Effects of U-Bending Deformation on Metastable Pitting Corrosion of Austenitic and Duplex Stainless Steel

Abstract

Effects of U-bending deformation on the metastable pitting corrosion of austenitic stainless steel (304 SS) and duplex stainless steel (2205 DSS) were investigated by microstructure characterization, electrochemical measurements and pitting morphologies observation. The metastable pitting behavior under tensile and compressive deformation in the exterior and interior surface of U-bended samples was both considered. The results show that the initiation frequency of metastable pitting for 304 SS increases in the order: tensile < as-received < compressive; while for 2205 DSS, as-received < tensile < compressive. It also reveals that the preferential sites of metastable pitting changed from γ phase to α phase due to U-bending deformation for 2205 DSS. Characteristic of transient signals and pitting morphologies were cluster distinguished and the correlation relationship was clarified. Type I transient signals exhibit the character of slow rise and fast decay, which are assigned to initiation, propagation and repassivation of metastable pits at inclusions. Type II transient signals show rapid rise followed by slow decay in current, which are related to micro-cracks propagation around metastable pits in 2205 DSS. Both tensile and compressive deformation promote the growth of few metastable pits, thus increase the transition probability from metastability to stability. In overall, both tensile and compressive deformation can degrade the pitting corrosion resistance of the experimental steels.

Keywords: U-bending deformation, metastable pitting, austenitic stainless steel, duplex stainless steel, transient signals