Study On Hollow Cathode Plasma Source Carburizing Of AISI304 Austenitic Stainless Steel

Hollow cathode plasma source carburizing was applied to treat the AISI304 austenitic stainless steel under the condition of 450 ?,500 Pa.Optical microscope,SEM,EPMA,XRD,Microhardness tester,electrochemical workstation and contact resistance measurement system were used to investigate the metallographic structure,surface morphology,carbon concentration-depth profiles,phase structure,Microhardness-depth profile,pitting corrosion resistant and surface contact resistance properties of the hollow cathode plasma source carburized AISI304 austenitic stainless steel,which are experiment basement for developing new plasma low-temperature carbonizing technique.The carburized surface of the austenitic stainless steel forms a C supersaturated metastable FCC phase(?c).The thickness of continuous and uniform modified layer is up to 20 ?m,the highest C concentration and microhardness at the surface is 6.5 at.%and HV0.25 N 7.5 GPa,respectively.XRD results show that ?c and a bit of Cr7C3 were formed on the surface of carburizing layer,and with the deeper from the surface goes,the yc peak moving to the original austenitic stainless steel position gradually.Under the surface of 4?m,the Cr7C3 peak disappeared,means the carbide only formed on the surface.The anodic polarization curve in 3.5%NaCl solution shows that the ?c modified layer under surface 8 ?m owns three steps:active dissolution,passivation and transpassivation,without pitting corrosion completely,means the corrosion resistance property is increased significantly.The bad corrosion property at the surface is caused by carbide,while,the corrosion property of carbonized layer under surface 4 ?m is closed to original stainless steel.The EIS results of yc in 3.5%NaCl solution shows nearly capacitance characteristics.The resistance of passivation film increase to 1.688×105 ? cm2 from original austenitic stainless steels' 3.05×104 1? cm2.The active carbon atom changed the structure of the double layer,lead to a higher activation energy of the metal cations reaction,and reduce the active point of pitting corrosion on the surface.The Mott-Schottky curve of passive film in 3.5%NaCl solution,donor concentration,acceptor concentration and flat-band potential are 1.24×1021 cm-3?1.622×1021 cm-3 and-237 mV(SCE),respectively,which are all lower than the original austenitic stainless steel.the active carbon atoms take the positions of defects in passive film,increases the its density,prevent Cl-moving into the passive film.Compared with original austenitic stainless steel sample,the contact resistance of carbonized layer decrease from 12 m? cm2 to 6 m? cm2 under 160 N/cm2 pressure condition.The trace amounts of carbide in the yc increase the conductivity,which is good for the electrical contact property.