Effect of hydrostatic extrusion on passivity breakdown on 303 austenitic stainless steel in chloride solution

The stability of the passive film formed on austenitic 303 stainless steel in the as-received state and after severe plastic deformation by hydrostatic extrusion (HE) leading to nanostructurization was investigated in an aggressive environment containing Cl- ions by anodic polarization. Transmission electron microscopy (TEM) and stereological image analysis were used to examine structural changes introduced by HE. Surface analytical techniques such as light microscopy, scanning electron microscopy, Auger electron spectroscopy (AES), and scanning Auger microscopy (SAM) were used to characterize the morphology, grain size, and chemistry of the surface, including local characterization of nonmetallic inclusions and their surface before and after HE. SAM analysis revealed discontinuities of the passive oxide film on MnS inclusions. TEM and scanning transmission electron microscopy examinations confirm a drastic reduction of grain size accompanied by a distinct refinement of the size of sulfide inclusions in the HE matrix. These changes in the HE 303 stainless steel are apparently responsible for its reduced pitting resistance compared to the as-received material.