Materials Science and Engineering A 372(2004)
180–185
Micro-electrochemical polarization study
on 25%Cr duplex stainless steel
Chan-Jin Park a ,∗,Hyuk-Sang Kwon a ,M.M.Lohrengel b
a
Department of Material Science and Engineering,Korea Advanced Institute of Science and Technology (KAIST),
373-1Guseong-dong,Yuseong-gu,Daejeon 305701,South Korea
b
Institute for Physical Chemistry and Electrochemistry,Heinrich-Heine University Duesseldorf,Duesseldorf 40225,Germany
Received 5September 2003;received in revised form 3December 2003
degradeAbstract
Micro-electrochemical characteristics of 25%Cr duplex stainless steel (DSS)were investigated by a new electrochemical technique,a micro-droplet cell.Anodic polarization tests were carried out in pH 5.6acetate buffer solution for annealed DSS solution,for the alloy aged at 850◦C,and for the alloy aged at 475◦C.The anodic peak current density for the ferrite (␣)phase was measured to be higher at slightly lower potential compared with that for austenite (␣)phase in solution annealed DSS,suggesting that ␣was electrochemically more active than ␥,which has a lower Ni content.The polarization curve for the region where the sigma ()phase coexisted with secondary austenite (␥2)phase,(+␥2),in the alloy aged at 850◦C showed two anodic current peaks,which possibly corresponded to those for ␥2and ,respectively.The anodic peak current density and passive current density for the region of (+␥2)in the alloy aged at 850◦C were higher,and the current scattering corresponding to the initiation of metastable pitting corrosion was more evident than those for ␣and ␥in the solution annealed alloy.This indicates that the (+␥2)region was probably electrochemically active,and thus sensitive to active dissolution.The ␥2phase containing lower contents of N,Cr,and Mo compared with the primary austenite (␥)phase as well as the Cr-depleted zone around the phase appeared to be the main cause of the degradation of the stability of the pas
sive film.An anodic current peak was also observed in the polarization curve for the region that had experienced spinodal decomposition of ferrite phase to Fe-rich ␣phase and Cr-rich ␣ phase (␣+␣ ).The anodic current peak for the region of (␣+␣ )in the alloy aged at 475◦C was found at lower potential and the peak current density was higher than those for primary ␣and ␥in a solution annealed alloy.The precipitation of ␣ phase induced the formation of a microscale Cr-depleted region,which is active and hence degrades the stability of passive film.©2004Elsevier B.V .All rights reserved.
Keywords:Duplex stainless steel;Micro-droplet cell;Sigma ()phase;Alpha prime (␣ )phase;Passive film
1.Introduction
Characterization of the passive film on a metal surface is traditionally in the domain of electrochemical methods.Because of the low lateral resolution of electrochemical stan-dard methods,however,only macroscopic averaged proper-ties could be obtained.Therefore,important texture or phase depending characteristics of polycrystalline material have rarely been investigated.Except for single crystals,which are specially controlled during solidification,most metals
∗
Corresponding author.Present address:Stainless Steel Research Group,Technical Research Laboratories,POSCO 1,Goedong-dong,Nam-gu,Pohang 790785,Gyeongbuk,South Korea.Tel.:+82-54-220-7833;fax:+82-54-220-6915.
E-mail address:kr (C.-J.Park).consist of many grains and,sometimes,two or three phases with various crystallographic orientations or phase compo-sitions.Accordingly,the macroscopic electrochemical be-havior is simply the sum of the contributions of the different grains or phases,multiplied by their individual degree of coverage.Sometimes,some special grains or phases showing completely different properties compared with others may dominate the electrochemical response of a macroscopic sample.
A new electrochemical device,the capillary-based droplet cell [1,2],as shown in Fig.1,provides facilities for micro-electrochemical investigations at high resolution.Small electrolyte droplets are positioned on the sample sur-face,enabling a spatially resolved surface analysis or modi-fication.The small area of the working electrode determined by the tip size of a capillary with a diameter of 20–600m
0921-5093/$–see front matter ©2004Elsevier B.V .All rights reserved.doi:10.1016/j.msea.2003.12.013
C.-J.Park et al./Materials Science and Engineering A372(2004)180–185
181
Fig.1.Typical experimental setup of a micro-droplet cell;the droplet cell contains a micro-reference electrode and a thin gold wire as a counter electrode.
enables the investigation of localized corrosion or passiva-tion of small areas within a single grain or phase.In addi-tion,the device has a smaller resolution than some special probe STM),but enables a complete range of potentio-static(or dynamic)and galvano-static(or dy-namic)techniques including impedance spectroscopy.This is accomplished by providing a convenient three-electrode system consisting of the working electrode of interest,a thin gold wire as a counter electrode(CE),and a micro-reference electrode(RE).This method has already been applied to the single grains of polycrystalline materials such as Al,Ta,Nb, Hf[3],and Zn[2],and some evidence of texture-depending characteristic of grains were found in those materials.
In contrast to pure metals,alloys have rarely been in-vestigated using the micro-droplet cell because of their complexity in structure and chemical composition.Duplex stainless steel(DSS)is potentially a good candidate for micro-electrochemical investigations with the droplet cell, because this alloy is composed of an approximately equal volume fraction of ferrite(␣)and austenite(␥)phase,and, someti
mes,secondary phases such as sigma()or alpha prime(␣ )phase[4–6].The excellent resistance to stress corrosion cracking(SCC)of this alloy is attributed to the synergistic effect of␣and␥[7,8].In addition,secondary phases in DSS are known to degrade the corrosion resis-tance of the alloy due to nearby formation of Cr and/or Mo
depleted zones[4–6].
Thus far,single phase alloys with similar chemical com-
positions and crystal structures to those of␣,␥orin DSS
have been used to investigate the electrochemical behaviors
of each phase.However,these single phase alloys can con-
tain grain boundaries and inclusions,and such defects may
unexpectedly affect the experimental results.Therefore,the
optimal method is to directly measure the electrochemical
properties of a single phase or local area containingfine
precipitates while excluding the effects of other phases or
defects in DSS;the micro-droplet cell enables such an ap-
proach.
The objective of the present work is to elucidate the in-
dividual electrochemical characteristics of␣,␥,and other
secondary phases of duplex stainless steel using the droplet
cell.Solution annealed DSS is basically composed of␣and ␥,where␣is relatively richer in Cr and Mo but poorer in Ni and N than␥.This difference in chemical compositions
between␣and␥can make a difference in their electrochem-
ical behaviors.Generally,␣is considered to be more ac-
tive than␥,which has a lower Ni content.With an aging at
850◦C,the ferrite phase in DSS is decomposed tophase
and secondary austenite(␥2)phase.Thephase has very
high contents of Cr and Mo and induces the depletion of
Cr and Mo in the region around the phase and the resultant
degradation of the stability of the passivefilm.In addition,
for the DSS aged for a long of time at475◦C,the alloy
undergoes the spinodal decomposition of ferrite phase into
a Cr-rich␣ and Fe-rich␣phase.This also can lead to the
degradation of passivefilm.The present work,cover these
influences of each phase in DSS on the electrochemical be-
haviors and the stability of passivefilm of the alloy.
2.Experimental
The duplex stainless steel used in the present work was
melted in a30kW induction furnace.The alloy was cast
in shell molds,and cylindrical castings with a diameter of
27mm and height of130mm were obtained.Table1shows
the chemical composition of the casting.The castings were
homogenized for2h at1250◦C and then solution annealed
for1h at1050◦C.Aging treatment for the specimens was
additionally performed for10h at850◦C and for300h at
475◦C,respectively.The specimens were polished up to
Table1
Chemical composition of the duplex stainless steel casting
Cr24.37 Ni 6.76 Mo 1.50 W 3.23 Mn 1.19 Si0.62 N0.26
182 C.-J.Park et al./Materials Science and Engineering A 372(2004)180–185
1m surface finish,and some specimens were then etched at 5V in 10%KOH solution for metallographic observations.For polarization tests,polished specimens without etching were used,since the chemical composition or roughness of the surface can be changed by the selective dissolution of alloying elements during the etching process.However,␣and ␥could be easily distinguished with optical microscopy,because the hardness of each phase was different;thus re-sulti
ng in small differences in height at phase boundaries during mechanical polishing with diamond suspension.Potentiodynamic anodic polarization tests were carried out in a pH 5.6acetate buffer solution and in a 0.5M H 2SO 4+0.01M KSCN solution using the droplet cell.A capillary with an inner diameter of 30m (area:7.1×10−6cm 2)was used for the measurement.The tip size of the capillary could be estimated from the plateau current in cyclic voltammograms on pure aluminum (Al)in a pH 6ac-etate buffer solution at a scan rate of 100mV/s.A plateau current of 250A corresponds to an specimen exposure area of 1cm 2.Thus,the diameter of the capillary made for the ex-periment in the present work was obtained by measuring the plateau current in the cyclic voltammogram on a Al sample using the droplet cell equipped with the capillary and con-verting the measured plateau current to the capillary area.
3.Results
3.1.Investigation of solution annealed alloy
Fig.2shows an optical micrograph of a solution annealed alloy etched in 10%KOH solution.The dark phase indicates ferrite (␣)and the bright phase austenite (␥)in this figure.The volume fraction of ␣and ␥was approximately 50:50,and the grain size was large enough for measurement with a capillary with a diameter of 30
m.
Fig.2.Optical micrograph of solution annealed alloy etched in 10%KOH solution.
1E-8
1E-7
1E-6
1E-5
1E-4
P o t e n t i a l [V v s H g /H g O ]
Current density [A/cm 2
]
Fig.3.Polarization curves of austenite (␥)and ferrite (␣)phase in pH 5.6acetate buffer solution.
Fig.3shows the individual polarization curves for ␣and ␥in the pH 5.6acetate buffer solution.In the be
ginning of po-larization,the current values were scattered.Metastable pit-ting corrosion on surface defects such as micropores formed during solidification or inherent inclusions may have been the cause of this scattering.An anodic current peak was observed near 500mV with increasing potential for both phases.This peak may be attributed to the active dissolution of alloying elements.For the ferrite (␣)phase,the anodic current peak was found at lower potential and the peak cur-rent density was higher than that for the austenite (␥)phase.The lower corrosion potential of ␣compared with that of ␥also confirmed that ␣was electrochemically more active.These results arise from differences in the chemical compo-sitions of each alloy;␣is relatively rich in Cr and Mo while ␥is rich in Ni and N [5,9].3.2.Investigation of σand γ2
Precipitates with a size less than 10m are too small for micro-electrochemical measurements using a droplet cell with a capillary diameter of 30m and can only be mea-sured with the surroundings;for example ␣ phase with the surrounding ferrite matrix and phase with the adjacent eu-tectoid secondary austenite (␥2).
Fig.4shows an optical micrograph of an alloy aged for 10h at 850◦C.Ferrite (␣)was fully decomposed to and ␥2after the aging.The dark phase indicates and the small bright phase enclosed by phase is ␥2in the figure.
Polarization tests on the region where and ␥2coexisted (+␥2)were conducted in a pH 5.6acetate buffer solution using the droplet cell,and the results are shown in Fig.5.The polarization curves for ␣and ␥in the solution annealed alloy are also presented in Fig.5for comparison.In partic-ular,the polarization curve for (+␥2)showed two anodic current peaks,which appeared to be associated with the ac-tive dissolution of ␥2and ,respectively.For (+␥2),the
C.-J.Park et al./Materials Science and Engineering A 372(2004)180–185
183
Fig.4.Optical micrograph of an alloy aged for 10h at 850◦C,
etched in
10%KOH solution.
anodic peak current density and passive current density was higher,and the current scattering in the passive range was more evident than those for ␣and ␥in the solution annealed alloy.It has been established that secondary austenite (␥2)is poorer in N,Cr,and Mo compared with primary austenite,since it originates from the primary ferrite phase and Cr and Mo concentrate in neighboring phase [4].Therefore,␥2and Cr-depleted zones around the phase can degrade the stability of a pa
ssive film.The cyclic voltammograms on ␥and (+␥2)regions in 0.5M H 2SO 4+0.01M KSCN so-lution (Fig.6)confirmed that the (+␥2)region was very susceptible to active dissolution.3.3.Investigation of (α+α )
␣ Phase can form as a result of spinodal decomposition of ferrite to Fe-rich ␣and Cr-rich ␣ phases,and such phases
1E-8
1E-7
1E-6
1E-5
1E-4
P o t e n t i a l [V v s H g /H g O ]
Current density [A/cm 2
]
Fig.5.Polarization curve for the region where and ␥2coexist (+␥2)in an alloy aged for 10h at 850◦C compared with that for ␣and ␥in the solution annealed alloy,in pH 5.6acetate buffer solution.
-6
-4-20246810
C u r r e n t d e n s i t y [m A c m -2
]
Potential [V vs Hg/HgSO 4/0.5M H 2SO 4]
Fig. 6.Cyclic voltammograms of ␥and (+␥2)region in 0.5M H 2SO 4+0.01M KSCN solution.
have a very fine size of about 10nm as shown in Fig.7.The symbol ␣in this case may be confused with ␣indicat-ing a primary ferrite phase in other sections.However,␣is currently used in both cases in the literature,thus we have not changed the notation.Polarization tests were conducted on the region where the ferrite phase was decomposed into ␣and ␣ (␣+␣ ).Fig.8shows the polarization curve for (␣+␣ )in an alloy aged for 300h at 475◦C,compared with that for primary ␣and ␥in a solution annealed alloy.An anodic current peak was also observed in the polarization curve for (␣+␣ ).In addition,for the (␣+␣ )region,the anodic current peak was found at a lower potential,and the anodic peak current density and passive current density were higher than those for primary ␣and ␥.The lower corrosion potential of (␣+␣ )region compared with than that of pri-mary ␣and ␥also confirms that the (␣+␣ )region is elec-trochemically active.The precipitation of ␣ phase induces the formation of a Cr-depleted,Fe-rich region (␣)around it,and this region degrades the stability of passive film [6].
4.Discussion
Thus far,some efforts [8,10,11]have made to investigate the individual electrochemical behaviors of each phase in DSS.In the early stage of the research,single phase alloys with similar chemical comp
ositions and crystal structure to those of ␣and ␥or in DSS was used to estimate the elec-trochemical characteristics of each phase.However,these single phase alloys contain grain boundaries and inclusions,and such defects may unexpectedly affect the experimental results.Especially,phase or grain boundary effects should be taken into account.Perren et al.[11]found the superpo-sition of the two polarization curves of the single phases (␣and ␥)in the active range to a new polarization curve of the phase boundary region,which contains a significant frac-tion of each phase,in their study using their own micro-cell.
184 C.-J.Park et al./Materials Science and Engineering A 372(2004)
180–185
Fig.7.TEM micrographs of an alloy aged for 300h at 475◦C,(a)␥+(␣+␣ )and (b)(␣+␣ ).
1E-8
1E-7
1E-6
1E-5
1E-4
P o t e n t i a l [V v s H g /H g O ]
Current density [A/cm 2
]
Fig.8.Polarization curve for the region where ferrite underwent spinodal decomposition to Fe-rich ␣and Cr-rich ␣ in an alloy aged for 300h at 475◦C,compared with that for ␣and ␥in the solution annealed alloy,in pH 5.6acetate buffer solution.
Higher fractions of one single phase in the measured area reduced the relative contribution of the other phase to the active region.In addition,it is not usual that two activation peaks corresponding to ␣and ␥,respectively,are obtained in the polarization curve on the solution annealed DSS in the works using traditional three-electrode system with a working electrode area of some cm 2.However,different ac-tivation peak position and peak current density for ␣and ␥were clearly observed in the present work.
5.Conclusions
1.In the polarization curve for ferrite phase,an anodic cur-rent peak was observed at lower potential and the anodic peak current density was higher compared with those in the curve for the austenite p
hase in solution annealed du-plex stainless steel,suggesting that ferrite was electro-chemically more active than austenite,which has a lower Ni content.
2.The polarization curve for the region where and ␥2
coexisted (+␥2)in the alloy aged at 850◦C showed two anodic current peaks,which appeared to be associated with the active dissolution of ␥2and ,respectively.For (+␥2),the anodic peak current density and passive current density were higher,and the current scattering in the passive range was more evident than those for primary ␣and ␥in the solution annealed alloy,indicating that the (+␥2)region was electrochemically active,and thus sensitive to active dissolution.
3.An anodic current peak was also observed in the polariza-tion curve for the region where ferrite was decomposed to Fe-rich ␣and Cr-rich ␣ (␣+␣ )in the alloy aged at 475◦C.In addition,for the (␣+␣ )region,the anodic peak was found at a lower potential and the anodic peak current density and passive current density were higher than those for primary ␣and ␥.The precipitation of ␣ phase induces the formation of a Cr-depleted,Fe-rich re-gion,and this area is not only susceptible to active disso-lution,but also degrades the stability of the passive film.
References
[1]M.M.Lohrengel,A.Moehring,M.Pilaski,Frescenius J.Anal.Chem.
367(2000)334.
[2]C.J.Park,M.M.Lohrengel,T.Hamelmann,M.Pilaski,H.S.Kwon,
Electrochim.Acta 47(2002)3395.
[3]A.Moehring,M.M.Lohrengel,in:A.B.Ives,J.L.Luo,J.Rodda
(Eds.),Proceedings of the Eighth International Symposium on Pas-sivity of Metals and Semiconductors,vol.PV 99–42,The Electro-chemical Society,Pennington,NJ,2000,p.114.[4]J.O.Nilsson,Mater.Sci.Tech.8(1992)685.[5]J.S.Kim,H.S.Kwon,Corrosion 55(1999)512.[6]C.J.Park,H.S.Kwon,Corros.Sci.44(2002)2817.
[7]H.D.Solomon,T.M.Devine Jr.,in:Proceedings of the Conference
on Duplex Stainless Steels,ASM,Ohio,1983,p.693.
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