Electrochemical Corrosion Characteristic Of AISI304Austenitic Stainless Steel Modified By Plasma Source Nitriding In Borax Buffer Solution

Plasma Source Nitriding technology was used for the treatment of AISI304austenite stainless steel. The phase structure, surface morphology, nitrogen concentration-depth and microhardness of the modified layer on AISI304austenite stainless steel were characterized by X-ray diffractometer, optical metallurgical microscope, electron probe microanalyzer and microhardness tester. The pitting corrosion resistance and semiconductor property of the passive film on the modified layer in a Borax Buffer Solution with a pH value of8.4were investigated by the measurement of anodic polarization curves, electrochemical impedance spectroscopy and Mott-Schottky curves.The results indicate that the nitrided layer consisted of monophase γN phase with a peak nitrogen concentration of23at.%and a thickness of17μm was formed on AISI304austenite stainless steel by Plasma Source Nitriding. The microhardness of the modified layer reached to HVo0.25N10-13GPa,was5times higher than of the original sample. The anodic polarization curves of the original and modified samples in a Borax Buffer Solution with a pH value of8.4were spontaneous passivation process. The corrosion potential Ecorr and the breakdown potential Ept of the γN phase layer increased from-301mV (SCE) to-226mV (SCE) and from946mV (SCE) to978mV (SCE), repectively. The self-corrosion current density Icorr and passivation current density Iss of the γN phase layer decreased approximately an order of magnitude, compared with the original sample. After immersing in Borax Buffer Solution with a pH value of8.4for1h, the diameter and Modulus|Z|of the capacitance arc of the γN layer was larger than the original sample, and the phase angle platform gets higher and wider. Fitted by R(QR) equivalent circuit, the passive film resistance Rp of the γN layer increased approximatively2orders of magnitude than the original sample, and Y0which characterises the capacitance of the passive film decreased correspondingly, indicated that the passivation film of the γN layer has higher insulation resistance and better compactness. With the immersation time increased to24h, the passive film resistance Rp of the γN layer was stable to a magnitude of105Ω·cm2, and the passive film of the γN layer has good stability. After immersing in the Borax Buffer Solution with a pH value of8.4for1h, both of the passive films on the original sample and the γN phase layer had characteristics of n-type and p-type semiconductors;with the frequency of1kHz, the donor density of the γN phase layer decreased approximately1order of magnitude and the acceptor density has little change, and flat band potential changes negatively, compared with the original sample. The passive film formed on the γN phase layer in the Borax Buffer Solution with a pH value of8.4has better insulation conductivity than the original sample, leading to more excellent corrosion resistance.