This work focuses on the factors causing appearance of a steady and continuous vapour-gas envelope which functions as medium for plasma electrolytic saturation of metal and alloys with interstitial elements (nitrogen, carbon, and boron). It is established that second critical voltage associated with transition from the current oscillation mode to the stable heating is determined by anion emission from boiling electrolyte in the envelope and heat transfer conditions in the system. Stability of the interface electrolyte–envelope is provided by the energy liberation in the envelope due to the passage of current. Second critical voltage promoting the anion emission is calculated on the base of Gouy– Chapman model and Tonks–Frenkel aperiodic instability. Theoretical dependence of critical voltage on the electrolyte concentration is confirmed experimentally. The influence of the electrolyte concentration on the second critical voltage is explained by the ability of the electrolyte to emit anions. Effect of solution flow rate on this voltage accounts for heat transfer conditions. It should be noted that the anion emission explains the influence of electrolyte composition on the weight change of the anode sample, limit heating temperature (1000C) due to the limited emissivity of electrolyte, discrete current in the case of a small surface anode, and high-frequency pulse of the current.