Characterization of submicron-size layer produced by pulsed bipolar plasma electrolytic carbonitriding

The feasibility of developing a submicron-size layer on nitriding steel using pulsed bipolar plasma electrolytic techniques was investigated. Plasma electrolysis was performed in duty cycle of 80% and process duration of 600 s, with an electrolyte composition of 85.8 wt% urea, 12 wt% water and 2.2 wt% sodium carbonate. It was found that this technique reduces the grain size in both compound and diffusion layers. The average grain size of about 110 nm was observed in the white layer. The presence of Fe3O4 phase in low frequencies was confirmed by X-ray studies. γ′-Fe4N rather than ε-Fe2-3N was found to be the dominant phase at higher frequencies. The results suggest that the size of the cavities formed on the surface is reduced to 300 nm. Characterization of the corrosion behavior of the layer by electrochemical impedance spectroscopy technique revealed that the capacitive properties of the surface layer (Cc) in samples processed at high frequencies were reduced. Hence, it showed better corrosion resistance compared to the samples processed at low frequencies.