Preparation Of Nanostructured Surface Layer Of AISI 304 Stainless Steel And Its Pitting Corrosion Resistance

Surface nanocrystallization(SNC)technology aims to fabricate nanostructured surface layer so as to improve the mechanical properties,such as strength and hardness,and to optimize the surface activity and chemical heat treatment performance of materials.Up to now,several different SNC techniques have been developed,among which plastic deformation method has drawn great interest owing to its simple process,low cost,and the absence of sudden changes in structure and composition between surface layer and substrate.However,heavy plastic deformation often degrades the surface quality,such as increasing the surface roughness and occuring microcracks,which is not good for the practical service and also influences the correct evaluation of the properties of nano-structured surface layer.Therefore,here we in this study,selected an austenitic stainless steel as the research object,to fabricate high-quality nanostructured surface layer by Ultra-Sonic Rolling Treatment(USRT)and Pipe Inner Surface Grinding(PISG).The microstructure evolution mechanism was characterized in detail,and the pitting corrosion behavious in 0.6 mol/L NaCl solution was studied.The following coclusions were reached:1)USRT obtained high-quality nanostructured surface layer in AISI 304 stainless steel.The 4 passes of amplitude 8?m USRT refined the surface grains down to 42 nm,leading to a hardened layer of 600?m thick.The hardness was increased from 190 HV to 450 HV,and The surface roughness is reduced from 1.9?m to 0.3?m(Ra).2)The mechanism of good surface quality was proposed to be an asperity peak flattening process.The general process of grain refinement of AISI 304 stainless steel by USRT is revealed:i)generation of planar dislocation arrays and deformation twins,ii)formation of twin bundles,iii)initiation and development of SBs,and iv)formation of ultrafine-and nano-laminated structure.3)The nanostructured surface layer of AISI 304 stainless steel had high pitting potential.The potentiodynamic polarization curves of nanostructured surface and coarse-grained samples show passivation behavior.Compared with the annealed coarse-grained samples,the surface layer of nanostructures has higher pitting potential and wider passivation interval.4)A large number of defects caused by grain refinement on stainless steel surface was an important factor affecting pitting corrosion resistance.By calculation,the effective diffusion coefficient D_eff of Cr on the nano-surface layer of 304 stainless steel was 2.9×10³times that of annealed coarse-grained samples.Passiva film formed on the surface of 304 stainless steel in NaCl solution had a double-layer structure,the inner layer was rich Cr and the outer layer was rich Fe.After corrosion,the roughness of nanostructure surface was lower and the thickness of passivation film was increased.The structure of passivation film was mainly amorphous and contained nanocrystals inside.