Research On Pitting Corrosion Resistance Of S32205 Duplex Stainless Steel Subjected To Laser Shock Peening

Corrosion damage is the main form of key components failure in offshore and chemical industry.The surface properties of the material are the key factors which affect its corrosion resistance.S32205 duplex stainless steel is an important raw material for marine engineering and chemical components.Its performance determines the service life of these components.Therefore,the surface modification of S32205 duplex stainless steel that improves its corrosion resistance in complex environments is extremely important for better performance of offshore and chemical equipment.Laser shock peening（LSP）is a non-contact surface modification method.It utilizes ultra-high pressure plasma shock wave to shot the target material.During the process,the surface undergoes intense plastic deformation that generates compressive residual stress and induces grain refinement and even phase transformation,which significantly improve the mechanical properties and corrosion resistance.This essay systematically investigates the effect of LSP on the surface integrity and corrosion resistance of S32205 duplex stainless steel,revealing the mechanism of improved pitting corrosion resistance of LSPed S32205.The main contents and results are as follows:（1）Based on the laser shock wave and plastic deformation theory,the optimum laser parameters are analyzed and calculated in this study,and a reasonable test plan is proposed.The changes in surface integrity of specimens before and after LSP are systematically investigated by X-ray techniques,micro-hardness tester and confocal microscope.The results indicate that the surface roughness of the LSPed specimens increases compared with the as-received one.Moreover,LSP removes the tensile stress in the surface layer and produces compressive residual stress in austenite and ferrite phases.The peaks value of compressive residual stress in both phases increase as laser energy increases.Gain refinement is found in the surface of the LSPed specimens,and increased dislocation density and micro-strain in both phases are discovered.The microhardness of the LSPed specimens decreases from the surface to the matrix due to grain refinement and dislocation increment.Martensitic phase transformation occurs during the process and noσphase is produced.（2）Electrochemical experiment is implemented on as-received and LSPed specimens in 4mol/L Na Cl solution.The results show that the open circuit potentials and self-corrosion potentials of the LPSed specimens positively shift and passive current densities and current densities reduce.Moreover,the pitting potentials and impedance radiuses increase,which proves that the pitting corrosion resistance improves after LSP.Meanwhile,the density of corrosion pits decreases,and chromium content on the surface rises.Combining residual stress strengthening with grain refinement theory,the mechanism is theoretically summarized.The improved pitting resistance of LSPed S32205 is attributed to that compressive residual stress enhances the self-healing ability and denseness of the passive film,and grain refinement provides more nucleation sites for passive film,which improves its uniformity.The formation of a more homogeneous and dense passive film could resist pitting corrosion more effectively（3）Electrochemical experiment is implemented on as-received and LSPed specimens in 0.5mol/L H₂SO₄ solution.The results suggest that the open-circuit potentials,self-corrosion potentials and pitting potentials of LSPed specimens increase.Decreased passive current density and self-corrosion current density and increased impedance radiuses are also observed.Reduced pitting density and enriched chromium on LSPed specimen’s surface demonstrate that pitting is suppressed.Based on grain refinement theory,the reason for enhanced anti-pitting corrosion performance of LSPed S32205 is that the acidic solution causes an increase of Cr/Fe ratio in the passive film,and the grain refinement provides more diffusion paths for Cr element and promotes the formation of chromium oxide.The enrichment of chromium oxide and chromium in the passive film leads to a more resistant passive film.