E. N. Lubnin

Properties and preparation of amorphous chromium carbide electroplates

Abstract The electrochemical and corrosion behavior of chromium electroplates formed in sulfuric acid solutions of Cr(III) in the presence of oxalates was studied by measuring steady-state polarization curves in 0.5 M H 2 SO 4 solution. These electroplates demonstrate no active dissolution region and their open-circuit potentials are located in the passivity region, i.e. shifted substantially in the positive direction as compared with those of metallurgical chromium and electroplates from standard chrome-plating baths. The results of X-ray photoelectron spectroscopy (XPS) studies evidence that the passive metal surface layer several nanometers thick consists of chromium oxides with incorporated carbides formed during electroplating. It is supposed that the peculiarities of the corrosion and electrochemical behavior of the deposits under study can be attributed to the presence in them of chromium carbides, which operate as cathodic agents. At the same time, the formation of these carbide compounds during the cathodic deposition of chromium electroplates from sulfuric acid Cr(III) solutions in the presence of sodium oxalate is a result of electrocatalytic activity of metal chromium. The latter assumption is confirmed by XPS analysis of surface layers formed during the exposure of chromium to sulfuric acid solutions containing organic substances.

Corrosion–Electrochemical Behavior of Chromium Deposits Obtained from Sulfuric Acid Solutions Containing Oxalates

The corrosion–electrochemical behavior of chromium electrodeposits, which are formed in sulfuric acid solutions of Cr(III) containing oxalates, is studied by taking steady-state polarization measurements in a 0.5 M H2SO4 solution. No region of the metals active dissolution is observed for such coatings, and the open-circuit potential is localized in the passivity region, i.e. it is substantially displaced in the positive direction as compared with that for metallurgic chromium or the coatings deposited from standard chromium-plating electrolytes. According to XPS data, the surface layer of the passive metal a few nanometers thick includes oxide compounds of chromium and also carbides formed during the coating electrodeposition. Specific features of the corrosion–electrochemical behavior of the deposits are attributed to the presence of carbide compounds of chromium in them, with the compounds playing the role of a cathodic alloying agent.

Structural characteristics and catalytic activities of nanocrystalline Ni-Mo-B coatings obtained by catalytic electroless reduction

The structures, chemical states of elements, and catalytic activities of Ni-Mo-B alloys with different molybdenum contents, which were obtained by catalytic electroless reduction of metal ions, were studied. The rates of the partial reactions (heterogeneous hydrolysis of dimethylamine borane, reduction of nickel ions, and evolution of molecular hydrogen) were found to make a bell-shaped curve when plotted versus the concentration of molybdate ions in solution.

Surface Layers on Chromium in Sulfuric Acid Solutions of Organic Substances: An X-ray Photoelectron Spectroscopy Study

Surface layers formed at a contact of chromium with sulfuric acid solutions containing some organic substances (sodium oxalate, dimethylformamide, methanol) are analyzed by the X-ray photoelectron spectroscopy method. The layers are shown to contain products of deep destruction of organic molecules, i.e. carbon species and those containing functional groups. The data obtained point to the electrocatalytic activity of chromium in cathodic processes. The results of this study may provide a deeper insight into the mechanism of carbon incorporation (in the form of carbide compounds as well) into amorphous chromium deposits formed in sulfuric acid baths containing some organic substances.

X-ray photoelectron spectroscopy of chromium layers electroplated from oxalate-sulfate solutions

The chemical composition of chromium deposits obtained from acidic (pH 1.5 to 1.7) chromium (III) oxalate-sulfate solutions was studied with the use of x-ray photoelectron spectroscopy. From the solutions containing more than 100 g/l Cr2(SO4)3·6H2O, an amorphous carbide close to Cr23C6 with inclusions of metallic chromium clusters is deposited on the cathode. From less acidic solutions poorer in chromium, metallic chromium is deposited.

Distribution of Components of Binary Nickel-Chromium Alloys Electrodeposited from Sulfate-Oxalate Solutions

The composition and distribution of elements over the thickness of electrodeposited nickel—chromium layers are studied by x-ray photoelectron and Auger electron spectroscopy techniques. During a layer-by-layer etching, the Cr : Ni ratio varies with a period of 2–5 nm. This period and the amplitude of the ratio of concentrations of components increases with the current density. During alloy deposition on a copper cathode, the first to deposit is nickel, which exerts a catalytic effect on the chromium deposition, which in turn exerts a catalytic effect on the oxalate reduction to carbon. Once a deposit thicknesses of ~100 nm is reached, the alloy composition becomes constant. The deposits contain considerable amounts of carbon (in the form of graphitized structures and carbides), which increases from 0.6 to 2 wt % with the current density. The surface of the growing alloy 50–70 nm thick is rich in nickel and chromium hydroxides and organic compounds containing carboxyl groups.

X-ray photoelectron spectroscopy of chemically deposited amorphous and crystalline Ni-B coatings

The chemical state of elements and the composition of surface and bulk layers of Ni–B coatings with the volume concentration of boron of 3.7 and 27.1 at. % obtained by electroless catalytic reduction are studied with x-ray photoelectron spectroscopy combined with layer-by-layer profile analysis. Up to 90% of nickel and 80% of boron are shown to be reduced on the surface of the coatings. The effect of the conditions of heat treatment in a vacuum and in air on the composition of the surface layer is analyzed. The original nonuniformity of the in-depth distribution of the elements is found to be most of all affected by heating in air at a temperature of 700°C that provides the surface formation of a B2O3 oxide layer containing no nickel. A stabilized composition of the alloys is observed at a depth of 10 to 60 nm depending on the conditions of heat treatment.

Chromium electroplating from sulfate-oxalate solutions containing nanoparticles of alumina and silicon carbide

The effect of the introduction of Al2O3 and SiC particles into the base chrome-plating sulfate-oxalate Cr (III) bath on the electrochemical characteristics of chromium deposition, as well as the structure and mechanical properties of deposits, is studied. It is shown that Al2O3 particles are incorporated only into the surface film and are not detected in the bulk of the coating. Particles SiC are incorporated into both the surface film and the deposit. Introducing the particles into the bath makes the cathode polarization at a fixed current density, as well as the current efficiency and the bath throwing power, increase.

Corrosion and degradation of platinum-based electrocatalysts in concentrated phosphoric acid

The degradation of nanodimensional platinum catalysts in concentrated phosphoric acid and the related changes in the kinetics of cathodic oxygen reduction have been studied in the course of accelerated corrosion tests. The results of structural and electrochemical investigations showed that the stability of such electrocatalytical systems substantially depends on the type of supporting carbon and the metal phase composition.

Composition and structure of cobalt-molybdenum-boron alloys produced by chemical catalytic reduction with dimethylamine borane

The co-reduction of cobalt and molybdate ions with dimethylamine borane in pyrophosphate solutions is shown to produce Co-Mo-B alloy coatings. Molybdate is reduced concurrently with cobalt to metal forming a solid solution in cobalt matrix with a hexagonal lattice. The alloy deposits have a pronounced [001] texture with the easy magnetization direction normal to the coating surface. The composition of alloys is heterogeneous in their thickness. The alloys contain cobalt and molybdenum hydroxide admixtures, as well as intermediate products of molybdenum reduction, in the superficial layers.