Study On Microstructure And Corrosion Behavior Of Sensitized Duplex Stainless Steel

Duplex stainless steel?DSS?has two-phase structure of ferrite???and austenite???.Therefore,it shows good mechanical properties and excellent corrosion resistance,which was widely used in industrial production.The?phase can not only improve the stress corrosion resistance of the material,but also reduce the cracks produced in the welding process.In addition,?phase can also improve the strength of the material.However,?phase also have a lot of negative effects on DSS.When sensitized at high temperature,the?phase will precipitate,which greatly reduces the corrosion resistance of the material.When DSS works at high temperature and with aggressive ions,some harmful chromium rich precipitates will inevitably be produced in the?/?phase boundary,such as carbonitride,?phase,?phase and?₂ phase.These precipitates will destroy the structure of?:?=1:1,resulting in chromium depletion areas,which are easily attacked by aggressive ions.Thus,inducing pitting and intergranular corrosion,and seriously reducing the corrosion resistance and mechanical properties of the material.As we all know,pitting corrosion and intergranular corrosion are not only the most common corrosion in DSS,but also the most destructive corrosion.Therefore,it is necessary to study the pitting and intergranular corrosion of DSS,which was very important for the research and development of new stainless steel.Based on the above background,the microstructure and local corrosion behavior of economical duplex stainless steel LDX 2404 and super duplex stainless steel UNS S32750were studied in this paper.Using XRD,SEM,TEM,CPT,DL-EPR to study the intergranular corrosion behavior of LDX 2404 sensitized at 800?for various time.In addition,the microstructure and local corrosion behavior of UNS S32750 after different time of sensitization at 900?were studied.The main research content is as follow:1)The intergranular corrosion sensitivity of LDX 2404 was evaluated by double ring electrochemical potentiodynamic reactivity?DL-EPR?method,and the conditions for testing intergranular corrosion were optimized.The relationship between the precipitates and IGC sensitivity of LDX 2404 sensitized at 800?for different time was studied by optimized DL-EPR method.The results showed that the optimal conditions of DL-EPR were:2M H₂SO₄+0.75M HCl,scanning rate:1.5 mv/s,solution temperature:30?.The Ra value increases with the extend of aging time and reaches the maximum at 100 h.The increase of Ra is due to the formation of chromium rich precipitates,which lead to the increase of chromium deficiency around them.Moreover,because the formation of chromium rich precipitates is more important than the diffusion of Cr and Mo from adjacent matrix to chromium poor area,the grain boundary corrosion sensitivity shows no sign of recovery.2)The critical pitting temperature?CPT?method was used to study the relationship between the pitting properties and the precipitates sensitized by UNS S32750 at 900?for different times.The microstructure analysis shows that with the increase of aging time,the volume fraction of?phase decreases and the volume fraction of precipitates increases.The main precipitates are?and Cr₂N phases,the precipitation mechanism is???+?₂ and??Cr₂N+?₂.The values of CPT are closely related to the microstructure.With the increase of aging time,CPT value first decreases and then increases,the pitting resistance first decreases and then recovers.When the aging time extended from 10 h to 100 h,most of?phase has changed,so the pitting resistance appears obvious repair phenomenon.The main mechanism is the supplement of Cr to the chromium poor area,which improves the pitting resistance and slightly increases the CPT value.3)The relationship between the precipitates and intergranular corrosion of UNS S32750 sensitized at 900?for different time was studied by double ring electrochemical potentiodynamic reactivation?DL-EPR?method.The results show that the DOS value is closely related to the microstructure.With the increase of sensitization time,the Ra value increases,there is no healing and the degree of intergranular corrosion increases all the time.This is because DOS evaluation technology is not only affected by the weakest chromium poor area,but also affected by the whole surface structure of the specimen.