t—The results of plasma-electrolytic polishing of steel specimens after their anodic diffusion saturation with elements, which form interstitial solutions, are considered. The structure and phase composition of the modified layers are studied using the X-ray diffraction analysis and electronic microscopy. The surface roughness and corrosion resistance of specimens in the 3.5% sodium chloride solution are measured. Various morphology of oxide layer is revealed after nitrocarburizing, nitriding, boronizing and borocarburizing of carbon structural steels 20 and 45. The versions of treatment and electrolyte compositions are determined that reduce the surface roughness from 1.00 μm for the control specimen to 0.23 μm after borocarburizing in the solution of boric acid, glycerol and ammonium chloride followed by polishing in the sulfate electrolyte and to 0.40–0.42 μm after nitriding in the ammonia and ammonium chloride solution followed by polishing in the sulfate or chloride electrolyte. It is shown that the corrosion current density of steel 45 can be reduced by almost 4 times after nitriding and polishing in the sulfate or chloride electrolyte.