Oxygen reduction on a duplex stainless steel

Corrosion Science\ Vol[ 39\ No[ 5\ pp[ 738Ð759\ 0887Þ 0887 Elsevier Science Ltd[ All rights reserved[\ Pergamon Printed in Great Britain[

9909Ð827X:87 ,*see front matter

PII] S9909Ð827X"87#99993Ð2

OXYGEN REDUCTION ON A DUPLEX STAINLESS STEEL

S[ LJ[ GOJKOVICł\ S[ K[ ZEC EVICł\ M[ D[ OBRADOVICł and D[ M[ DRAZ ICł

Faculty of Technology and Metallurgy\ University of Belgrade\ Karnegijeva 3\ P[O[ Box 383\ 00990 Belgrade\Yugoslavia[

Abstract*O1 reduction on duplex stainless steel was investigated in neutral and alkaline solutions with andwithout the presence of chloride ions[ All polarization curves showed hysteresis with higher reaction rate inpositive going sweep than in the negative one[ Tafel slopes of about −89 mV dec−0 in neutral and alkalinesolutions without chloride ions and about −049 mV dec−0 in NaCl solution were obtained[ Reaction rate is almostindependent on pH in solutions of pH ³ 01 while in more alkaline solutions reaction order with respect to OH−

ions of about −0 was obtained[ Reaction rate and kinetic parameters for O1 reduction on duplex stainless steelwere compared to data for O1 reduction on pure Fe and great similarity was found[ Þ 0887 Elsevier Science Ltd[All rights reserved

Keywords] A[ oxygen\ stainless steel\ B[ reduction

INTRODUCTION

Duplex stainless steel consists of austenite and ferrite as the main phases and some amountof non!metallic inclusions and secondary phases[ Non!metallic inclusions and secondaryphases are mostly located at the main phase boundaries and inside the ferrite[ In comparisonwith single phase stainless steel containing only austenite or ferrite\ the duplex stainlesssteels have a signi_cantly larger surface of the phase boundaries and\ therefore\ lowerdensity of non!metallic inclusions and secondary phases on it[ Hence\ it can be expectedthat the duplex stainless steel has a higher corrosion resistance than austenite or ferritesteels[0

In an earlier work\1 anodic behavior of duplex stainless steel in chloride media wasinvestigated[ As it could be expected\ that material is much more resistive against anodicdissolution than pure iron[2 Since corrosion behavior of most materials is determined byits anodic dissolution rate and rate of cathodic reaction "hydrogen evolution or oxygenreduction#\ the aim of this work was to examine oxygen reduction on duplex stainless steelin neutral and alkaline media with and without chloride ions and to compare these resultswith kinetics of oxygen reduction on iron[

EXPERIMENTAL METHOD

The chemical composition of steel used in the investigation is given in Table 0[Cylinder!shaped specimen "4mm dia# was cut o} from the cast block\ water quenched

Manuscript received 29 August 0885^ in amended form 20 December 0886

738

S[ Lj[ Gojkovic� et al[749

Table 0[ Chemical composition of duplex stainless steel

Element C Si Mn S P Cr Ni Mo Cu

Content:mass) 9[97 9[72 9[62 9[903 9[91 11[4 6[7 1[2 2[4

from 0099>C after 0 h holding time and sealed in a Te~on holder for rotating disk electrode"RDE#[

In some experiments disk made of Fe "Puratronicþ\ 88[884) Fe# was employed[Experiments were carried out in a three!compartment cylindrical glass cell with Pt!wire

spiral as counter and saturated calomel electrode as a reference electrode[ All potentials arereferred to a standard hydrogen electrode "SHE#[

Prior to each experiment the stainless steel disk was mechanically polished with siliconcarbide papers "399 down to 3:9 grit#\ washed with ethanol in an ultrasonic cleaner\ thenpolished with 9[2mm alumina water suspension and _nally rinsed with pure water in theultrasonic cleaner[ By analogy with our earlier experiment with pure iron\3 after mechanicaltreatment electrode was held at the potential from the beginning of H1 evolution "e[g[ atpH02[7 this potential was −0[9V:SHE for 09min in N1 saturated electrolyte#[ That elec!trode surface will be signed as prereduced surface[ However\ while Fe surface is bare afterthat treatment\4 Cr containing stainless steel surface is still covered by a thin layer ofchromium oxide[5 Further pretreatment of the electrode surface was dependent on whetherthe electrolyte contains chloride ions or not[ In solutions without chloride ions\ electrodewas treated potentiostaticaly for 04min in N1 saturated electrolyte at the potential moreanodic than corrosion potential "preanodized surface# while in chloride electrolyte electrodewas left at corrosion potential for 0 h in O1 saturated electrolyte "precorroded surface#[After such electrode treatment and saturation of the electrolyte with O1 "in the case ofelectrolyte without chloride ions#\ polarization curves for O1 reduction were obtained bylinear sweep technique with slow sweep rate of 9[1mV s−0[

In experiments when H1O1 reduction was investigated\ H1O1 stock solution was addedin electrolyte after electrochemical pretreatment and polarization curve in N1 atmospherewas recorded[

The electrolytes used were borate bu}er solutions of pH6[3\ 7[3 and 8[7\ NaOH solutionswith concentrations of 9[909\ 9[09 and 0[9M "pH01[9\ 01[8 and 02[7# and 9[4M NaClsolution[ Solution were prepared with pure water of high resistivity "Q!Millipore# andanalytical!grade chemicals[ Experiments were carried out at constant temperature of 17>C[

EXPERIMENTAL RESULTS

Neutral and alkaline solutions without chloride ionsCyclic voltammo`rams[ Cyclic voltammograms in N1 saturated solutions were re!

corded in order to establish possible changes in electrode surface with potential whichcould interfere with O1 reduction[ Steady state cyclic voltammogram of stainless steel inborate bu}er of pH7[3 is shown in Fig[ 0[ It can be seen that there is no current peaksif anodic potential limit is more negative than 9[6V[ The absence of passivation peakindicates that stainless steel is a self!passivating material which is passive at the opencircuit potential[ At potentials more positive than 9[6V oxidation of alloying elements

Oxygen reduction on a duplex stainless steel 740

Fig[ 0[ Cyclic voltammograms of duplex stainless steel in borate bu}er pH 7[3 with di}erentpositive potential limits[ Sweep rate 49 mV s−0\ rotation rate 899 rpm\ N1 atmosphere[

"possibly Cr# occurs and cathodic current peaks are present in the potential region whereO1\ if present\ would be reduced[ On the basis of that voltammogram potential of 9[44Vwas chosen as a potential for anodic pretreatment which provides reproducible electrodesurface without unnecessary complications because of reduction of oxides of alloyingcomponents[

Cyclic voltammograms in borate bu}ers pH6[3 and 8[7 were similar to that on Fig[0\ but in alkaline solutions pH dependent anodic and cathodic current peaks wereobserved "Fig[ 1#[ In solution of pH02[7 signi_cant current density corresponding tooxide reduction was observed in the potential region where O1 reduction occurred[Potential value for the anodic pretreatment in that solution was 9[04V[

Fig[ 1[ Cyclic voltammograms of duplex stainless steel in NaOH solution of di}erent pH values[Sweep rate 49 mV s−0\ rotation rate 899 rpm\ N1 atmosphere[


Fig[ 2[ Polarization curves for O1 reduction on prereduced and preanodized duplex stainless steelin borate bu}er solution pH 6[3[ Sweep rate 9[1 mV s−0\ rotation rate 899 rpm[

Hysteresis[ Polarization curves at prereduced and preanodized electrode surface inborate bu}er pH6[3 are shown in Fig[ 2[ Signi_cantly lower reaction rate was observedat preanodized surface[ The same phenomenon is characteristic also for the O1 reductionon pure Fe[6 However\ limiting current density on pure Fe was not dependent on theelectrode pretreatment while at duplex stainless steel lower limiting current densitieswere observed at the preanodized surface indicating that lower number of electrons wasexchanged per O1 molecule[ The same dependence of reaction rate and limiting currentdensity on the electrode pretreatment were obtained in all examined solutions[

Tafel plots[ On the basis of polarization curves recorded in solutions pH6[3 to 02[7Tafel plots\ corrected for di}usional e}ects "assuming3 _rst order kinetics with respectto O1# were obtained "Fig[ 3#[ Tafel slopes at kinetic current densities greater than about9[90mAcm−1 are close to −89mVdec−0[ pH dependence of the kinetic current densitiesat constant potential value within Tafel region "Fig[ 4# could be approximated with twolines having slopes of 9 "at pH³ 01# and −0 "at pH× 01# which are simultaneouslyvalues of reaction order with respect to OH− ions[ It has to be stressed that currentdensities at Fig[ 4 are normalized values\ i[e[ they are divided by the concentrations ofdissolved oxygen7\8 to cancel dependence of O1 solubility on concentration of the solution[

It is interesting to compare voltammogram in pH02[7 "Fig[ 1# with Tafel diagramfor O1 reduction in the same solution "Fig[ 3#[ Net cathodic current at the voltammogramoccurs at 9V and that value coincide with beginning of the O1 reduction[ Signi_cantincrease in cathodic current at the voltammogram "at about −9[4V# is close to beginningof the Tafel region for O1 reduction[ Limiting current density is established at −9[7Vwhere _lm is almost reduced[

Tafel plots for O1 reduction in 0M NaOH at duplex stainless steel and Fe\ shownin Fig[ 5\ are almost the same[ pH dependence of the reaction rate is quite similar3

except that change in reaction order with respect to OH− ions occurs at pH¼01 insteadof pH¼00[1 for Fe[

Levich plot[ In order to calculate the number of exchanged electrons per O1 molecule\


Fig[ 3[ Tafel plots for O1 reduction on preanodized duplex stainless steel in solutions of variouspH[ Sweep rate 9[1 mV s−0\ rotation rate 899 rpm[

Fig[ 4[ Dependence of the O1 reduction current on preanodized duplex stainless steel on thesolution pH at −9[449 V[ Data taken from Fig[ 3[

polarization curves at di}erent rotation rates of RDE were recorded in 0M NaOH[Figure 6 shows that the waves are obtained at higher rotation rates of RDE instead oflimiting current density plateau[ Decreasing of limiting current density at the expectedplateau could be explained by the reduction of passive _lm and decreasing of numberof transferred electrons with changes in electrode surface[ When the limiting currentdensities for O1 reduction from the maximums of the waves were plotted against rotationrates of RDE\ Levich plot as shown in Fig[ 7 was obtained[ The limiting current densityfor O1 reduction on Fe are also shown[ Number of exchanged electrons per O1 moleculewas calculated using data7\8 c"O1#�9[66 mol m−2\ D"O1#�0[66 = 09−8 m1 s−0 andn�0[95 = 09−5 m1 s−0 and values of 1[8 and 2[5 were obtained for stainless steel and


Fig[ 5[ Tafel plots for O1 reduction on preanodized duplex stainless steel and Fe in 0[9 M NaOHsolution[ Sweep rate 9[1 mV s−0\ rotation rate 899 rpm[

Fig[ 6[ Polarization curves for O1 reduction on preanodized duplex stainless steel in 0[9 M NaOHsolution at various rotation rate[ Sweep rate 9[1 mV s−0[

iron\ respectively[ Value of 1[8 for the stainless steel is valid for potential region wheredi}usion control of reaction is established[ At more positive potentials where reactionis under mixed activation di}usion control\ number of exchanged electrons could beobtained from the Levich!Koutecky plot[09 However\ reproducibility of polarizationcurves depicted in Fig[ 6 and narrow potential region where reaction is under activation!di}usional control did not allow to determine dependence of current density on rotationrate in that potential region precisely[

Reduction of H1O1[ Number of exchanged electrons per O1 molecule on stainless steelindicates that O1 is reduced via H1O1 as an intermediate which is only partially reduced


Fig[ 7[ Levich plots for O1 reduction on preanodized duplex stainless steel and Fe in 0[9 M NaOHsolution[

to OH! ions[ Polarization curve for H1O1 reduction on stainless steel in N1 atmosphere"Fig[ 8# shows that H1O1 and O1 reduction take place in almost the same potentialregion and that decreasing of the limiting current density value at the expected plateauis also characteristic for H1O1 reduction[ It seems that the electrode surface becomesless active for H1O1 reduction at potentials more negative than −9[64V which leads tonumber of exchanged electrons in O1 reduction lower than 3[

Chloride solutionCyclic voltammo`rams[ The _rst and the steady!state cyclic voltammograms of

stainless steel in 9[4M NaCl are depicted in Fig[ 09[ An anodic peak is observed on the

Fig[ 8[ Polarization curves for O1 and H1O1 reduction on preanodized duplex stainless steel in0[9 M NaOH solution[ Sweep rate 9[1 mV s−0\ rotation rate 899 rpm[


Fig[ 09[ First and steady state cyclic voltammograms of duplex stainless steel in 9[4 M NaClsolution[ Sweep rate 49 mV s−0\ rotation rate 899 rpm\ N1 atmosphere[ Inset] Time dependence ofcorrosion potential of duplex stainless steel in three repeated experiments in 9[4 M NaCl[ Rotation

rate 899 rpm\ O1 atmosphere[

_rst voltammogram but corresponding cathodic peak is not observed till the beginningof H1 evolution[ After cycling the potential\ anodic peak vanishes\ because passive _lmis formed[ Time dependent corrosion potential which value increases "inset in Fig[ 09#shows self!passivation ability of duplex stainless steel in chloride solution[

Hysteresis[ Polarization curves at prereduced surface and surface which was held atcorrosion potential for 0 h indicated that prereduced surface is more active with respectto O1 reduction\ what was established in solutions without Cl− ions\ too[

Tafel plots[ Tafel plots\ corrected for di}usional e}ects for O1 reduction on stainlesssteel are plotted together with corresponding Tafel plots for O1 reduction on Fe "Fig[00#[ The same Tafel slope of about −049mVdec−0 in the same potential region isobtained on stainless steel\ irrespective whether surfaces were prereduced or held atcorrosion potential[ Reaction rates at stainless steel and Fe on prereduced surface arealmost the same\ except that corrosion potential of stainless steel is more positive[ Whenboth electrodes were held on corrosion potential before the experiment\ Tafel slopes atFe were about −009mVdec−0 and corrosion potential was for 9[3V more negative thancorrosion potential of stainless steel[ The di}erence in corrosion potential and Tafelslope values are attributed to passivation of stainless steel at corrosion potential in NaClsolution while Fe does not form passive _lm under these conditions[ O1 reduction onstainless steel takes place on oxide surface while on Fe bare metal surface probably ispresent[

Levich plot[ Limiting current densities for O1 reduction at stainless steel\ taken fromthe polarization curves recorded at the surface which was held at corrosion potentialbefore the experiment\ are plotted against rotation rate of RDE\ together with the datafor Fe "Fig[ 01#[ One could see that di}erence between two sets of data are within theexperimental error[ Using data for concentration of dissolved oxygen in 9[4M NaCl00

c"O1#�9[827molm−2\ di}usion coe.cient01 D"O1#�1[64 = 09−8 m1 s−0 and kinematic


Fig[ 00[ Tafel plots for O1 reduction on prereduced and precorroded duplex stainless steel and Fein 9[4 M NaCl solution[ Sweep rate 9[1 mV s−0\ rotation rate 899 rpm[

Fig[ 01[ Levich plots for O1 reduction on precorroded duplex stainless steel and Fe in 9[4 M NaClsolution[

viscosity02 n�7[50 = 09−6 m1 s−0\ number of exchanged electrons of 2[4 is obtained[Certainly\ this value depends on the validity of literature data of c"O1# and D"O1#[Levich!Koutecky plot were made for stainless steel in potential region of mixed activation!di}usion control and almost the same Levich slopes i[e[ number of exchanged electronsas at limiting current density were obtained[

DISCUSSION

The kinetics of O1 reduction on duplex stainless steel and on pure Fe are very similar[Reaction rate in the positive going sweeps were higher than in the negative going sweeps at


both electrode materials[ These hysteresis are caused by the irreversibility of transformationsin oxide _lms[

The limiting current density at stainless steel is lower than that at Fe which is causedby the lower activity to H1O1 reduction at stainless steel in that potential region[ However\at more positive potentials\ reaction rates at both materials are almost the same[ Tafel slopesof about −89mVdec−0 in solutions without Cl− ions and values of about −049mVdec−0 inchloride solution are characteristics of both stainless steel and Fe[ These value of Tafelslopes are probably in~uenced by the presence of the passive _lm at stainless steel and Fe\because Tafel slope value of −009mVdec−0 is obtained at bare metal surface of pre!corroded Fe[

Present results of O1 reduction on duplex stainless steel ought to be compared with therelated papers from the literature[ Unfortunately\ there is no paper dealing with O1 reductionon duplex stainless steel\ but only at commonly used single phase stainless steel[ Wroblowaand Quaderi03 established that in alkaline solutions O1 reduction on cold! and hot! rolledsteel "SAE ( 0995# is characterized by two well separated waves which indicate thatelectrode surface is not active for H1O1 reduction at potentials about 9[4V more positivethan corrosion potential[ In contrast to this\ duplex stainless steel is almost completelyinactive for O1 reduction in region of about 9[4V negative with respect to corrosionpotential[ Current densities are lower than 09mAcm−1 and are slightly dependent on thepotential[ At potentials where current density for O1 reduction rises\ electrode surface isactive for the H1O1 reduction that is con_rmed by experiments with H1O1 reduction in N1

atmosphere[Babic and Metikos!Hukovic04 studied O1 reduction on AISI 293 stainless steel in chloride

solutions of pH3Ð09[ They observed single!wave polarization curves with limiting currentdensities corresponding to four!electron reduction[ Tafel slopes were between −004 and−079mVdec−0[ Values obtained in the anodic scan direction were always lower than thosein the reverse scan direction[ This is completely di}erent from our results which showedthe same Tafel slopes of about −049mVdec−0 irrespectively of the potential scan direction[On the Fe surface we observed dependence of the Tafel slope on the potential scan directionbut it was opposite to those in the paper of Babic and Metikos!Hukovic^ lower Tafel slopevalues at Fe correspond to cathodic scan direction and higher Tafel slope values to anodicdirection[ However\ the hysteresis in current density values in both papers showed thatreaction is faster in anodic than in cathodic scan direction[

Okuyama and Haruyama05 was dealing with O1 reduction in borate bu}er pH6Ð8 onFe!07mass) Cr alloy[ They observed limiting current density dependent on the pH butnot on the rotation rate[ It is entirely di}erent from the results discussed above[

The di}erent characteristics of O1 reduction presented in di}erent papers could beascribed to the di}erent stainless steel samples[ All the samples were of the similar chemicalcomposition "with Cr and Ni as the main alloying elements#\ but their phase structure wasdi}erent[ The presence of austenite and ferrite as the main phases in the stainless steel usedin the present paper could be responsible for that[ It would be interesting to investigate theO1 reduction and anodic dissolution of the stainless steels of the exactly the same chemicalcomposition but di}erent phase structure to check this hypothesis and to establish whichfactor\ chemical composition or phase structure\ is predominant for the corrosion stabilityof the stainless steel[

The similarity in kinetics of O1 reduction on Fe and stainless steel are not quite expectedhaving in mind results of Ramasubramanian et al[5 and Da Cunta Belo et al[06 that passive


_lm on stainless steel in potential region where O1 is reduced is enriched in Cr in thecomparison with bulk of that material[ Our results could be explained by two assumptions[The _rst is that rates of O1 reduction at passive Fe and Cr are similar to each other[ Thesecond is that Cr oxide in not active for O1 reduction leading reaction occurs at Fe oxideonly[ This assumption explains the similar kinetic parameters for O1 reduction at Fe andstainless steel but somewhat lower current densities for O1 reduction at stainless steel thanat Fe[

Higher corrosion stability of duplex stainless steel in comparison to Fe is probably dueto slower anodic dissolution reaction and more positive pitting potential and it is notin~uenced by the kinetics of cathodic reaction[

CONCLUSIONS

The following conclusions may be drawn from the experimental results]

"0# Reaction rate of O1 reduction on duplex stainless steel is higher on prereducedsurface than on preanodized as well as precorroded[

"1# Tafel slope on preanodized surface in solutions without Cl− ions is about−89mVdec−0[ Reaction rate does not depend on concentration of OH− ions insolutions of pH³01 while in more alkaline solutions reaction order with respect toOH− ions is −0[

"2# Tafel slope in chloride solution is about −049mVdec−0 on both prereduced andprecorroded surface[

"3# Number of exchanged electrons per O1 molecule is about 1[8 in solutions withoutCl− ions and about 2[4 in Cl− containing solutions[

"4# The kinetics of O1 reduction on duplex stainless steel and on pure Fe are verysimilar[ Higher corrosion stability of duplex stainless steel in comparison to Fe isprobably due to slower anodic dissolution reaction and it is not in~uenced by thekinetics of cathodic reaction[

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Oxygen reduction on a duplex stainless steel

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