Characterization of the effects of hydrostatic extrusion on grain size, surface composition and the corrosion resistance of austenitic stainless steels

Passivity breakdown on austenitic Types 303 and 316 stainless steels in the as-received state, and after heavy plastic deformation by hydrostatic extrusion (HE), was investigated in an aggressive environment containing Cl− ions. Microscopic, surface analytical and electrochemical methods were used to characterize changes in structure and chemistry of the surface of austenitic stainless steels introduced by HE. TEM and stereological image analysis were used to examine structural changes introduced by the HE treatment. The effects of surface oxide films on pit nucleation in stainless steel were investigated by anodic polarization in a borate buffer + 0.1 M NaCl solution. Surface analytical techniques such as light microscopy, SEM, AES and SAM were employed to characterize the morphology, grain size, and chemistry of the surface, including local characterization of nonmetallic inclusions and their surface before and after HE modification. The results confirm a drastic reduction of grain size due to the HE process and a distinct reduction of the size of sulfide inclusions occurring in the Type 303 matrix. Moreover, a discontinuity of the surface oxide film on MnS inclusions was found using a local SAM analysis. These factors apparently result in a lowering of the resistance of Type 303 HE to pit nucleation as compared to the as-received material. On the contrary, Type 316 stainless steel, where only few nonmetallic inclusions were present and MnS inclusions are absent, has shown a higher resistance against passivity breakdown after the HE process. A tentative role of the above differences in the stability of these materials in a Cl−-containing electrolyte is discussed.