Electrochemical Corrosion Behavior Of Direct Current Pulsed Plasma Source Ion Nitriding Stainless Steel

developed in our laberatory. The phase microstructure, composition and surface morphology of 316 austenitic stainless steel andγN phase layer were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM); the hardness of 316 austenitic stainless steel andγN phase layer are studied by microhardness tester; the passivation of high-nitrogen f.c.c. (γN) phase formed on AISI 316 austenitic stainless steel and the semiconductor characteristics of the passive film were studied by using anodic polarization, electrochemical impedance spectroscopy (EIS), and Mott-Schottky plot, to explore the pitting corrosion mechanism of the stainless steel. The self- passivation of theγN phase layer in 3.5 % NaCl solution was observed. The results indicate that:At a fixed nitriding pressure, the content of nitrogen in surface increases with the increase of the permeability nitrid temperature, from 4.15 at.% of 400℃to 24.72 at % of 450℃.At a fixed nitriding temperature applying 250V negative bias consideraloly enhanced, the thickness ofγN layer improved its uniformity. And it showed the uniform structure.The optimal working parameters of direct current pulsed plasma source ion nitriding 316 stainless steel are 250V as well as negative bias 450℃500 Pa, 6h,γΝphase layer is about 13μm thick, and the average microhardness of nitried 316 stainless steel isΗV0.1N1.39GPa, five times higher than that of 316 stainless steel.Both the original and nitrided presented self-passivation in 3.5% NaCl solution. At 200mV, the 316 stainless steel showed pitting breakdown.γN phase layer had no pitting breakdown. With the marinated time extension, the EIS of 316 stainless steel andγN phase layer in 3.5% NaCl solution shows that, the diameters of the capacitive increase, the phase angle platform widened, highly increasing. After 3h immersion, the |z|-logf ofγN phase layer low-frequency peak change flat, phase angle increased height closer to 90°, the passive film ofγN-phase-modified layer was more compact than 316 stainless steel.The Mott-Schottky plot shows that the passivation films ofγN/316 stainless steel behave as n-type and p-type semiconductor in the potential range about and below the flat band potential. It showed that: the passive film had two parts, the endothecium was Cr2O3, the deep space was Fe2O3, compare to the 316 stainless steel, the capacitance of space charge, donor density and accept density ofγN decreased by 1 order. The flat-band moved negatively, the value of oxygen vacancy diffusion coefficient increased by 1 order, it restricted the self-catalysed- The surface modification of 316 stainless steel has been investigated by the direct current pulsed plasma source ion nitriding (DCPPSIN) apparatus LDL-50, which designed and acidification, decreased the disfigurement of passive film, increased the capacitive character and the oxidation and protect capability of passive film toned up.