| To address resource saving, high quality, and good corrosion resistancerequirements, duplex stainless steels have been developed in the past decades,particularly lean duplex stainless steels，which are available at a relatively lowcost because of the addition of low amounts of Ni in comparison with austenitestainless steels and other duplex stainless steels. LDX, which are characterizedby ferrite () and austenite (γ) phases in almost equal volume fraction, presentdesirable mechanical properties and corrosion resistance as structural materialsin extensive fields, because of their advantageous combination of highmechanical strength and good corrosion resistance in various aggressiveenvironments，and in the future it is substitute for304austenitic steel. However,under the condition of the actual working condition, such as the artifacts withelimination stress annealing treatment, servicing at high temperature for a longtime, even slow cooling from high temperature, as well as the improper handlingof the welding heat affected zone, hot working, etc., the device will be oftenbetween300℃and1200℃. At the same time, precipitate phase in phase boundary or grain boundary, phase ratio, austenitic dissolve and precipitate,which make microstructure change, and uneven distribution of alloy elements intwo phases. Evently it leads to corrosion resistance of two phase different, whichaffect the corrosion resistance of materials.In this paper, in order to obtain the required microstructure, the S32101were heat treated from300℃to1200℃. Then we have electrochemicalcorrosion in Cl-and H+medium: potentiodynamic scanning, double looppotentiodynamic electrochemical reactivation (DL-EPR) and electrochemicalimpedance spectroscopy (EIS). Finally we use first principles to explain theselective dissolution. The results show that:The S32101samples were aged for2h between300℃and900℃andthen water quenching. From the point of metallographic microstructure，eutectoid transformation and produce precipitation, which were carbon nitrideand secondary austenite γ2. As the aging temperature from300℃heating up,microhardness increased parabolicly; and the corrosion resistance presented aparabolic downward trend, passivation gradually became poor. Whentemperature arrived700℃, the corrosion resistance was worst. Thentemperatures continued to rise to900℃, microhardness slightly decreased;corrosion resistance increased with a parabolic trend. After the aging treatmentof700℃for different times, as the aging time prolonged, the precipitation wereprecipitated in the/γ phase boundary or intracrystalline and then graduallygrow up and diffused to the phase, precipitation were becoming more and more. Microhardness increased linearly. The corrosion resistance declinedlinearly, and the density of passivation film was becoming worse. Not onlyphase was corroded, and grain boundary of γ phase has also been significantlycoarse.The samples were annealed for1h between950℃and1200℃and thenwater quenching. From the point of metallographic microstructure, the change ismainly the change of the relative amounts of and γ. And with the redistributionof all alloying elements in each phase, therefore it will impact on the corrosionresistance. Cl-corrosive environment: from900℃to1200℃, pitting werefirstly occurred in/γ phase interface, and gradually expand to the phase, sophase eventually were corroded. Specificly to see various corrosion degree weredifferent with temperatures. from900℃to1050℃, change of phase ratioplayed a leading role: with temperatures rised, austenite gradually dissolved, andcorrosion resistance were gradually improved, the nucleation of pitting werepriorityly took place in the/γ phase boundary and then expanded to phase,until γ phase was terminated. When the temperature over1100℃, the phase ratioand austenite transformation affect the corrosion resistance: at1100℃, due tothe dissolution of austenitic, the local ferrite area existed some very small size ofthe thin strip austenitic distribution on the ferrite grain boundaries. And pittingwas nucleated in/γ phase boundary, the degree of corrosion was most serious.Temperatures continue to rise, the small size of the thin strip austenite willcontinue even dissolve completely. When the temperature is above1150℃, because of the slow cold speed, grain boundary in phase would precipitate tinyacicular or irregular shape of secondary austenite γ2. Pitting was nucleated at the/γ2phase boundary and continued to diffusion, and small γ2was firstlycorroded, and then was corroded, but γ phase without any change. H+corrosive environment: the corrosion occurred in the/γ phase boundary andor γ grain boundary.From the atomic level building Fe-Cr-Ni, Fe-Cr-Mn structure model, fccand bcc of two structures are the thermodynamic stability structure. From thestate density at Ef: for Fe10Cr4NiMn(2101), the DOS at Efof the γ with fcc arelower than that of, illustrating electrochemical activity of γ is lower than thatof, that is to say, corrosion resistance of γ is higher than that of, thereforewill be corrosded. For Fe9Cr4Ni2Mo(2205), the DOS at Efof the γ with fcc arehigher than that of, illustrating electrochemical activity of γ is higher than thatof, that is to say, corrosion resistance of γ is lower than that of, therefore γwill be corrosded. From the energy difference of two phase of the state densityat Efof2101and2205, respectively, it is17.5electrons/eV,0.8electrons/eV, theenergy difference of2101is higher than that of2205. Thus at same conditions,the corrosion resistance of2205is much higher than2101. System after joiningN and Mo, state density at Efreduce, illustrating electrochemical activityweakened, and the corrosion resistance is improved. So adding Mo, N or elsealloying element that can improve the solubility of N, increase the corrosionresistance of the material. |
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