Surface working of 304L stainless steel: Impact on microstructure, electrochemical behavior and SCC resistance

The effect of surface working operations on the microstructure, electrochemical behavior and stress corrosion cracking resistance of 304L stainless steel (SS) was investigated in this study. The material was subjected to (a) solution annealing (b) machining and (c) grinding operations. Microstructural characterization was done using stereo microscopy and electron back scattered diffraction (EBSD) technique. The electrochemical nature of the surfaces in machined, ground and solution annealed condition were studied using potentiodynamic polarization and scanning electrochemical microscopy (SECM) in borate buffer solution. The stress corrosion cracking resistance of 304L SS in different conditions was studied by exposing the samples to boiling MgCl2 environment. Results revealed that the heavy plastic deformation and residual stresses present near the surface due to machining and grinding operations make 304L SS electrochemically more active and susceptible to stress corrosion cracking. Ground sample showed highest magnitude of current density in the passive potential range followed by machined and solution annealed 304L SS. Micro-electrochemical studies established that surface working promotes localized corrosion along the surface asperities which could lead to crack initiation.

► Machining/grinding produce extensive grain fragmentation near the surface of 304L SS. ► Machining/grinding result in martensitic transformation near the surface of 304L SS. ► Machining/grinding drastically reduce the SCC resistance of 304L SS in chloride. ► Machining/grinding make the surface of 304L SS electrochemically much more active. ► SECM study reveal that preferential dissolution takes place along surface asperities.