A method for calculation of an average local rate A¯ of potentiostatic nucleation in an arbitrary time range of an electrode process has been proposed. An example of analysis of Zn nucleation from a zincate electrolyte on a pyrolytic carbon electrode according to Scharifker and Mostany's kinetic model has been used as an illustration of the proposed method. A growth or decrease of A¯ during the nucleation period has been explained by activation of the electrode surface or depletion of more active center of the electrode, respectively.

The process of Zn monolayer electrocrystallization on an iron electrode from a zincate electrolyte including the transfer from Zn underpotential deposition (UPD) to Zn phase growth under supersaturation conditions (OPD) has been studied by the galvanostatic potential transient method. In the OPD region, an analysis of potential transients E(t) has been carried out on the basis of the conception of summation of the charge transfer and crystallization overpotentials. OPD of a Zn monolayer from both the pure electrolyte and one containing an inhibiting additive of polymeric tetraalkylammonium salt does not involve nucleation and has been interpreted as development of growth sites formed on a Zn-Fe alloy surface under UPD conditions. An assumption concerning inheritance of peculiarities of Zn intercalation into the Fe crystal lattice by the charge transfer and crystallization stages of Zn monolayer growth during the UPD-OPD transfer has been strengthened.

A b s t r a c tThe thermodynamic and kinetic peculiarities of metal nucleation under galvanostatic conditions are considered. The alternative expressions for supersaturation in electrochemical systems in terms of overall overpotential (Kaischew, Budevski, Malev, Polukarov, de Levi at al.) or crystallization overpotential (Fischer, Volmer, Schottky, Vetter, Lorenz, Gerischer at al.) are discussed. It is shown that incorrect use of expression for supersaturation in terms of overall overpotential leads some authors to unreasonable interpretation of experimental data. This leads to negative impact on electrochemical nucleation theory development. Some kinetic theories (Schottky, Baraboshkin, Kashchiev) take into account changes of supersaturation and atoms consumption by growing nuclei during the current pulse. However this corrections are difficult to identify. Computer modeling of galvanostatic phase formation (Isaev, Baraboshkin, Volegov) correctly describes the time dependence of the main parameters of nucleation. Moreover, applying of this approach in data processing of experimental overpotential transients requires the kinetic and thermodynamic constants of the process determination. Gutsov`s kinetic model is still more suitable for the analysis of experimental overpotential transients, because this theory takes into account the influence of crystallization overpotential on the rate of phase formation. А н о т а ц і я Розглянуто дискусійні аспекти термодинаміки і кінетики фазоутворення в гальваностатичному режимі електролізу. Проаналізовано альтернативне вираження пересичення в електрохімічних системах через сумарну перенапругу електродного процесу η або перенапругу стадії кристалізації η к . Застосування еквівалентної пересиченню величини η негативно вплинуло на розвиток теорії електрохімічної нуклеації. Врахування в деяких кінетичних моделях фазоутворення впливу пересичення та витрачання атомів на ріст зародків пов'язане з великою кількістю наближень, які іноді важко навіть оцінити. Раціональною для аналізу гальваностатичних транзієнтів η залишається модель Гуцова, що враховує вплив на швидкість нуклеації η к . Ключові слова: гальваностатичне фазоутворення; пересичення; перенапруга кристалізації; адатоми; транзієнт перенапруги. А н н о т а ц и я Рассмотрены дискуссионные аспекты термодинамики и кинетики фазообразования в гальваностатическом режиме электролиза. Проанализированы альтернативные выражения пересыщения в электрохимических системах через суммарное перенапряжение электродного процесса η или перенапряжение стадии кристаллизации η к . Использование эквивалентной пересыщению величины η оказало негативное влияние на развитие теории электрохимической нуклеации. Учет в некоторых кинетических моделях фазообразования влияния пересыщения и потребления атомов растущими зародышами связан с большим числом приближений, которые иногда трудно даже оценить. Рациональной для анализа гальваностатических транзиентов η остается модель Гуцова, учитывающая влияние на скорость нуклеации η к . Ключевые слова: гальваностатич...

Potentiostatic formation of zinc nuclei on an isotropic pyrolitic carbon electrode from a zincate solution with no additive and an electrolyte containing polymeric tetraalkilammonium salt (TAS) was studied. The process was analyzed taking into account an influence of hydrophobic and hydrophilic parts (zones A and B, respectively) of the electrode surface on the charge transfer and crystallization stages. Due to specific features of TAS adsorption on the surface of zones A and B, nucleation results in formation of two groups of nuclei. A little number of nuclei (N~10 5 cm -2 ) is formed on those parts of zones B which are "free" from the additive. On sites of zones A and B "occupied" by TAS, nucleation occurs at a high supersaturation and a large number of nuclei (N~10 8 cm -2 ) is formed. The number of nuclei in the both electrolytes is controlled by nucleation exclusion zones.

A novel method of calculation of the average local rate A¯ of potentiostatic nucleation within a short time interval at an arbitrary moment of the electrode process has been proposed. This method has been applied to Zn nucleation on a zirconium electrode from zincate electrolyte using the kinetic model developed by Scharifker and Mostany (J. Electroanal. Chem., 177, 13 (1984)). An increase of A¯ during the nucleation period has been explained by activation of the oxidized electrode surface, while a decrease of A¯ can be caused by consumption of more active centers of the electrode. We assume that formation of new active nucleation centers is caused by the stage of reduction of the surface Zr oxides.