Corrosion behavior of So-W coatings obtained from citrate electrolyte (with W content in the coating of about 24 att. %) in the following corrosive environments was studied by the methods of electrochemical impedance and polarization measurements: 0.6M sodium chloride (3.5 mass%), 0.1 M sodium sulfate (Na2SO4), 1 M hydrochloric acid (HCl) and 0.5M sulfuric acid (H2SO4). As a comparison, JIS-SK5 steel, which acts as a substrate during deposition, was taken. It is shown that the corrosion current of Co-W coatings in three corrosive media (sodium chloride, hydrochloric acid and sodium sulfate) is close to 20 +/- 3 mu A/cm(2). When exposed to acidic environments, the corrosion potential of Co-W coatings shifts to the anodic region reaching values of -0.260 V, and in neutral environments (chloride and sulfate) it reaches significantly higher values of -0.76 V. The high passivating ability of the Co-W coating in used acids is demonstrated, where the corrosion current is more than 100 times (in the case of sulfuric acid) and 17 times (in the case of hydrochloric acid) less than that of the SK-5 steel substrate. The maximum polarization resistance of So-W coatings is observed in sodium chloride and sulfuric acid environments and takes a maximum value of similar to 36000 Ohm center dot cm(2), and the minimum value of this coating shows similar to 3500 Ohm center dot cm(2) in hydrochloric acid environment.

The influence of temperature on the properties of nanocrystalline Co-W coatings obtained from a citrate electrolyte at pH 6.7 and deposited under natural convection conditions was studied. The influence of the electrolyte temperature on the morphology, composition, structure and roughness of coatings, as well as their corrosion and tribological properties, is demonstrated. It is shown that the structure of the coating can change from the X-ray amorphous to the crystalline one when the deposition temperature of the coating is increased to 90 ºС. The average value of the mass wear of Cо-W coatings formed at 80 ºС and measured under the linear friction condition of 0.08 mg, and 62 times lower than that of a Cr coating (4.95 mg) and 84 times lower that at the surface of steel SK-5 (6.71 mg). When testing by the reciprocating wear method, the value of the volumetric wear of Co-W coatings obtained in the temperature range 50–70 ºС was 0.00109 mm3, which is 55 times lower than the wear value of a Cr coating (0.0596 mm3) and 41 times lower than that of the steel SK-5 surface (0.0449 mm3). The paper also shows a slight decrease in the currents of corrosion coatings and a decrease in the dispersion values of the corrosion potentials obtained after mechanical polishing of the surface of the coatings at temperatures in the range of 20–90 ºС. It is shown that under the corrosive effect of 3.5% sodium chloride, an increase in the deposition temperature of the coating is accompanied by a slight in-crease in the corrosion current.

The electrospark deposition (ESD) technique is a low-heat-input process that has great potential for coating applications and the restoration of damaged high-value parts. Carbon steels are commonly used as a substrate material for ESD coatings. However, we demonstrated that carbon steels could be used successfully as the electrode tool for the ESD process. Furthermore, ESD coatings commonly have a high as–deposited roughness. In view of this, in order to reduce the roughness of the ESD coatings, electrodeposition as a tool to alter surface morphology was investigated. Hence, the micro-leveling power of several electrolytes for Ni, Fe-W, Fe, and Cr electrodeposition were evaluated. The maximum leveling effect was detected for Ni electroplated from the Watts electrolyte. Thus, the novel hybrid coatings based on an ESD layer and a subsequent layer of electrodeposited Ni were obtained. ESD layers were obtained by using the following electrode tools as anodes: several types of carbon steels (St20, St30, and St45), alloys T15K6 (WC + TiC + Co), CuNiZn; and NiCr. The morphology and structure of the obtained hybrid coatings with an electrodeposited Ni top-layer was analyzed and compared to ESD coatings from the point of view of their wear and corrosion behavior. The wear rate of the novel ESD coatings based on carbon steels was comparable with coatings obtained using the NiCr electrode tool. Moreover, for all the studied cases, the corrosion resistance of the hybrid coatings was higher than for their ESD counterparts and close to electrolytic chromium.

— The article presents the results of surface modification of austenitic stainless steel by electrolyte-plasma nitriding under anodic and cathodic polarity of the product being treated. The morphology and surface roughness, phase composition, and microhardness of diffusion layers, as well as tribological and corrosion properties, were studied. The effect of physicochemical processes during anodic and cathodic treatments on the features of the formation of a modified surface and its operational properties is demonstrated. The anodic treatment increased the hardness of the nitrided layers up to 1150 HV, halved the surface roughness, improved the wear resistance by a factor of 166, and decreased the coefficient of friction by a factor of 1.6. The cathodic treatment increased the hardness only to 580 HV and improved the wear resistance by a factor of 13.6. Both treatments were shown to decrease pitting corrosion in a sodium chloride medium.

The possibility of increasing the wear resistance and corrosion resistance of the surface of low-carbon steel after cathodic plasma electrolytic boronitrocarburizing in a solution of boric acid, glycerin, and ammonium chloride, and subsequent anodic plasma electrolytic polishing in a solution of ammonium sulfate through the formation of a modified structure has been demonstrated. The modified structure consists of a dense oxide layer and a diffusion layer underneath, which contains up to 0.87

The possibility of increasing the hardness to 1420 HV and the corrosion resistance of the CP-Ti surface using a combined plasma electrolytic treatment consisting in anodic plasma electrolytic nitrocarburising in a solution of ammonia, acetone and ammonium chloride at 900 °C and subsequent plasma electrolytic polishing is shown. The morphology, surface roughness, phase composition, structure and microhardness of the modified layer were studied. The corrosion characteristics of the treated surface were studied through potentiodynamic tests and electrochemical impedance spectroscopy. It has been shown that an increase in the surface roughness has a negative effect on the corrosion resistance. The proposed plasma electrolytic polishing makes it possible to remove the outer porous oxide layer, providing increased corrosion resistance. The highest reduction in the corrosion current density, by 13 times compared to CP-Ti and by two orders compared to a plasma electrolytic nitrocarburising sample, is achieved after plasma electrolytic polishing in a solution of ammonium fluoride (4%) at 300 V for 3 min.

Experimental study of the anodic dissolution of titanium and its alloys over a wide range of current densities, including pulsed currents (up to 100 A/cm2), under controlled hydrodynamic conditions and surface temperature in nitrate and chloride solutions, showed that the process is mediated by electrochemical formation of an anodic oxide film (AOF), which undergoes chemical dissolution. The AOF has a bilayer structure (two barrier films: at the interface with the metal and solution). It is described by PDM-III (Point Defect Model). Under certain conditions, it is possible to achieve a steady state in which the film growth rate is compensated by the rate of its chemical dissolution (during a pulsed treatment). In this case, there is a 100

It is shown that at high-speed pulsed galvanostatic anodic dissolution of chromium-nickel steels Kh3N35VT (CSN17335), Kh18N10 (AIS1 304) in the regions of pulse durations of 20–2000 ms and current densities of 1–100 A/cm2 in electrolytes for their electrochemical dimensional machining (ECDM) (chloride, nitrate, and mixed chloride-nitrate with an electrical conductivity of 0.15 S/cm), a significant part of the charge (up to 50%) is spent on the formation of a passivating oxide film having a semiconductor character. As a result, the film, not the alloy, is subject to electrochemical treatment. As a result, the current output of the ECDM process of these materials under pulsed conditions is ~ 50–70%, depending on the composition of the alloy. The speed of the process increases with the transition to a constant processing current because of the destruction of the film due to its thermokinetic instability (“thermal explosion”) caused by an increase in the surface temperature.

It is shown that high-speed anodic dissolution of Kh18Ni10 chromium-nickel steel (Cr18Ni10) in a concentrated nitrate solution occurs through an anode oxide film (AOP), the formation of which is carried out as a result of charge transfer, and the dissolution is chemically. The condition of constant film thickness is achieved due to the equality of the rates of these two processes (electrochemical formation of the film and its chemical dissolution) and is realized under pulsed conditions until the boiling point of the electrolyte is reached at the film-solution interface. When thermokinetic instability of the anodic oxide film occurs at the film-solution interface at surface temperatures exceeding the boiling point of the electrolyte, the interaction of the dissolving surface of the alloy free of the film with the solution (electrolyte) is observed. The dissolution rate in this case may exceed the Faraday rate (anomalous anodic dissolution) as a result of the destruction of the film, for the formation of which the leaked charge was spent. The presence of a barrier film at the metal-film interface is the reason that the change in the surface temperature and dissolution rate is achieved only by changing the heat transfer conditions of the metal surface. The porous part of the film in contact with the solution is a "heat seal" (limits the heat removal towards the electrolyte), as a result of which a change in the electrolyte flow rate does not affect the surface temperature. Based on the results of calculating heat fluxes, the influence of the temperature of the metal surface in contact with air (the outer part of the surface of the dissolving metal) on the heat transfer coefficient during natural convection and rotation at a speed of 1000 rpm of the electrode was estimated, as well as the ratio of the corresponding heat fluxes. An increase in convective heat removal during rotation leads to a decrease in temperature in the anode treatment zone and, as a consequence, to a decrease in the dissolution rate.

The structural-phase composition and some properties of medium-carbon steel modified surface after cathodic plasma electrolytic nitriding in a solution of ammonium chloride and ammonia have been studied. It is shown that cathodic nitriding of a steel surface is accompanied by high-temperature oxidation with the formation of oxides FeO, Fe2O3, and Fe3O4, as well as nitrogen diffusion and quenching with the formation of FeN, Fe3N, and Fe4N phases, martensite and the retained austenite. The competing effect of surface erosion by the actions of discharges and high-temperature oxidation on the morphology and roughness of the surface was revealed. It was established that the maximum microhardness of the modified layer reaches 1040 HV, and the corrosion current density of the nitrided surface decreases by a factor of 1.5–2.4.

Effect of plasma electrolytic polishing on the wear and corrosion resistance of medium carbon steel after its plasma electrolytic boriding was investigated. An X-ray diffractometer was used to analyse the phase composition of the modified surface; the layer structure was investigated using an electron microscope. The layer microhardness and surface roughness were measured by standard methods. Wear resistance of steel was studied under dry friction conditions with an alumina counter-body. Corrosion resistance of samples was tested in a 3.5

It is shown that, in high-rate pulsed galvanostatic anodic dissolution of type CSN17335 and AISI 304 chromium–nickel steels in electrolytes for electrochemical machining (ECM) (chloride, nitrate, and mixed chloride–nitrate solutions with a conductivity of 0.15 S/cm) using microsecond pulses with a duration of 20–2000 μs and current densities in the range of 1–100 A/cm 2 , a substantial fraction of charge (up to ~40%) is spent on the formation of a passivating oxide film with a semiconducting behavior. The electrochemical treatment therefore directly involves the oxide film, not the alloy. As a consequence, the current efficiency of ECM of these materials is ~60–70%, depending on the alloy composition. When using direct current, the rate of machining increases as a result of the oxide film breakdown due to its thermokinetic instability (“thermal explosion”) caused by a rise in the surface temperature.

The possibility of using an aqueous non-toxic electrolyte of ammonium nitrate and glycerin for the cathodic plasma electrolytic nitrocarburizing of low-carbon steel is considered in this paper. Surface morphology and roughness, element and phase compositions, and microhardness of the modified layer were investigated. Kinetic calculations of the processes of nitrogen and carbon diffusion into the steel surface are proposed, taking into account their mutual influence. Wear resistance was studied under dry friction conditions with tool alloy steel as a counter-body. Corrosion studies are performed using potentiodynamic polarization curves in 3.5% sodium chloride solution. The plasma electrolytic nitrocarburizing in an aqueous electrolyte with ammonium nitrate and glycerin is established to increase surface hardness up to 980 HV due to the formation of a nitrocarburized layer with 1.35 ± 0.12% carbon and 0.32 ± 0.08% nitrogen concentration. The influence of erosion in electrolyte plasma and high-temperature oxidation on the morphology and surface roughness is shown. The presence of a dense oxide layer, low surface roughness, and high hardness of the diffusion layer favor a decrease in the friction coefficient by 1.3 times, weight wear by 1.8 times and corrosion current density by 1.4 times.

The paper discusses a possibility of increasing the wear and corrosion resistance of a low carbon steel surface after cathodic plasma electrolytic boronitrocarburising in a solution of boric acid, glycerin, and ammonium chloride, with the subsequent anodic plasma electrolytic polishing in an ammonium sulfate solution due to the formation of a modified structure consisting of a dense oxide layer and a diffusion layer below it, which contains up to 0.87% carbon, 0.80% nitrogen, and 0.87% boron upon reaching microhardness up to 970±20 HV. The competitive influence of the surface erosion under the action of discharges and high-temperature oxidation on the morphology and roughness of the surface is revealed. A positive effect of reducing the surface roughness during the formation of a dense oxide layer on the surface and a hardened diffusion layer under it on reducing the friction coefficient and mass wear, as well as reducing the roughness and additional oxidation of the surface during polishing on reducing the corrosion current density, has been established.

The paper presents the results of studies on modifying the surface of Ti6Al4V titanium alloy by combined exposure to cathodic nitriding and anodic polishing in electrolysis plasma. The morphology and roughness of the surface, microhardness of the modified layer have been investigated. Wear resistance was studied under dry friction conditions. The effect of combined treatment on corrosion resistance of Ti6Al4V alloy was examined by means of potentiodynamic polarization in Ringer’s solution. It has been established that cathodic nitriding at 750 °C for 10 min leads to the hardening of the surface layer up to 820 HV with an increase in roughness by 2 times and wear resistance almost 3 times. Subsequent anodic plasma electrolytic polishing of the nitriding surface in solution of ammonium sulfate leads to a decrease in roughness and friction coefficient with an increase in corrosion resistance.

The influence of technological parameters of plasma electrolytic nitriding and polishing on the wear resistance and corrosion resistance of medium-carbon steel is considered. The morphology and roughness of the surface, phase composition and microhardness of the modified layer have been investigated. Wear resistance was studied under dry friction conditions with bearing steel as counter-body. It was found that plasma electrolytic polishing removes the loose part of the oxide layer and provides a two-fold decrease in surface roughness compared with untreated steel, and 2.8 times compared with the nitrided one. Combined processing at optimal technological parameters leads to an increase in microhardness up to 1130 HV, an increase in wear resistance by 70 times, and a decrease in the corrosion current density by almost 3 times in comparison with untreated steel.

The investigation of Fe-W alloys is growing in comparison to other W alloys with iron group metals due to the environmental and health issues linked to Ni and Co materials. The influence of Na2WO4 concentration in the range 0 to 0.5 M on bath chemistry and electrode reactions on Pt in Fe-W alloys’ electrodeposition from citrate electrolyte was investigated by means of rotating disk electrode (RDE) and cyclic voltammetry (CV) synchronized with electrochemical quartz crystal microbalance (EQCM). Depending on species distribution, the formation of Fe-W alloys becomes thermodynamically possible at potentials less than −0.87 V to −0.82 V (vs. Ag/AgCl). The decrease in electrode mass during cathodic current pass in the course of CV recording was detected by EQCM and explained. The overall electrode process involving Fe-W alloy formation may be described using formalities of mixed kinetics. The apparent values of kinetic and diffusion currents linearly depend on the concentration of Na2WO4. Based on the values of partial currents for Fe and W, it was concluded that codeposition of Fe-W alloy is occurring due to an autocatalytic reaction, likely via the formation of mixed adsorbed species containing Fe and W compounds or nucleation clusters containing both metals on the electrode surface.

The possibility of realizing surface plasma electrolyte modification of the steel surface by successive diffusion saturation and polishing by changing the value of the applied voltage is shown. The composition, structure and properties of the formed coating have been studied.

The structure of alpha + beta-titanium alloy, its microhardness, surface roughness, and wear and corrosion resistance after anode plasma electrolytic borocarburising (PEB/C) in electrolyte containing boric acid, glycerol and ammonium chloride were investigated. An X-ray diffractometer and scanning electron microscopy were used to characterize the phase composition and structure of the modified surface. Tribological properties of treated titanium alloy were evaluated using a ball-on-disc tribometer under dry testing conditions. The effect of electrolyte compositions on corrosion resistance of the PEB/C samples was examined by means of potentiodynamic polarization in Ringer's solution. It was shown that the electrolyte containing boric acid, glycerol and ammonium chloride provided the saturation of alloy with boron, carbon and oxygen and formation of TiO2 with rutile structure and boron/carbon solid solution in titanium. The anode PEB/C resulted in a decrease in surface roughness by 2.5 times, corrosion current density by 9 times, wear rate by 54 times and an increase in microhardness to 1075 HV10.

A change in the surface morphology of a titanium alloy under the influence of its erosion as well as dissolution and oxidizing for cathodic or anodic carburizing was studied. An electronic microscope was applied to investigation of surface morphology and cross-section of modified specimens. The roughness of samples surface and their microhardness were measured. It was found that the decrease in the samples weight after cathodic or anodic carburizing was comparable. The surface roughness after cathodic carburizing increases more than after anodic one, but it can be reduced by 1.5 times by plasma electrolytic polishing.