This study investigates the effect of electrolytic plasma processing on the degree of defective layer removal from a damaged layer obtained after manufacturing operations. Electrical discharge machining (EDM) is widely accepted in modern industries for product development. However, these products may have undesirable surface defects that may require secondary operations. This work aims to study the die-sinking EDM of steel components followed by the application of plasma electrolytic polishing (PeP) to enhance the surface properties. The results showed that the decrease in the roughness of the EDMed part after PeP was 80.97%. The combined process of EDM and subsequent PeP makes it possible to obtain the desired surface finish and mechanical properties. In the case of finishing EDM processing and turning, followed by PeP processing, the fatigue life is enhanced without failure up to 109 cycles. However, the application of this combined method (EDM + PeP) requires further research to ensure consistent removal of the unwanted defective layer.

SOME ASPECTS OF ELECTROLYTIC-PLASMA PROCESSING TECHNOLOGY

A technological device (pilot model) for electrolytic-plasma processing of various surfaces has been developed. Some theoretical calculations and processing recommendations have been done. This technology contributes to significant savings in human and energy resources and achieving the required surface quality of parts, and preliminary processing, and is very usefullbefore vacuum-arc deposition of protective coatings

Reaction pathway of nitrate and ammonia formation in the plasma electrolysis process with nitrogen and oxygen gas injection

The plasma electrolysis method using N2 and O2 injection is an effective and environmentally friendly solution for nitrogen fixation into nitrate and ammonia. The reaction pathway, the effect of the N2 and O2 gas injection composition are important parameters in understanding the mechanism and effectiveness of these processes. This study aims to determine the formation pathway of nitrate and ammonia by observing the formation and role of reactive species as well as intermediate compounds. Two reaction pathways of NOx and ammonia formation have been observed. The NOx compound formed in the solution was oxidized by ∙OH to NO2, followed by the production of a stable nitrate compound. The ammonium produced from the ammonia pathway was generated from nitrogen reacting with ∙H from H2O. The amount of NH3 formed was lesser compared to the NOx compounds in the liquid and gas phases. This indicates that the NOx pathway is more dominant than that of ammonia. The gas injection test with a ratio of N2/O2 = 79/21 was the most effective for nitrate formation compared to another ratio. The results of the emission intensity measurement test show that the reactive species ∙N, ∙N2*, ∙N2+, ∙OH, and ∙O have a significant role in the nitrate formation through the NOx pathway, while the reactive species ∙N and ∙H lead to the formation of NH3. The highest nitrate product was obtained at a ratio of N2/O2: 79/21 by 1889 mg L−1, while the highest ammonia product reached 31.5 mg L−1 at 100% N2 injection.

Effect of carbon and nitrogen diffusion zone on growth mechanism of boride layer produced on Q235 Steel

The purpose of this study was to indicate the effect of the presence of the carbon and nitrogen diffusion zone below the boride layers of Q235 low-carbon steel. The boride layers were produced by cathodic plasma electrolysis treatment (PET). Based on the results, the boride layer thickness on the treated samples at 260 V and 300 V varied from 10 μm to 22 μm. The XRD and GDOES analysis demonstrate that the boride layer on the treated samples mainly consists of single Fe 2 B phase, and the top loose layer mainly contain Fe 2 O 3 , Fe 3 C, Fe 4 N, and BN phases. The boride layers exhibited excellent scratch resistance compared to the bare Q235 steel. In addition, the plasma electron temperature was around 3000–4500 K, and the frequency spectra of acoustic signals were related to the voltage-time and current-time curves. At the high temperature and strong electric field, the decomposition reaction of electrolyte and the growth mechanism of the boride layers were discussed. Schematic of reactions in vapor gaseous envelope around the Q235 steel cathode during the plasma electrolysis process. • The boride layers on Q235 steel were produced by cathodic plasma electrolysis treatment. • The boride layers exhibited excellent scratch and adhesion resistance. • The OES and air acoustic spectra were measured. • Decomposition reactions of electrolyte and growth mechanism of boride layer were discussed.

Fast and catalyst-free conversion of protein-rich biomass using plasma electrolysis

As a byproduct of daily life and many modern industries, protein-rich waste or biomass is increasingly produced worldwide and emerges as an environmental burden if not properly managed. Here, we demonstrate a sustainable plasma electrolysis (PE) process for high-performance cleaner energy production from protein-rich biomass (PRB). With rapid external electricity delivery to drive electrical discharges directly into liquid, a localized and intensified reacting environment can be created, well suited to waste conversion into higher-value products, thus enabling an electrified biorefinery. Using protein-rich foods (here, pork liver and soy protein) as examples, this study investigates the PE-enabled, catalyst-free conversion of PRB in polyethylene glycol 200 (PEG 200), aiming to understand the conversion process, product compositions and migration and transformation of organic elements and metal ions. Results show that the conversion of the pork liver reaches 95.1% at 10 min, generating a liquid product with a high heating value (HHV) of 26.88 MJ/kg. Highly reactive oxygen species generated during the discharge, such as O and OH radicals, lead to the precipitation of metal ions from the feedstock and the transfer of nitrogen to the gas phase. This study provides a novel and promising Power-to-X process for the valorization of protein-rich feedstock under mild conditions.

Facile synthesis of reduced graphene oxide@Co3O4 composites derived from assisted liquid-phase plasma electrolysis for high-performance hybrid supercapacitors

Reduced graphene oxide@Co3O4 (rGO@Co3O4) composites are widely developed as promising electrode materials; however, their practical applications are restricted owing to their complex synthesis process. In the current work, we developed a rapid, efficient, and facile method for the one-step synthesis of rGO@Co3O4 composites via assisted liquid-phase plasma electrolysis. During the plasma electrolysis process, uniform and ultrafine Co3O4 particles are grown in-situ on rGO. The rGO@Co3O4 composites present a specific capacitance as high as 1249.0F/g at 1 A/g and a capacitance retention of 89.7 % over 10,000 cycles in a three-electrode system. The superior electrochemical performance is ascribed to the synergistic effect of the high specific capacitance Co3O4 and highly conductive rGO, which is conducive to promoting the transportation efficiency of ions/electrons and reducing the volume change of Co3O4 in the charge–discharge cycle. Asymmetric supercapacitors are also assembled using rGO@Co3O4 composites as the positive electrode and rGO as the negative electrode. The asymmetric supercapacitors achieve a specific capacitance of 72.3F/g at 1 A/g, display an energy density of 23.6 Wh/kg at a power density of 0.4 kW/kg, and exhibit superior cycle stability with a capacitance retention of 88.2 % over 10,000 cycles at 5 A/g. The current work provides a rapid, efficient, and facile method for the one-step synthesis of rGO-transition metal oxide electrodes as advanced energy storage devices.

Electrochemical and biological properties of mono- and bilayer nitride coatings deposited on Ti–6%Al–4%V alloy

Mono- and bilayer coatings were deposited by plasma nitriding process, cathodic plasma electrolytic deposition, and cathodic arc evaporation methods to improve the corrosion resistance and biological properties of the Ti–6Al–4V substrate. Coatings characterizations include phase analysis, surface morphology, and film microstructure observations were done using X-ray diffraction (XRD) analysis, atomic force microscope, and field emission scanning electron microscopy (FESEM), respectively. The results confirmed the formation of titanium nitride phases (TiN and Ti2N) in plasma nitriding and cathodic arc evaporation processes and deposition of oxy-nitride phases (TiNxOy) in plasma electrolytic nitriding process. The electrochemical study of coated and uncoated samples in a simulated body fluid (SBF) solution revealed the lower corrosion current density for bilayer nitride coating in comparison to the monolayer coating and substrate. Lower penetration of solution and more stable phases such as TiO2 improved the corrosion resistance of bilayer coatings. Hemocompatibility and MTT assay were performed on the coated and uncoated sample to determine the blood compatibility and cell viability. The statistical results showed better cell growth on the bilayer nitride coatings after three days.

CO2 conversion via coupled plasma-electrolysis process

Surplus renewable electricity used to convert CO2 into CO, the building block of liquid fuels, advances the energy transition by enabling large-scale, long-term energy storage and the synthesis of fuel for long-haul transportation. Among the various technologies developed, renewable electricity driven conversion of CO2 by high-temperature electrolysis and by plasmolysis offer a tantalising potential. High-temperature electrolysis is characterized by high-yield and energy-efficiency and the direct separation of the CO2 dissociation products CO and O2. However, the difficulty to break the carbon-oxygen double bond poses challenging requirements on electrode materials. CO2 plasmolysis on the other hand, offers a similar energy efficiency, does not employ scarce materials, is easy to upscale, but requires efficient gas separation and recuperation because the produced CO remains mixed with O2 and residual CO2. Here, we demonstrate that the coupling of the two processes leads to a renewable-electricity-driven route for producing CO from CO2, overcoming the main bottleneck of CO2 plasmolysis. A simulated CO2 plasmolysis gas mixture is supplied to a high-temperature electrolyser to separate the product gases electrochemically. Our results show that the product stream of the coupled-process contains 91% less oxygen and 138% more CO compared with the bare plasmolysis process. Apart from upgrading the produced gas mixture, this coupled approach benefits from material stability. Durability tests (~100 h) show better stability in coupled operation when compared with conventional CO2 electrolysis. Synergy between plasmolysis and electrolysis opens up a novel route to efficient CO2 conversion into valuable CO feedstock for the synthesis of long-chain hydrocarbons.

Effect of voltage polarity and supply frequency on the properties of plasma contacting liquid electrodes and gold nanoparticle synthesis

Plasma contacting with liquid (PCL) provides many charged particles and reactive species into the liquid. The difficulty in controlling or selecting each specific species has significantly limited its applications in industry. Here, we present a study on using voltage polarity to regulate the type of charged particles absorbing from the plasma into the liquid. A detailed understanding of the processes at the plasma–liquid interface and electrolysis due to switching in voltage polarity was determined via a visual pH observation, measuring the concentration of H2O2 and solvated electrons. The results indicated that changes in voltage polarity strongly affect the plasma properties, chemical properties and electrolysis process in liquid, and also in the types of reducing species for gold nanoparticle (GNP) synthesis. The results also showed that using a suitable frequency could improve the efficiency of absorption of H2O2 from plasma into the bulk liquid as well as the yield in the production of GNPs. The results provide a way to select desired species from the plasma to be transferred into the liquid for a distinct purpose and to accompany other properties in the system of PCL.

Plasma electrolytic processing of bimetals at the anodic process

Bimetals have a unique combination of functional characteristics, which are not inherent to components. Usually, the joining is the final procedure of the bimetals manufacturing because the conditions of heat or thermochemical treatment of the bimetal components are very different. Plasma electrolytic processing is a highly effective and environmentally friendly method to process surfaces of metals and alloys. This technology can be used for heat or thermochemical treatment as well as to clean or polish metal surfaces. Due to the voltage ranges of different modes of the anodic process of plasma electrolytic processing for various metals in the same electrolyte partially overlaps, the components of bimetals can be treated identically or differently. It is demonstrated that there exists an option of separate or simultaneous heat or thermochemical treatment, as well as polishing of one or both components of bimetals. For example, in Ti / Stainless Steel (SS) bimetal, the steel was heated up to 620°C, in Mo / SS one — up to 630°C, in Al / SS one — up to 720 ÷ 850°C without heating of the second component higher than 140°C. Also, an ability of simultaneous polishing of both components of Al / SS bimetal was shown. During 8 min polishing, the reflection coefficients of both surfaces increase from 0.5 ÷ 2 % to >40 %.

Evaluation of tribotechnical properties of modified surfaces after plasma electrolytic treatment

The article considers the study of the effect of plasma electrolytic processing on the tribotechnical characteristics of medium-carbon steel. Friction tests were carried out in dry friction mode. Electron microscope and profilometer were used to study the friction tracks. For a comprehensive assessment of the quality of the modified surface layer, the Kragelsky-Kombalov complex parameter was calculated. It was found that plasma electrolytic treatment leads to a decrease in the coefficient of friction and weight wear in comparison with hardened and untreated steel. It has been determined that the mechanism of wear of samples after plasma electrolytic treatment is fatigue wear at boundary friction and plastic contact.

Thermal processes during friction and wearing of medium carbon steel after plasma electrolytic processing

The thermal state of a frictional contact with varying normal loads and sliding speeds of a friction pair after plasma electrolytic processing is considered. It is shown that an increase in the temperature of the frictional contact decreases the coefficient of friction, and the wear rate of the tribointerface increases with an increase in the temperature of the frictional contact.

Combination of Plasma Electrolytic Processing and Mechanical Polishing for Single-Crystal 4H-SiC.

Single-crystal 4H-SiC is a typical third-generation semiconductor power-device material because of its excellent electronic and thermal properties. A novel polishing technique that combines plasma electrolytic processing and mechanical polishing (PEP-MP) was proposed in order to polish single-crystal 4H-SiC surfaces effectively. In the PEP-MP process, the single-crystal 4H-SiC surface is modified into a soft oxide layer, which is mainly made of SiO2 and a small amount of silicon oxycarbide by plasma electrolytic processing. Then, the modified oxide layer is easily removed by soft abrasives such as CeO2, whose hardness is much lower than that of single-crystal 4H-SiC. Finally a scratch-free and damage-free surface can be obtained. The hardness of the single-crystal 4H-SiC surface is greatly decreased from 2891.03 to 72.61 HV after plasma electrolytic processing. By scanning electron microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) observation, the plasma electrolytic processing behaviors of single-crystal 4H-SiC are investigated. The scanning white light interferometer (SWLI) images of 4H-SiC surface processed by PEP-MP for 30 s shows that an ultra-smooth surface is obtained and the surface roughness decreased from Sz 607 nm, Ra 64.5 nm to Sz 60.1 nm, Ra 8.1 nm and the material removal rate (MRR) of PEP-MP is about 21.8 μm/h.

Wettability and in-vitro study of titanium surface profiling prepared by electrolytic plasma processing.

Electrolytic plasma processing (EPP) was used to create hydrophilic surface profiles on titanium. The wettability, surface morphology characteristics and chemical composition of the treated samples were studied as a function of EPP processing parameters. The EPP profiled surfaces comprised of a characteristic "hills and valleys" morphology because of continuous surface melting and freezing cycles. A bimodal surface profile was produced with 2-3 μm height hills and valleys with nano-roughness (≤200 nm). The produced profile resulted in a significant contact angle decrease (from 38.7° to 5.4°). Ratios of actual surface area to projection area (r) and fraction of solid surface remaining dry (φ) were obtained from profilometry. The surface characteristics and large r values produced by EPP were able to induce hemi-wicking. Hence, EPP produced superhydrophilic surfaces on Ti. The bioactivity of EPP treated Ti was evaluated using cell free and MC3T3 cells in-vitro studies. The treated Ti surface significantly increased the bioactivity and formed stoichiometric hydroxyapatite after immersion in a bone cell culture medium for 21 days. Cells' attachment and proliferation studies indicated that EPP treated surface significantly enhances the cells' adhesion and growth after 24 and 48 h compared to the untreated surface. The results show that Ti surface profiling by EPP constitutes a promising method to potentially improve bone implant bonding.

Influence of electrolytic-plasma hardening modes on structure and hardness of 0.34Cr-1Ni-Mo-Fe steel

The article presents the results of studying the process of electrolytic-plasma hardening of 0.34Cr-1Ni-Mo-Fe steel by surface hardening, as well as the results of the current-voltage characteristics of the cathodic electrolytic-plasma process depending on the composition of the electrolyte. Temperature-time and special modes of electrolytic-plasma hardening of steel 0.34Cr-1Ni-Mo-Fe were determined. The optimal composition of the electrolyte for electrolytic-plasma hardening has been determined, providing a relatively high heating rate and high hardness of the steel surface. It has been determined that after the electrolytic-plasma hardening, the microhardness of 34KhN1M steel increases 2.9 times due to the formation of fine martensite. In this case, the basis of the material does not change, it consists of a ferrite-pearlite structure.

Distinctive features of electric current passing through vapour gaseous envelope in anodic plasma electrolytic processes

This article focuses on the study of the effect of anodic composition of electrolyte on electric conductivity of a vapour gaseous envelope, formed in the mode of anode stationary heating. The investigation methods were based on measuring current-voltage characteristics in aqueous solutions of ammonium chloride, chlorohydric acid and hydrobromic acid at equal anion concentrations. It has been established that conductivity in vapour gas medium is primarily determined by electric field intensity which varies depending on its composition and thickness, ion mobility considered as factor of secondary importance. It has been assumed that emissive capacity of aqueous solutions can be limited by a critical concentration of their components.