The Study Of The Corrosion And Passive Behavior Of 2205 Duplex Stainless Steel

Duplex stainless steel,which is composed of body-centered cubic ferrite(a)and face-centered cubic austenite(y),combines the advantages of both ferritic and austenitic stainless steel,and normally contain approximately equal phase fractions of austenite and ferrite phases.Duplex stainless steels are universally employed in oil,petrochemical,paper and marine industries due to their excellent mechanical properties and corrosion resistance.The properties are strictly dependent on the amounts of ferrite and austenite,on their morphology,composition and environment characteristics.The duplex stainless steel would be caused pitting corrosion in aggressive environment.Therefore,the study of the factors that affecting the corrosion resistance of duplex stainless steel is essential for further utilization and improvement of materials.Therefore,the main aim of the paper is to analyze the effects of some factors on corrosion resistance and passive behavior of 2205 duplex stainless steel by in-situ ETEM,Scanning Electron Microscope(SEM),X-Ray Electron Probe Microanalyzer(EPMA),Factsage 7.0 thermodynamics software,Electron Backscattered Diffraction(EBSD),X-ray photoelectron spectroscopy(XPS),Kelvin Probe Force Microscopy(SKPFM),On-line gas mass spectrometry and multiple electrochemical measurements.The main conclusions are as follows:1)The results showed that high annealing temperatures and prolonged isothermal aging time increased the ferrite content and diluted the key alloying elements in the ferrite,lowering the corrosion resistance of the ferrite owing to the transformation of ??? during the solid-solid multiphase interdiffusion annealing process.The pitting corrosion of 2205 in chloride solutions was initiated preferentially in the weaker phase or at the weaker side of the austenite-ferrite boundary.The depth or sizes of the pits that occurred on austenite were smaller than those on ferrite.The pitting potential and the corrosion resistance of the passive film on DSS 2205 were strongly influenced by the solution treatment temperature and isothermal aging time.At a temperature of 1075?,annealing for 2 h resulted in the largest impedance values,revealing the best stability of the film with a superior corrosion resistance.2)The mechanism of phase transformation ??? was investigate by the in-situ ETEM.The nucleation of ferrite phase in the austenite phase was observed.The form of phase transformation mechanism of ??? existed during solution is nuclearation growth.The elements of Cr and Mo migrated from the austenite phase to the boundary layer to cause the formation of the ferrite from the boundary layer.The alloy elements of Ni and N which are austenite formation and stabilizing elements were diffused from ferrite phase acrossed the boundary layer into the austenite phase.Therefore,the mechanism of the phase transformation ??? of the solid solution process is that the alloying elements diffused from the region with higher chemical potential to the region with lower chemical potential,and finally attain the steady state.At the same time,the growth of the ferrite phase along the(110)orientation was observed,and the ferrite and austenite phases grew by a layered form.Finally,the phase boundaries between the ferrite phase and the austenite phase become smooth,and the austenite phase distribut like island on the ferrite phase.3)The ratio of ferrite to austenite phase increases slightly with the content of ferrite stabilizing element Ti.The alloying element Ti has a refinement effect on the microstructure of 2205 duplex stainless steel,and can effectively suppress Cr-carbide,chromium-nitride and ? phase precipitation,which can significantly improve the pitting resistance of the duplex stainless steel and reduce the corrosion current densities.However,excessive Ti will cause coarse TiN precipitation,and the ratio of ferrite phase to austenite phase content will increase significantly and become unbalanced,which were detrimental to pitting resistance of the duplex stainless steel.The suitable content is 0.010 wt.%.The element of Ti was not found in the passive film of the duplex stainless steel with the addition of Ti,but the chromium in the oxides layer was enriched,the proportion of Cr2O3 was obviously increased,and the content of hexavalent molybdenum Mo6+ in the passive film was increased.As a result,both the donor and acceptor densities of the film are reduced,and the charge transfer resistance Rp at the interface of film/electrolyte is increased,which effectively improve the stability of the passive films on the duplex stainless steel and the corrosion resistance of the stainless steel.In addition,comparing the addition of the same content of Ti and Nb,it is found that both Ti and Nb can improve the pitting resistance of stainless steel,but the beneficial effect of Ti is better than that of Nb.4)The oxide inclusions in duplex stainless steel can cause pitting corrosion due to the heterogeneity with steel matrix.The O-Mn-Cr-Si-Ca inclusions in duplex stainless steel are divided into two parts,the outer part is rich in Cr.The hardness of the outer portion is higher than steel matrix but surface Volta potential is slightly lower than matrix around the inclusion.The surface potential of the inner Ca-rich and Si-rich portion is significantly lower than matrix and the outer portion.In the aggressive environment,the center of the inclusions with low surface potential becomes the active region,which preferentially dissolves and initiates pitting initiation.Under the same conditions,the Ce modified Ce-oxides inclusions became uniform,and the surface potential was higher,the activity was lower than the matrix.And the stability was improved,so as the corrosion resistance of the duplex stainless steel.Only when the applied potential is further increased or the environment become worse,the matrix at the interface between the inclusion and the stainless steel will dissolved,causing the initiation of pitting.O-Al-Ca-Si inclusions have a lower surface potential in the austenite phase and dissolve at lower potentials in the aggressive solution,while the surface potential of the inclusions in the ferrite phase is higher than that of the matrix.As a result,the corrosion behavior in the austenite phase mainly begins to dissolve from the oxide inclusions and propagates to the interface between the inclusions and the matrix,eventually leading to steady-state pitting of the stainless steel.The inclusions in the ferrite phase are relatively stable than the matrix,and the inclusions are preferentially dissolved at the interface between the inclusions and the ferrite.5)The effect of S2O32-on the corrosion behavior of 2205 duplex stainless steel in Cl-solution was studied.The mechanism of the effect of S2O32-on the pitting corrosion of stainless steel in Cl-solution was described.The results show that the addition of S2O32-reduces the pitting potential of duplex stainless steel in Cl-solution,and the concentration of S2O32-has a significant effect on the corrosion behavior of stainless steel.The selective dissolution of the ferrite phase is observed.The detrimental effect of S2O32-on the passive film depends on the anode potential.At lower potentials,Cl-competes with S2O32-,while at higher potentials,Cl-and S2O32-plays a synergistic effect on pitting corrosion,and S2O32-promotes the generation of steady-state pitting.Passivae films on duplex stainless steel surface in Cl--S2O32-solution exhibits n-type and p-type semiconductor properties,and effect the concentration of S2O32-on the donor and acceptor densities of passive film which affect the corrosion resistance in different potential show different behaviors.At high potential,S2O32-accelerates the dissolution of the passive film and promotes the oxidation of Cr(?)to Cr(?),resulting in a decrease in the Cr2O3 content in the film and a deterioration effect.The S2O32-is reduced to H2S during the dissolution of stainless steel at high anode potential to catalyze the dissolution of the duplex stainless steel.