ИСТИНА

The surface composition of alloys usually differs from their volume composition. The “driving force” and the mechanism of selective spontaneous accumulation of individual alloy components in the surface layer (surface segregation) also attract keen attention as regards both theory and practice. This phenomenon was actively and widely studied since the appearance of modern methods of monitoring the alloy surface at its boundary with vacuum, such as, first of all, the methods of XPS and Auger spectroscopy. The alloy interface with electrolyte solutions is much more complicated for experimental and theoretical investigation. It deserves mention that it is the surface segregation at the alloy/electrolyte interface that determines the electrocatalytic and corrosion behavior of alloys and is responsible for the processes of gradual modification of the metal surface structure and in its electrical, optical and other properties. In this report, the results of studying the kinetics and mechanism of surface segregation at the interface of the in situ renewed surface of binary-alloy electrodes at the interface with electrolyte solutions are briefly surveyed. The methods used are impedance spectroscopy, cyclic voltammetry and laser-induced temperature potential shift. The study objects represent alloys with different phase composition: two-phase alloys of the eutectic type (Sn-Pb, Ag-Bi); alloys in which intermetallic compounds can be formed (Ag-Sn); (Ag-Cd); solid solutions with unlimited mutual solubility of components (Au-Ag). It should be noted that the first (the main) component is characterized by the higher specific surface energy as compared with the second component present in the alloy as a small addition. In these systems, after a thin (~10 µm) layer was mechanically cut off from the metal surface in contact with electrolyte, i.e., the surface and the volume alloy compositions were leveled out, the processes of electrode surface enrichment with the atoms of surface-active alloy components, which are observed at room temperature, are “anomalously” fast for solid metals (with relaxation times are of the order of magnitude of tens of minutes). The rate of segregation processes is shown to substantially depend on the electrochemical parameters of systems under study. The results of studying the effect of segregation at the alloy/electrolyte interface are compared with the data obtained by Auger spectroscopy and XPS at the interface of these alloys with vacuum. Phenomenological models that describe the effects observed on the alloy types listed above are proposed and substantiated. The most important conclusion of this study is that in contrast to the metal/vacuum interface, the electrochemical experimental conditions, makes it possible to vary the rate and sometimes the mechanism and the direction of the surface segregation processes by changing different electrochemical parameters. The authors are grateful to the Russian Foundation for Basic Research for the financial support of this study (grant no. 15-03-05243).