Effect of the roughness produced by plasma nitrocarburizing on corrosion resistance of AISI 304 austenitic stainless steel

In this work, we studied the influence of surface roughness produced by sputtering during plasma nitrocarburizing treatments on corrosion resistance of AISI 304 austenitic stainless steel in an aqueous solution of sodium chloride. Plasma nitrocarburizing treatments were carried out in a gaseous atmosphere containing 80% N2 + 2% CH4 + 18% H2, at 400 Pa, for 2 h and at temperatures of 375 °C, 430 °C and 475 °C. Surface roughness after plasma nitrocarburizing was characterized using 3D confocal microscopy. Corrosion susceptibility of the plasma nitrocarburized surfaces was investigated by electrochemical impedance spectroscopy, potentiodynamic polarization, and scanning electron microscopy. The results showed that when increasing the temperature of the plasma nitrocarburizing treatment, both thickness and amount of precipitates increased, as expected. Analyses by 3D confocal microscopy also showed increased surface roughness with increasing plasma nitrocarburizing temperature. When comparing two plasma nitrocarburized specimens with similar precipitate-free structure, it was found that thinner thickness of the expanded austenite layer and lower surface roughness resulted in higher corrosion resistance.