Stress Corrosion Cracking Behavior And Mechanism Of 2205 Duplex Stainless Steel And The Heat Affected Zone

This work is focused on the stress corrosion cracking(SCC)behavior and mechanism of 2205 duplex stainless steel(DSS)and the weld heat affected zone(HAZ).Effects of acidification,prior hydrogen charging and polarization on passivation and SCC of 2205 DSS were studied.In addition,characteristics of the possible microstructure of HAZ were analyzed,and the specific researches on hightemperature HAZ(HTHAZ)and low-temperature HAZ(LTHAZ)were carried out for uncovering the microstructure evolution and SCC.The SCC behavior and mechanism of 2205 DSS and the HAZ were fully understood.Based on the above studies,a method of online monitoring the SCC early stage was proposed based on the corrosion big data technology.The main conclusions are as follows.SCC of 2205 DSS in acidified simulated seawater is dominated by localized anodic dissolution(AD).Hydrogen charging deteriorates passive film stability,and leads to AD and hydrogen damage;as a result,SCC is controlled by hydrogen facilitated anodic dissolution(HFAD)and hydrogen embrittlement(HE)in hydrogen pre-charging conditions.With hydrogen pre-charging,ferrite acts as preferential SCC initiation sites.In the SCC early stage,SCC tends to initiate and propagate in ferrite,while austenite blocks the crack growth.In the SCC late stage,austenite can also be penetrated.The variation of loading directions results in differences of stress and strain partitioning during SCC propagation,which leads to the diversity in hydrogen distribution and effects.When loaded along rolling direction(RD),austenite undertakes more plastic deformation than ferrite,and can capture more hydrogen;the hardening effect of hydrogen embrittles austenite.When loaded along transverse direction(TD),ferrite shares more plastic deformation and is hydrogen enriched;hydrogen enhances localized plastic deformation(HELP)in ferrite,which facilitates SCC propagation.SCC mechanism under various cathodic potentials can be classified into three types including AD,AD+HE and HE.A quantitative correlation of SCC susceptibility with the slow/fast-scan polarization current was established,which is applicable for assessing the SCC susceptibility with various applied potentials.Under applied potential from-450mV to-550mV,SCC is controlled by AD,and SCC susceptibility is low.When the applied potential is more negative than-650mV,hydrogen effects gradually stand out,which deteriorates passive film stability,inhibits repassivation and increases SCC susceptibility drastically.In particular,extremely serious pitting corrosion happens at the applied potential of-850mV due to the synergy of AD and HE.The pit and SCC initiation sites changes with various applied potentials.At open circuit potential(OCP),pits and SCC are prone to initiate in ferrite and phase boundaries;while with the negative shift of applied potential,the tendency of the initiation in austenite increases as a consequence of hydrogen enrichment in austenite with continuous dynamic hydrogen charging.HTHAZ shows the highest corrosion tendency among different locations in HAZ of 2205 DSS.The intrinsic reason is the variation in percentage of the two phases and the partitioning of constitutional elements.The higher fraction of ferrite lowers the content of Cr and Mo,and decreases the pitting resistance equivalent number(PREN).As a result,ferrite is vulnerable to pitting and preferred corrosion,which induces SCC initiation.In addition,coarse-grained ferrite and the high fraction promote SCC propagation.Furthermore,the change of the percentage of the two phases and the element partitioning increases the nitrogen content in austenite and causes excessive solution strengthening;when the strengthening effect reaches to some extent,the strength of austenite exceeds that of ferrite,which alters the stress and strain partitioning of the two phases and may influence the micro deformation mechanism.The harmful effect of low-temperature heat treatment on microstructure mainly lies in the detrimental precipitates,which accelerates hydrogen adsorption,deteriorates the passive film and promotes SCC initiation.Isothermal aging at 650℃facilitates Cr2N formation on phase boundaries and ferrite grain boundaries,which induces intergranular corrosion and minor crack branches along the boundaries.Isothermal aging at 850℃ leads to σ precipitation,where internal cracks are prone to initiate.Heat treatment at 1000℃ leads to the homogenization of alloying elements and the two phases without any precipitates;both the diffusible hydrogen content and the defect density in passive film decrease;therefore,isothermal aging at 1000℃ improves resistance in corrosion and SCC.The method of online monitoring the SCC early stage was proposed based on the corrosion big data.It is proved that this method can monitor the early stages of corrosion/SCC and assess the susceptibility of 2205 DSS.Values of instantons current can indicate the dynamic corrosion behavior evolution of 2205 DSS corrosion sensor with different microstructure.The energy distribution plots(EDP)based on wavelet analysis of corrosion current signals can identify the corrosion/SCC stages and degree,and the criterion for identifying different corrosion/SCC stages was given.The accumulative corrosion quantity reflects accumulative effects of corrosion,and can be used for assessing susceptibility of corrosion and SCC.