The relationship between the amount of hydrogen electrochemically introduced to Pd and the volume and linear sizes of the formed PdH system, as well as the intensity of cavitation that arises after saturation of Pd with hydrogen, has been determined. The results of the study allows one to describe the mechanism of origin of cavitation on PdH in spite of the impossibility to determine the composition of atmosphere in bulk defects (microvoids, microcracks, and dislocations), which are formed upon hydrogenation of Pd. A Harvey–Knapp model has been employed, which describes well the origin of cavitation on nonwettable surfaces with fine cracks. The model is based on the hypothesis that presence of cavitation cores are present in water in the form of fine air bubbles.

The gas evolution process on fully hydrogenated Pd (H : Pd = 0.73) cathodes is studied in a 1 M solution of NaOH at ambient temperature. A phenomenon of spontaneous gas evolution on the surface of PdH after electrolysis and transfer of the sample into a burette with a 1 M solution of NaOH or distilled water is discovered. The volume of the gas that spontaneously evolved on the surface of PdH is 0.22–0.24 cm3/cm2 in a 1 M solution of NaOH and 0.40–0.43 cm3/cm2 in distilled water. It is proposed that the spontaneously evolving gas is not hydrogen from PdH, but results from the cavitation process.

The currentless deposition of Ag + cations from silver nitrate solutions has been found on the surface of hydrogenated Pd (Pd−H). This process has been shown to proceed via a chemico-catalytic mechanism. It has been demonstrated that the major fraction of silver deposited onto the Pd-H system from the nitrate solutions in the first 0.5–1 h of holding in the solution. A minimal size of silver crystallites (26–40 nm) was found when deposited from the diluted nitrate solutions that contain 0.32–2.5 g/L of Ag. The study has revealed that the currentless Ag deposition on the Pd–H surface is accompanied by the formation of the colloid phase of Ag crystallites. It has been established that the Pd–H system could purify solutions of Ag + when the amount of hydrogen in Pd exceeded the amount of Ag + in the solution.

Установлено, что на поверхности насыщенного водородом Pd (Pd–H) происходит бестоковое осаждение катионов Cu2+, Ag+ и др. из растворов их солей. Сделано предположение, что этот процесс идет по химико-каталитическому механизму. Показано, что 80–90% меди осаждается на Pd–H за первый час контакта образца с раствором CuSO4. Установлено, что система Pd–H с тонким палладиевым покрытием способна глубоко очищать растворы от Cu2+ тогда, когда количество водорода в Pd превышает количество Cu2+ в растворе.

The currentless deposition of Cu2+, Ag+, and other cations from solutions of their salts takes place on the surface of hydrogen-saturated palladium (Pd-H). It is hypothesized that this process occurs via a catalytic chemical mechanism. It is demonstrated that 80–90% of the copper deposits onto Pd-H from a CuSO4 solution in the first hour of their contact. The Pd-H system with a thin palladium coating can finely purify solutions from Cu2+ when the amount of hydrogen in Pd exceeds the amount of Cu2+ in the solution.

The reason for the unstable influence of Cr(III) on the maximum rate of incomplete reduction of chromic acid is studied by a method of cyclic voltammetry and analytical investigations of the electrolyte composition. It is established that the decrease in the maximum rate of electrochemical reaction Cr(VI) → Cr(III) is due to the drop of concentration of free sulfate ions in the electrolyte. The drop of the concentration of free sulfate ions increases with the content of electrochemically synthesized ions of Cr(III) in the solution. The decrease in the concentration of sulfate ions is assumed to stem from the formation of unstable complexes of Cr(III) with sulfate ions. Sulfate ions regain their initial concentration with time and upon heating solution to 50–70°C, which facilitates dissociation of unstable sulfate complexes of Cr(III). It is shown that the concentration of sulfate ions in the solution remains invariant during chemical reduction of Cr(VI) to Cr(III), which points to the formation of inert complexes of Cr(III) that make no impact on the rate of incomplete reduction of chromic acid.

Effect of an alkylsulfo compound (ASC) on the cathodic process in chromate solutions is studied at 20–70°C. Below 40–50°C, ASC reduces the current efficiency for chromium (CE). Above 40–50°C, CE rises and then remains invariant with temperature. The ASC effect on the Cr deposition rate is attributed to variations in the sulfate concentration near the cathode, which are caused by the presence of ASC and its hydrolysis.

The rate of the trivalent chromium ion formation during the chromic acid reduction in the presence of sulfate ions is studied in a wide potential range by the RRDE method. At potentials of incomplete reduction of chromate ions, the reaction is attendant by a side process (probably, the hydrogen ion reduction). With a phase film covering the disk cathode, the ring current does not correspond to the number of Cr(III) ions formed at the disk, rather it characterizes the oxidation of products of a phase film comprising oxide-hydroxide compounds of Cr(III). The hypothesis about the formation of a phase cathodic film, presumably preceding the metallic chromium deposition, is confirmed experimentally. The oxide-hydroxide species of Cr(III), which are main film-forming substances, undergo phase transitions in a zone that depends on the sulfate ion concentration. The phase transitions whose products may reach the ring electrode occur in the near-electrode layer. The lower the sulfate ion concentration, the larger the extension of the layer.

Hydrogen content in chromium and phase structure of deposits are studied in relation to the time t(i) elapsed after introduction in formic acid into the chromic acid electrolyte. It is found that the hydrogen content of chromium increases with the increasing t(i), reaching a maximum corresponding to the hydrogen amount in chromium hydride. The higher the HCOOH additive concentration, the longer t(i), at which the hydrogen content in chromium reaches a maximum. The X-ray diffraction analysis confirms the conversion of bcc chromium into hcp chromium at an increase in the hydrogen content in the deposit and a decrease in the fraction of hcp chromium at t(i) corresponding to a decrease in the hydrogen content in chromium and to a decrease in the current efficiency after passing a maximum. The fact that the dependences of hydrogen content in chromium and the current efficiency on t(i) are similar is explained by variation in the thickness and density of the cathodic film. The latter is associated with the redox interaction between HCOOH and CrO3 and the concentration changes in the near-cathode layer. This explanation is supported by the effect of hydrodynamic conditions and the role of other factors affecting the HCOOH oxidation in the electrolyte and the diffusion limitations of the electrode process.

The effect of formic acid on the electrodeposition of chromium from chromate electrolyte was studied. The effect is a complex one; concentration and time dependences of current yield proved to be nonmonotonic. The effect of formic acid on electrodeposition is primarily related to changes in thickness of cathodic films due to buffering with formic acid and complexing of formic acid with Cr(III) ions.

The electroreduction of chromic acid was studied in solutions with added molybdate ions by recording potentiodynamic curves and by quantitative determination of the reaction products. It was found that the reduction of Cr3O10(2-) to Cr3+ ions is accelerated at potentials below those of chromium deposition when Na2MoO4, and particularly H2MoO4, is added. The rate of hydrogen evolution remains unchanged at the potentials of incomplete chromate ion reduction, while the current of molybdate ion reduction can practically not be measured. In solutions without sulfate ions, the rate of hydrogen evolution increases when molybdate ions are added to the chromic acid. The results obtained were explained in terms of conversion of the cationic form of Mo(VI) to an anionic form which occurs owing to the pH rise of the solution layer next to the cathode during reduction of the Cr3O10(2-) ions and owing to changes in the state of the electrode surface at the start of electrolysis resulting from the formation of reduction products of the molybdate ions.

The authors studied the effect of molybdenum on the surface structure and certain physicomechanical properties of the electrolytic chromium produced by Cr-Mo electrodeposition from CrO/sub 3/ solutions with added SO/sub 4//sup 2 -/ and SiF/sub 6//sup 2 -/ ions. The results indicate that the changes in hardness, hydrogenation, and internal stresses of the chromium coatings which occur upon alloying with molybdenum are chiefly due to structural changes, while the incorporation of Mo into chromium by itself does not markedly affect these characteristics. The structural changes of chromium which occur following the codeposition of molybdenum evidently are due, not only to a change in the relative concentration of anions in the cathodic film, particularly a decrease in the content of catalytic anions in the film, but also to special features in chromium electrocrystallization, an aspect which requires special consideration involving methods of investigating fine structure and surface composition.

It had been found previously that a codeposition of molybdenum with chromium leads to lower inhibition of metal crystallization, which to an appreciable degree is due to the lower fraction of catalytic anions found in the cathodic film when molybdate irons are incorporated into it. In this work x-ray photoelectron spectroscopy was used to study the chemical composition of the surface and the vertical composition profiles as functions of the amount of codeposited chromium and of electrolysis time, and thus to elucidate the influence of the specific features of MoO/sub 4//sup 2 -/ ion reduction on chromium crystallization.