The effect of plasma electrolytic polishing on the surface properties of nitrocarburised steel

Abstract

The effect of anodic plasma electrolytic polishing on the characteristics of low carbon steel (0.2 % C) after nitrocarburising was investigated. The formation of the modified layer including a hardened diffusion layer and a surface oxide layer occurs as a result of anodic plasma electrolytic nitrocarburising in an aqueous solution of glycerol (8 %), ammonium nitrate (5 %) and ammonium chloride (15 %) at 850 °C. Conducting a plasma electrolytic polishing of the surface leads to the removal of a loose part of the oxide layer (predominantly FeO), which affects the reduction the surface roughness of 2 times. The rate of the weight loss of steel in this process is 0.5 mg/s. The corrosion current density in a 3.5 % sodium chloride solution decreases from 41.0 μA/cm2 in an untreated sample to 32.6 μA/cm2 in the steel after nitrocarburising and polishing during 30 s and to 22–24 μA/cm2 when the second operation lasted 60-300 s due to the protective action of the nitride zone and the dense part of the oxide layer. Wear test shows that plasma electrolytic polishing enhances the wear resistance of a nitrocarburised sample by 1.8 times.