Abstract <p>Electrochemical heat treatment in which the electrical current runs through the gas phase is used to improve the process of saturation of impurity elements from the gas phase to metal surface at elevated temperatures. During an electrolytic-plasma surface treatment, a gas-vapor shell is formed around the material being processed (active electrode) owing to the boiling of the electrolyte. Under the heating mode, it is possible to separate water-based electrolyte with a temperature not higher than 373 K and the surface to be treated, the temperature of which may exceed 1400 K, by the gas-vapor film. In this paper, we carried out an analytical calculation of the thickness of the vapor-gas shell formed around the active electrode during the heating mode. This calculation is based on equal amount of evaporated electrolyte and the amount of the vapor withdrawn upwards along the surface of the active electrode due to the Archimedes force. Comparison of the results of calculation with the experiment confirmed the correctness of this method. We also analyzed the reasons for acceleration of the saturation of metal surfaces during both electrolytic-plasma treatment and other types of electrochemical heat treatment. It is shown that the processes of transfer of alloying elements through the gas, their adsorption, atomization, and absorption are significantly accelerated during the gas discharge. It is important that not only relatively light atoms of boron, carbon, nitrogen, or oxygen but also heavy atoms of Ti, V, Mo, W, etc., can be saturating elements. It is also shown that the cause of accelerated diffusion of alloying elements inside the metal being processed is not electromigration (“electronic wind”), whose contribution is insignificant, but nonstationarity and nonuniformity of current density distribution, and, consequently, temperature and elastic stresses.</p>