Electrolytic refining magnesium is carried out under argon environment to eliminate magnesium chloride hydrolysis process. The limit diffusion current densities in the porous diaphragms, impregnated with NaCl-KCl-MgCl2 fused electrolyte, for various electrolysis durations are determined as an important characteristic of process kinetics. The great time value for achievement of stationary state in magnesium dissolution is explained by absence of convective mass - transfer in the near - anode layer.