Electrolytic superconducting niobium coatings for a cryogyroscope rotor: creation and properties

Electrodeposition of niobium coatings on spherical substrates for a cryogyroscope rotor made of carbopyroceram was considered. A special design of the installation for obtaining coatings on spherical samples was created. The coatings were applied at a temperature of 750°C with the cathodic current density of 5·10−3 - 2–10−2 A·cm−2 and the electrolysis time of 8-12 h. It was found that the use of a cathodic current density of 2·10−2 A·cm−2 and higher is impractical, because there is a roughening of the coatings surface. The composition of electrolytic coatings was identified by XRD analysis. The macrostructure of niobium coatings was studied using electron microscopy. The concentration of metallic impurities in the niobium coatings was defined by spectral quantitative analysis. The content of gas impurities was determined by gas chromatography. The roughness, nonsphericity, and superconductive properties of niobium coatings were investigated.

Effect of Cathodic Polarisation Switch-Off on the Passivity and Stability to Crevice Corrosion of AISI 304L Stainless Steel

The effects of cathodic polarisation switch-off on the passivation of AISI 304L stainless steel in air and its crevice corrosion susceptibility in 3.5 wt.% NaCl aqueous electrolyte were investigated. Scanning Kelvin probe (SKP) data showed that the oxide film is significantly destabilised and the rate of steel passivation in air is slowed down. Thermal desorption analysis (TDA) highlighted that hydrogen absorption is proportional to the applied cathodic current density. A special crevice corrosion set-up was designed to realise simultaneous reproducible monitoring of potential and galvanic current to study the impact of prior cathodic polarisation on crevice corrosion onset.

ELECTROCHEMICAL DEPOSITION OF TIN–ZINC ALLOY FROM CITRATE–AMMONIA ELECTROLYTE

The cathodic processes of electrochemical deposition of a tin–zinc alloy in citrate–ammonia electrolytes have been investigated. The content of the main components of the investigated electrolyte (g/dm3): SnCl2·2H2O – 44, ZnO – 4, NH4Cl – 100, Na3C6H5O7 – 100. Wood glue (1.5 g/dm3) and neonol (4 ml/dm3) were added to the electrolyte as surfactants. It was found that high–quality coatings are deposited without heating and stirring only in the pH range from 6,0 to 7,0. The addition of these substances to the electrolyte is predicted to lead to inhibition of the reduction of metals, an improvement in the crystal structure of the deposit, but decreases the cathodic current efficiency. Hull cell studies showed that an electrolyte containing neonol as a surfactant showed better throwing power compared to other solutions. The dependence of the current efficiency of the alloy on the cathode current density showed that in the range of current densities from 0.5 A/dm2 to 4 A/dm2, the current efficiency decreases nonlinearly from 82 % to 52 %. The experimentally obtained dependence of the zinc content in the alloy on the cathodic current density showed the possibility of obtaining alloys with a zinc content of 8 % to 33 %. The obtained results allowed us to determine that for the deposition of an alloy with a zinc content of 20–25 %, which provides the best anti–corrosion properties of the coating, it is necessary to carry out the process at a cathodic current density of 1,5–2,0 A/dm2, while the current efficiency is about 70 %, and the deposition rate alloy is 0,44–0,54 μm/min. The received coatings have a semi–bright appearance, a fine–grained structure, light gray color, they are strongly adhered to the substrate.

Electrodeposition of Graphene Oxide Modified Composite Coatings Based on Nickel-Chromium Alloy

Composite electrochemical coatings (CECs) based on nickel-chromium alloy and modified with multilayer graphene oxide (GO) were obtained. The electrodeposition process of these coatings was studied in the potentiodynamic mode. The structure and the composition of nickel–chromium–GO CECs were studied by scanning electron microscopy and laser microspectral analysis. Nickel–chromium–GO CECs are dense and uniform. The carbon content in them increases when moving from the substrate to the surface. It was established that the addition of GO particles into the composition of electrolytic coatings with a nickel-chromium alloy results in the increase in their microhardness from 4423–5480 MPa to 6120–7320 MPa depending on the cathodic current density.

Electrodeposition of Copper and Brass Coatings with Olive-Like Structure

One method of creating a brass coating is through electrodeposition, which is most often completed in cyanide galvanic baths. Due to their toxicity, many investigations focused on the development of more environmentally friendly alternatives. The purpose of the study was to explore a new generation of non-aqueous cyanide-free baths based on 1-ethyl-3-methylimidazolium acetate ionic liquids. The study involved the formation of copper, zinc, and brass coatings. The influence of the bath composition, cathodic current density, and temperature was determined. The obtained coatings were characterized in terms of their morphology, chemical composition, phase composition, roughness, and corrosion resistance. It was found that the structure of the obtained coatings is strongly dependent on the process parameters. The three main structure types observed were as follows: fine-grained, porous, and olive-like. To the best knowledge of the authors, it is the first time the olive-like structure was observed in the case of an electrodeposited coating. The Cu-Zn coatings consisted of 19–96 at. % copper and exhibited relatively good corrosion resistance. A significant improvement of corrosion properties was found in the case of copper and brass coatings with the olive-like structure.

Electrochemical Toluene Hydrogenation Using Binary Platinum-Based Alloy Nanoparticle-Loaded Carbon Catalysts

A couple of toluene (TL) and its hydrogenation product, methylcyclohexane (MCH), are promising high-density hydrogen carriers to store and transport large amounts of hydrogen. Electrochemical hydrogenation of TL to MCH can achieve energy savings compared with hydrogenation using molecular hydrogen generated separately, and development of highly active catalysts for electrochemical TL hydrogenation is indispensable. In this study, binary Pt3M (M = Rh, Au, Pd, Ir, Cu and Ni) alloy nanoparticle-loaded carbon catalysts were prepared by a colloidal method, and their activity for electrochemical TL hydrogenation was evaluated by linear sweep voltammetry. Each Pt3M electrode was initially activated by 100 cycles of potential sweep over a potential range of 0–1.2 or 0.8 V vs. reversible hydrogen electrode (RHE). For all activated Pt3M electrodes, the cathodic current density for electrochemical TL hydrogenation was observed above 0 V, that is the standard potential of hydrogen evolution reaction. Both specific activity, cathodic current density per electrochemical surface area, and mass activity, cathodic current density per mass of Pt3M, at 0 V for the Pt3Rh/C electrode were the highest, and about 8- and 1.2-times as high as those of the commercial Pt/C electrode, respectively, which could mainly be attributed to electronic modification of Pt by alloying with Rh. The Tafel slope for each activated Pt3M/C electrode exhibited the alloying of Pt with the second metals did not change the electrochemical TL hydrogenation mechanism.

Electrodeposition of Copper and Brass Coatings with Olive-like Structure

One method of creating a brass coating is through electrodeposition, which is most often completed in cyanide galvanic baths. Because of the toxicity of cyanides, many are working on the development of cyanide-free baths, which are promising but not satisfactory to replace the classic methods. The purpose of the study was to explore a new generation of non-aqueous cyanide-free baths based on 1-ethyl-3-methylimidazolium acetate ionic liquids. The study involved the formation of copper, zinc, and brass coatings. The influence of the bath composition, cathodic current density, and temperature was determined. The obtained coatings were characterized by their morphology, chemical composition, roughness, and corrosion resistance. It was found that the structure of the obtained coatings is strongly dependent on the process parameters. The three main structures observed were fine-grained, porous, and olive-like. To the best knowledge of the authors, it is the first time the olive-like structure was observed in the case of an electrodeposited coating. The Cu-Zn coatings consisted of 19–96 at.% copper and exhibited relatively strong corrosion resistance. High improvement of corrosion properties was found in the case of copper and brass coatings with an olive-like structure.

The influence of chloride ion concentration on the localized corrosion of E717 Mg alloy

The localized corrosion behavior of E717 magnesium alloy immersed in chloride-containing electrolyte is investigated using an in-situ scanning vibrating electrode technique (SVET), coupled with time-lapse imaging (TLI). It is shown that initiation of localized corrosion in chloride-containing electrolyte is characterized by the appearance of discrete local anodes, corresponding with the leading edges of dark, filiform like features, which combine with time to produce a mobile anodic front. The size and growth rate of these features are highly dependent on the chloride ion concentration of the electrolyte. SVET-derived current density maps reveal that the corroded surface left behind the anodic front is cathodically activated, where cathodic current density values progressively decline with increasing distance away from the anodic leading edge. The intensity of localized anodes is highly dependent on the chloride ion concentration, where progressively higher local anodic current density values are observed with increasing chloride ion concentration along with progressively higher rates of volumetrically-determined hydrogen evolution. Breakdown potential, measured using time-dependent free corrosion potential transients and potentiodynamic polarization at neutral and elevated pH respectively, is shown to vary with the logarithm of chloride ion concentration and the time for localized corrosion initiation is progressively increased with decreasing chloride concentration. From the combination of results which are presented herein, the underlying reasons for the influence of chloride ion concentration on the localized corrosion characteristics of E717 alloy will be discussed.

Atomic Pt-Clusters Decoration Triggers a High-Rate Performance on Ni@Pd Bimetallic Nanocatalyst for Hydrogen Evolution Reaction in Both Alkaline and Acidic Medium

The development of inexpensive and highly robust nanocatalysts (NCs) to boost electrochemical hydrogen evolution reaction (HER) strengthens the implementation of several emerging sustainable-energy technologies. Herein, we proposed a novel nano-architecture consisting of a hierarchical structured Ni@Pd nanocatalyst with Pt-clusters decoration on the surface (denoted by Ni@Pd-Pt) for HER application in acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) mediums. The Ni@Pd-Pt NC is fabricated on a carbon black support via a “self-aligned” heterogeneous nucleation-crystal growth mechanism with 2 wt.% Pt-content. As-prepared Ni@Pd-Pt NC outperforms the standard Pt/C (30 wt.% Pt) catalyst in HER and delivers high-rate catalytic performance with an ultra-low overpotential (11.5 mV) at the cathodic current density of 10 mA∙cm−2 in alkaline medium, which is 161.5 mV and 14.5 mV less compared to Ni@Pd (173 mV) and standard Pt/C (26 mV) catalysts, respectively. Moreover, Ni@Pd-Pt NC achieves an exactly similar Tafel slope (42 mV∙dec−1) to standard Pt/C, which is 114 mV∙dec−1 lesser when compared to Ni@Pd NC. Besides, Ni@Pd-Pt NC exhibits an overpotential value of 37 mV at the current density of 10 mA cm−2 in acidic medium, which is competitive to standard Pt/C catalyst. By utilizing physical characterizations and electrochemical analysis, we demonstrated that such an aggressive HER activity is dominated by the increased selectivity during HER due to the reduced competition between intermediate products on the non-homogeneous NC surface. This phenomenon can be rationalized by electron localization owing to the electronegative difference (χPt > χPd > χNi) and strong lattice mismatch at the Ni@Pd heterogeneous binary interfaces. We believe that the obtained results will significantly provide a facile design strategy to develop next-generation heterogenous NCs for HER and related green-energy applications

Influence of Current Density on the Microstructure of Carbon-Based Cathode Materials during Aluminum Electrolysis

Sodium expansion plays an important role in cathode deterioration during aluminum electrolysis. In this work, the sodium expansion of semigraphitic cathode material has been measured at various cathodic current densities using a modified Rapoport apparatus. We have studied the microstructural changes of carbon cathodes after aluminum electrolysis using high-resolution transmission electron microscopy (HRTEM). Because of an increasing trend toward higher amperage in retrofitted aluminum reduction cells, an investigation is conducted both at a representative cathode current density (0.45 A/cm2) and at a high cathodic current density (0.7 A/cm2). The results indicate that the microstructures of carbon cathodes can be modified by Joule heating and electrostatic charging with higher current densities during aluminum electrolysis. With the penetration of the sodium and melt, zigzag and armchair edges, disordered carbon, and exfoliation of the surface layers may appear in the interior of the carbon cathode. The penetration of the sodium and melt causes remarkable stresses and strains in the carbon cathodes, that gradually result in performance degradation. This shows that increasing the amperage in aluminum reduction cells may exacerbate the material deterioration of the cathodes.