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ACKNOWLEDGMENT

The research has been sponsored by the German Ministry ofResearch and Technology within the German-Norwegian jointproject on erosion-corrosion in multiphase systems and within thenational research and development program Corrosion and Corro-sion Protection. The excellent cooperation with the partners, espe-cially with F. Durst, LSTM, Univ. Erlangen-Nurnberg, West Ger-many, of this project is gratefully acknowledged.

The authors thank the following firms that contributed equip-ment material and financial support to the project: KSB, Franken-thal, Hoechst AG, Frankfurt, Mannesmann Rohrenwerke AG, Dus-seldorf (FRG).

REFERENCES

1. U. Lotz, E. Heitz, Werkst. und Korr. 34(1983): p. 45.2. B.T. Ellison, C.J. Wen, AICHE Symp., Series no. 204, 77(1981): p. 161.

3. Z. Zemnura, Corr, Sci. 8(1968): p. 703.4. K.D. Efird, Corrosion 33(1977): p. 3.5. E. Heitz, G. Kreysa, C. Loss, J. Appl. Elektrochem. 9(1979): p. 243.6. B.K. Mahato, L.y. Cha, L.H. Shemilt, Corr. Sci. 20(1980'): p. 421.7. 8. Poulson, Corr. Sci. 23(1983): p. 391.

8. J. Postlethwaite, Corrosion 42(1986): p. 514.9. T. Kohley, E. Heitz, in "The Use of Synthetic Environments for Corrosion

Testing," ASTM STP 970, ed. P.E. Francis and T.S. Lee (Philadelphia, PA:ASTM, 1987), p. 235.

10. C. Pitt, Y. Chang, Corrosion 42(1986): p. 312.11. G.C. Pini, P. Li De Anna, Electrochim. A. 22(1977): p. 1423.12. H. Schlichting, Boundary Layer Theory (New York, NY: McGraw-Hill, 1979).13. T. Sydberger, U. Lotz, I. Electrochem.Soc. 129(1982): p. 276.14. I. Finnie, Wear 19(1972): p. 81.15. H. Uetz, ed., Abrasion und Erosion (Wien, Gemiany: Carl Hanser Verlag

Munchen, 1986).16. S. Selmer-Olsen, Paper K 4, Intern. Conf. on Mufli-Phase-Flow, The Hague,

Netherlands, 18-20 May, 1987.17. C. de Waard, D.E. Milliams, Corrosion 31(1975): p. 177.18. G. Schmitt, CORROSION/83, paper no. 43 (Houston, TX: NACE, 1983).19. A. Wieckowski, E. Ghati, Electrochim. A. 28(1983) p. 1619.20. U. Lotz, T. Sydberger, Corrosion 44,11(1988): p. 800.21. U. Lotz, M. Scholimaier, E. Heitz, Werkst. u. Korr. 36(1985): p. 163.22. C.P. Wang, American Scientist 65(1977): p. 289.23. F. Durst, A. Melling, J.H. Whitelaw, Principles and Practice of Laser-Doppier-Ane-

mometry (Academic Press, 1980).24. F. Durst, private communication.25. Z. Werkstofftechnik, in preparation.26. I. Finnie, J. Holak and y. Kabil, Joumal of Materials 2,3(1967): p. 682.27. I. Finnie, D.H. McFadden, Weer 48(1978): p. 181.28. I.M. Hutchings, Mechanical and Metallurgical Aspects of the Erosion of Metals,

CORROSION/79, paper no. (Houston, TX: NACE, 1979).29. J.G.A. Bitter, Wear 6(1963): pp. 5 and 169.

Pitting Corrosion of Duplex Stainless Steels

R. Sriram and D. Tromans*

ABSTRACT

The pitting corrosion of two duplex stainless steels was studied inchloride solutions. Phase compositions were determined by mi-cro-analytical techniques and correlated with pitting behavior. Thestudy showed that the pitting potential and the preferential pittingof the ferrite or austenite phase was dependent upon partitioningof the elements Cr, Mo, and N between the two phases. Alloyingconsiderations leading to improved pitting resistance were dis-cussed and it is proposed that the beneficial effects of alloyednitrogen were due to surface enrichment of N atoms.

KEY WORDS: chlorides, fatigue, localized corrosion, nitrogen,paper machine, solute partitioning, suction rolls, white water.

INTRODUCTION

The trend of the pulp and paper industry toward recycling of pro-cess effluents (miII closure), together with new procedures forbrightening paper, has caused paper machine components to besubjected to an increasingiy corrosive environment (white water).Chloride levels have increased and oxidized sulfur species (e.g.,thiosulfates) are frequentiy present. At the same time, newer pa-per machines have been designed to operate at higher productionspeeds and loads, leading to an increased tendency to encounter

Submitted for publication November 1988; revised July 1989.

Department of Metals and Materials Engineering and the Pulp and Paper

Research Institute of Canada, Pulp and Paper Centra, University of British

Columbia, Vancouver, Canada V6T 1W5.

corrosion fatigue cracking of suction press rolls.' Efforts to mini-mize the fatigue problem have led to a move from bronze roll ma-terial toward stainless steels (SS). These include conventionalaustenitic and martensitic SS and more recently, the newer duplexSS. However, localized corrosion (e.g., pitting and crevice corro-sion) of SS is a common problem in white water systems 2 and isaggravated by the changes that have occurred in the white waterchemistry.

It is becoming more widely recognized that localized corro-sion is a major cause of fatigue-crack initiation and premature fail-ure of SS suction rolls because the locally corroded sits act asstress raisers. Pits caused by aggressive white waters have beenassociated with fatigue cracking by Nathan, 3 Bowers,4 andYeske. 5 Consequently, pitting behavior may be as important aconsideration as mechanical properties when selecting optimumSS alloys for suction rolls. This leads to the necessity for morecomplete information on the pitting behavior of duplex SS becausetheir suitability for suction press roll application is now being madeprimarily on mechanical property considerations.

Duplex steels are composed of austenite (y) and ferrite (a)

phases and their pitting behavior may differ from single-phase al-loys because of solute partitioning effects and the presence of a/y

interfaces. For example, while MnS inclusions and sensitizationeffects often cause localized corrosion of single-phase commercialsteels 6'7 ' 8 additional pitting sites occur in duplex steels at the a/'y

phase boundariès, 9 ' 10 possibly due to segregation effects. 11 Also,phase transformations, like sigma phase formation, result in re-gions depleted in chromium and molybdenum. This may influencethe location of pitting sites.' 2 Nitrogen content is Known to affect

0010-9312189/000235/$3.00/0804 © 1989, National Association of Corrosion Engineers CORROSION-October 1989

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the corrosion resistance of duplex steels. 13,14 The lower solubilityof nitrogen in the a-phase 15 suggests that nitrogen additionsshould have a preferential effect on pitting resistance of they-phase. Other factors known to affect the preferential corrosion ofeach phase are environment and heat treatment. 16•"

The present study was concerned with an evaluation of thepitting behavior of two commercial duplex stainless steels havingdifferent «/y phase proportions and different alloy chemistry. Theenvironments of interest were a neutral chloride solution (1 MNaCI) and a synthetic white water. A major objective was to pro-vide a better understanding of the factors, particularly phase com-position, controlling pitting behavior of duplex steels. From this, therelationship between alloy chemistry and service performance ofduplex steels in white water service may be understood more fully.

EXPERIMENTAL PROCEDURE

Specimens and SolutionsThe starting materials were in the form of a 12.5-mm-thick

wrought plate and 25-mm-thick sections that were cut from a largecasting. The chemical composition and microstructure of the castand wrought duplex SS are given in Table 1 and Figures 1(a) and1(b), respectively.The cast microstructure consist of 65 to 70%ferrite with islands of Widmanstatten austenite developed alonggrain boundaries and within grains. The wrought microstructureconsists of approximately equal proportions (50%) of ferrite andaustenite phases that are elongated along the rolling direction.The etchant used was Kallingst' ) reagent, an acid chloride solution(1.5 g of CuCl 2 , 33 mL of HCI, 33 mL of alcohol and 33 mL of dis-tilled water) that etches ferrite dark and austenite light.

Square specimens of 100-mm 2 working area were cut fromboth steels and a high-purity nickel wire spot welded to the backface for electrical connection. The nickel wire was insulated withpolytetrafluoroethylene (PTFE) tubing and the end of the tubesealed to the specimen. The specimens were mounted in a cold-mount epoxy resin and the working surface was metallographicallypolished to a 1-µm alumina finish. The exposed specimen/epoxyinterface was covered with lacquer to prevent crevice corrosion.Tests were run in 1 M NaCI solution and a synthetic white waterwhose composition is given in Table 2.

Supplementary tests were conducted on unmounted and un-lacquered specimens. All faces of the specimen were polished, aNi wire spot welded to one edge, and the specimen partially sub-merged in the test solution. This completely eliminated interfacesbetween the meta) and a coating material and prevented any pos-sibility of crevice corrosion interfering with the experiment. Thisprocedure was used primarily on the cast steel, which was particu-larly prone to crevice corrosion.

Pitting TestsSingle-cycle potentiodynamic pitting scan (PPS) tests were

conducted in a single-compartment cel), using a platinum counterelectrode and a microprocessor controlled-poteniostat (EG&GPARCt2> Model 350A) operating at a scan rate of 0.5 mV/s. Thepotential was controlled with respect to an external saturated calo-mel electrode (SCE) that was connected to the test solution via aKCI salt bridge and a PTFE Luggin capillary that terminated 1 mmfrom the specimen. A porous zirconia plug was fitted in the end ofthe Luggin capillary. The PPS tests were not commenced until astable free corrosion potential was obtained. The potential wasthen scanned in the noble direction. The scan direction was re-versed once a preset threshold current density of 5 x 10" 3 A/cm2

was reached.In most PPS tests, nitrogen purging of the test solution was

conducted throughout the experiment. The solutions were not

0) Metals Handbook, vol. 8 (Metals Park, OH: ASM, 1973): p. 99.

(2) EG&G Princeton Applied Research Corporation (PARC), Princeton, NJ.

CORROSION—Vol. 45, No. 10

purged during PPS tests on partially submerged specimens to pre-vent turbulence. However, a nitrogen atmosphere was maintainedabove the solution.

Long-term pitting tests were conducted on mounted and pol-ished specimens that were immersed in the test solutions for peri-ods of 100 to 200 h. Potentiostatic control was exercised through-out each test, using either an ECOt 3) Model 549 or HokutoModel (4> HA-21 1 A potentiostat. At the end of the exposure period,specimens were removed, washed successively with distilled wa-ter and alcohol and stored in a desiccator for later examination.Some of the specimens were etched before the long-term immer-sion test in order to locate the position of phase boundaries priorto pit initiation.

Microscopical TechniquesThe genera) appearance of the microstructure and pitting

morphology was studied by optical microscopy and conventionalscanning electron microscopy (SEM) using 20 KeV excitation. Thedetailed microstructure of the steels in terms of second-phase par-ticles and non-metallic inclusions was determined by transmissionelectron microscopy (TEM). Thin specimens (25 by 25 by 3 mm)were cut from the two steels and mechanically polished to 0.1 mmthickness with a surface finish of 600 grit using SIC papers. Discsof 3 mm diameter were cut from the thin sheets using a spark ero-sion machine. The discs were subsequently electropolished andthinned using a jet polisher (Tenupolt5)-2) until a perforation ap-peared. The electrolyte was 5% perchloric acid and 95% aceticacid. The thinned areas around the perforations were observed byTEM at 200 KeV (Hitachitsi model H-800).

Micro-Chemical Analysis and Phase IdentificationThe composition of phases and partitioning of elements in

both steels was studied by quantitative energy dispersive spec-troscopy (EDS) using EDS spectrometers interfaced to both thescanning and transmission electron microscopes. In the case ofTEM-EDS studies, a scanning transmission beam control systemproduced a very fine diameter (-8 nm) electron beam forextremely small area microanalysis. A beam excitation of 200 KeVwas used in the TEM-EDS analysis to minimize beam broadeninginteractions in the specimen. Identification of phases during SEMobservations of pitted surfaces was assisted by EDS analysis oflocal areas. It is well known t2 that chromium and molybdenumpreferentially partition to ferrite, leaving behind an austenite con-taining reduced chromium and molybdenum compared to the aver-age alloy composition. Similarly, nickel preferentially partitions toaustenite. 12

Scanning Auger electron spectroscopy (AES) was used todetermine the distribution (partitioning) of nitrogen in the twophases of high-nitrogen wrought steel. The specimens were ini-tially polished and etched to locate the position of the phases andphase boundaries. Micro-hardness indentations were then placedon the phases. The samples were subsequently repolished to re-move the etched layer. AES analyses were performed on the re-gions around the indentations using a PHI-SAM 7t 595 system.

RESULTS

PPS TestsTypical single-cycle PPS diagrams in 1 M NaCI are shown for

the cast and wrought steels in Figures 2(a) and 2(b), respectively.The potential at which pits initiate, E 5, was determined by the po-tential value at which the anodic current underwent a rapid

13^ ECO Instrurnents, Newton, MA.

(4)Hokuto Denko (Hokto) Co. Ltd., Tokyo, Japan.

(5)Struers K/S, Copenhagen, Denmark.

(6) Hitachi Ltd., Tokyo, Japan

(7i Perkin-Elmer, Physical Electronics Division, Eden Prairie, MN.

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TABLE 1Composition of Steels (wt%)

Steel C Cr Ni Mo SI Mn P $ Cu N(A) Fe

cast duplex 0.06 20 5.0 2 0.8 0.6 0.03 0.01 4.5 0.02 balwrought duplex 0.03 22.4 5.8 2.6 0.37 1.7 ND(a)ND — 0.14 bal

(A) Determined from Fusion analysis using LECO analyzer.(ejND = Not determined.

TABLE 2Composition of Synthetic White Water

Proportions in „yChemical

Conc. of MasterTest Solutions

PPm InSolutlon (mL) Test Solutions

NaCI 3.29 g/L 10 32.9HCI 0.1 N 3.16 11.5Na2SO4 1.479 g/L 100 147.9Na2S2O3 1.412 g/L 100 141.2H20 — 1000 —

(A) Parts per million by weight.

increase on the forward scan. Pitting of the cast steel occurredrelatively easily. There was an initial rapid increase in current at+200 mVscE and a continuing increase thereafter. Thus, the pit-ting potential was identified as +200 mVscE and confirmed laterby long term potentiostatic tests. The E, j, value of the wroughtsteel was much higher, but its precise value was masked by an-odic currents corresponding to oxygen evolution. For example, ananodic polarization curve for Pt in 1 M NaCI is superimposed onFigure 2(b) (dashed line), and shows that the oxygen evolutioncurrent closely coincides with the increased anodic current on thePPS diagram of the wrought steel. However, the wrought steelexhibited pits after the PPS test. Therefore, it is concluded thatE,j, of the wrought steel must be greater than or equal to +900

mVSCE•The potential at which pits repassivated, E rp , during the PPS

test was determined from the potential at which closure of the an-odic hysteresis loop occurred during the reverse scan. Thus basedon Figures 2(a) and 2(b), E m was near —140 mVscE for the caststeel and no lower than +900 mVSCE for the wrought steel. Thesmall hysteresis effect on the wrought steel, relative to the caststeel, was a reproducible phenomenon. It is believed that the truenature of the hysteresis currents due to pitting are masked by oxy-gen evolution currents. Nevertheless, the results clearly show thatEp„ and Em of the wrought steel are more noble than the corre-sponding potentials for the cast steel, demonstrating that thewrought steel has superior pitting resistance.

At the end of the cyclic pitting scan in 1 M NaCI the micro-structure showed preferential dissolution of austenite in the caseof cast steel, and ferrite in the case of wrought steel, as shown inFigures 3(a) and 3(b). In the cast steel, the austenite phase wasdiscontinuous, because of the solidification process, and readilyidentified as islands. No SEM-EDS analysis was required forphase analysis. However, in the wrought steel due to phase con-stituents being present in almost equal amounts with similar mor-phology, a SEM-EDS analysis of Cr, Ni and Mo was performed toconfirm the identification of phases on the pitted surface. The re-sult of the analyses are given in Table 3. Comparison of the phaseanalysis with the pitted regions confirmed that the ferrite dissolvespreferentially.

FIGURE 1. Light micrograph showing Microstructures.(a) Cast duplex SS.(b) Wrought duplex SS.

Transmission Electron MicroscopyThe cast steel showed numerous small precipitates that were

either spherical in shape (diameter of <10 µm) or ellipsoidal. Theprecipitates were present in the a phase, at a—a-phase bound-aries, and at a—y-phase boundaries, as shown in Figures 4 and 5.The -y-phase regions were completely devoid of precipitates asshown in Figure 5. The phases were identified by TEM-EDS anal-

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1.200

0.800W0N

m0.400

c

0.000

-0.400

10 102 t0' iO4 los 10` 10710` IÓ to`

Current Density, uAlm 2

a

1.200

w 0.800

U

10 0.400cmó

0.000

-0.400

10 toe 10`i0 ic? 106 10,108 10 10 0

Current Density, uA/m 2

FIGURE 2. Single cycle potentiodynamic pitting scans in 1 M NaCI.(a) Cast duplex SS.(b) Wrought duplex SS. Dashed line indicates oxygen evolution

current on Pt electrode.

ysis of their composition. These compositions are listed in Table 4for both steels.

Table 4 shows that the small precipitates in the cast steelwere Cu-rich. However, considering the difficulty of analyzing verysmall precipitates without obtaining contributions to the EDS spec-trum from the surrounding matrix, it is concluded that the analysesindicate that the precipitates are composed principally of copper.This is consistent with the analysis of others. 18 Note that in Fig-ures 4 and 5, some of the precipitates have fallen out of the speci-men due to preferential dissolution of the adjoining matrix duringthe thinning process.

The wrought steel showed no precipitates in either of thephases or at the phase boundaries, consistent with the absence ofcopper in the alloy (see Table 1). Also, no nitrides were detected,despite the high nitrogen content in this alloy.

Long-Term Exposure TestsA cast steel specimen exposed to 1 M NaCI solution at the

freely corroding potential showed pitting at the a-y boundary and

CORROSION —Vol. 45, No. 10

FIGURE 3. Appearance of pitted surfaces. Pits appear as blackregions. (Light micrograph)(a) Proferential pitting of austenite phase (light areas) in cast duplex

SS.(b) Preferential pitting of ferrite phase (dark areas) in wrought

duplex SS.

TABLE 3SEM-EDS Analysis of a- andy-Phases in Wrought Steel

Cr Ni No

(wt%) (wt%) (wt%)

ferrite (a) 25.53 4.43 3.86austenite (-y) 21.98 6.94 2.33

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FIGURE 4. TEM micrograph of cast steel showing a-a interface.Note Cu precipitates in both regions and at interface.

FIGURE 5. TEM micrograph of cast steel showing a-y interface.Note Cu precipitates in a-phase and at interface.

propagation into the austenite, as shown in Figure 6. During thetime period the corrosion potential rose Erom —200 mVscE to —40

mVSCE , which was 100 mV more noble than the repassivation po-

tential (Erp = —140 mVSCE). Figure 7 shows a series of micro-graphs of cast duplex SS exposed to 1 M NaCl for 24 hours atdifferent controlled potentials. The sequence a,b,c in Figure 7shows the initiation of pits at the a-y boundary, propagation intothe austenite and eventual dissolution of austenite. Similar pittingbehavior was obtained when the cast steel was exposed to syn-thetic white water. However, the times to pit initiation were 5 to 6

times longer (120 to 140 h). Wrought steel showed no pitting,even at potentials of +900 mVSCE in 1 M NaCI solution and syn-thetic white water. This observation shows that the Epi, of wroughtSS must be greater than 900 mVscE, consistent with the PPS test.

Nitro gen DistributionThe AES analyses of the wrought duplex SS (high N)

showed that the atom ratio of Fe:N was —20 in the -y-phase and—60 in the a-phase. This corresponds to a nitrogen enrichment ofthree times in the -y-phase and is consistent with the reported lowsolubility of N in the a-phase 15 .

Note that based on the composition of the wrought steel inTable 1, the average Fe:N ratio should be —124:1. If all the N en-ters the -y-phase only, the Fe:N ratio in this phase should be 62:1.This is higher than the observed value of 20:1. The discrepancy isattributed to difficulties in obtaining absolute atom ratios by theAES technique when the solute concentration falls below 1 at%,which is the case for N (i.e., 0.14 wt°/a = 0.55 at% N). Therefore,attention must be directed to relative differences in atom ratio be-tween the phases, rather than absolute ratios, in order to confirmpartitioning effects.

No AES analyses were conducted on the cast steel becausethe residual N content was too low.

DISCUSSION

Alloying EffectsThe resuits clearly show that the y-phase is most susceptible

to pitting in the cast duplex SS whereas a-phase is more suscepti-bie in the wrought duplex SS. Also, the long term tests show thatthe wrought material is more resistant to pitting than the cast alloy,and PPS tests show that Epit and E,P of the wrought alloy aremore noble than those of the cast material. All of these observa-tions can be rationalized in terms of phase composition, particu-larly the distribution of the elements Cr, Mo and N.

It is well established that the pitting resistance of SS is im-proved by raising the Cr and Mo content. 19 The analyses clearlyshow that the ferrite phase in both steels is enriched in Cr andMo. This accounts for the higher resistance of the a-phase to pit-ting in the cast material. It also accounts for the high E P;1 and E,^values of the a-phase in the wrought material. However, anotherfactor must be important in the wrought steel because the ferritepitted preferentially, despite the observed partition of Cr and Mo.

The wrought steel contained high nitrogen content relative tothe cast steel. The results show that alloyed nitrogen in thewrought steel is partitioned preferentially in the y- phase. It isknown that N has a beneficial effect on the pitting behavior of sin-gle-phase austenitic SS. 20,21 Therefore, it may be concluded thatthe beneficial effect of N in the duplex SS is conferred primarily onthe -y-phase. This raised E P;t and E rp of -y-phase to values morenoble than the a-phase, so that when pitting is observed in chlo-ride-containing solutions, it occurs preferentially in the a-phase.This clearly indicates that good pitting resistance of duplex steelsrequires both high N, which benefits the -y-phase only, and high Crand Mo to benefit a-phase by preferential partitioning of Cr andMo.

Consistent with the present study, other workers havereported that N raises the E p;1 of duplex SS to more noble

TABLE 4TEM-EDS Analysis of a- and y-Phases

in Cast and Wrought Steels

Cast (wt%) Wrought (wi%)Cr Ni Mo Cu Cr Ni Mo

ferrite (a) 22.37 2.76 2.25 1.01 26.1 3.16 2.73austenite (y) 17.55 4.54 1.61 4.27 22.41 5.97 1.94precipitate 14.37 4.14 0.64 34.91

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FIGURE 6. SEM micrograph showing preferential pitting of auste-nite in cast steel in 1 M Nacl solution at free corrosion potential. Pitsinitiate at a--y interface and propagate info -y region.

values. 22 and that pitting occurs preferentially in the a phase ofhigh-N duplex SS and in the -y-phase of low-N duplex 13 . 23 SS.

Role of Nitro genThe mechanism by which N confers additional corrosion re-

sistance to duplex SS is not clear. Truman et al. 21 have studiedthe effect of N additions on pitting corrosion resistance of a seriesof austenitic alloys. They found that with sufficient Cr and Mo con-tents a large increase in pitting resistance is seen with smal) addi-tion of N, due to synergistic effects. They speculated that the in-creased availability of adsorbed N in the atomic form at thesurface favored metal nitride formation, which then increased thecorrosion resistance.

Osozowa et al. 24 have proposed that when alloyed nitrogendissolves, it consumes protons in the pit to form ammonium ions.This prevents a local lowering of pH and helps to repassivate thepit before it propagates. Jargelius and Wallin also believe that thebeneficial role of N is due to the formation of NH4 ions. 25 Ther-modynamic considerations26 show that it is possible for molecularnitrogen to be reduced to ammonia. However, Pourbaix 26 haspointed out that the reaction is extremely irreversible and that N ispractically non-reducible in solution. Similar considerations couldwell apply to N that exists in solid solution in the steel; i.e., kineticconsiderations severely retard formation of ammonium ions fromalloyed nitrogen. Obviously, when pitting does occur in the N-con-taining alloys, the nitrogen atoms must dissolve (react) to producea species consistent with the thermodynamics of aqueous equilib-ria. Hence, it is not surprising that NH4 ions are detected in theaqueous environment after pitting has occurred. 24,25 However, webelieve it is the rate at which N reacts with the environment andnot the reduction of N, per se, that accounts for the beneficial ef-fect of alloyed nitrogen on pitting resistance.

Based on the preceding considerations, it is now proposedthat in the initial stages of pit growth on N-alloyed SS, the metalions will preferentially dissolve, due to the irreversibility of N reduc-tion, thereby enriching the surface with N atoms. This enrichmentprogressively decreases the sites at which metal dissolution oc-

CORROSION —Vol. 45, No. 10

FIGURE 7. Effect of 24-h potentiostatic tests on pitting of cast steelin 1 M NaCI. (Light micrographs) Pits appear as black areas.(a) –200mVscE; Pit initiation at a-y interface.(b) OmVscE; Pit propagation into austenite.(c) +200mVscE Dissolution of austenite.

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curs. Surface diffusion of N atoms to kink and ledge sites will fur-ther retard dissolution (i.e., pit growth is inhibited). Some indirectevidence in support of the N enrichment model was obtained inthe present work. It was found during AES analysis that wroughtduplex SS surfaces etched in Kallings reagent (i.e., acid chloridesolution) gave higher N signals than unetched surfaces. It shouldbe noted that a surface enrichment effect has been proposed byNewman27 to account for the beneficial effect of Mo on pitting re-sistance of SS.

Effect of CopperThe preferential nucleation of pits at the a/y interfaces (eg.

Figure 6) of the cast steel could be associated with Cu precipitatesat these sites. Whenever an interfacial precipitate encounters thefree surface, the local integrity of the passive film will be affected,leading to easier film breakdown and pit initiation. This explainsthe somewhat surprising occurrence of pitting in the cast steel inFigure 6, where the free corrosion potential rose to a value >Erpbut < Epit , as defined by the pitting scan experiments. The Cu-richprecipitate may not weaken the film sufficiently for pits to initiate atpotentials <E P;t in the pitting scan test, but may weaken the filmsufficiently for pits to initiate at these potentials in long-term expo-sure tests. The pits then grow into the -y-phase because of thelower pitting resistance of this phase arising from its alloy chemis-try.

It should be noted that Cu is often added to stainless steelsto raise the strength level by precipitation hardening and to im-prove corrosion behavior under freely corroding conditions in non-oxidizing media. It has been observed that both pitting currentdensity and corrosion rate are decreased. 22 Also, cavitation ero-sion resistance in seawater contaminated with H 2S is improved byCu addition. 22 However, in view of the possible role of Cu in pitinitiation in chloride solutions, it would appear that its presencemay be detrimental in white water environments.

Industrial ConsiderationsCast duplex SS of similar composition to that used in the

present study is being used as material for suction press rolls inthe pulp and paper industry. It is now clear that such material haspoor pitting (and crevice) corrosion resistance. Pitted sites will leadto early and easy fatigue-crack initiation, leading to reduced ser-vice lifetimes. Therefore, it is recommended that the alloy compo-sition of such steels should be modified by raising the Mo contentand adding N to obtain a composition similar to the pit-resistantwrought alloy. The alloy modifications wilt probably require precau-tionary measures to prevent sigma-phase formation and nitrideprecipitation during cooling of the casting.

CONCLUSIONS

The pitting studies on a cast duplex SS and a wrought duplexSS lead to the following conclusions regarding the behavior of du-plex steels in aqueous chloride solutions:

► Preferential pitting of either the a-phase or -y-phase may occur.This is due to partitioning of the alloy elements between the twophases. There is an enrichment of Cr and Mo in the cc-phase ofboth cast and wrought alloys and of nitrogen in the -y-phase of thewrought alloy.

In the absence of alloyed nitrogen (cast steel), enrichment of Crand Mo in the a-phase causes the y-phase to be most susceptibleto pitting. When alloyed nitrogen is present (wrought steel), enrich-ment of nitrogen in the y raises the pitting resistance of this phaseabove that of the Cr and Mo enriched a-phase.

It is proposed that the beneficial effect of nitrogen on the pittingresistance of the -y-phase is due to a surface enrichment of nitro-gen, possibly because of the irreversibility of nitrogen reduction.

ACKNOWLEDGMENTS

The authors wish to thank the Department of Physics, SimonFraser University, for conducting the AES analysis. One of the au-thors (RS), wishes to acknowledge the financial support of thePulp and Paper Research Institute of Canada via the award of aPAPRICAN scholarship.

REFERENCES

1. R. Yeske, Materials Performance 26,10(Oct.1987): pp. 30-37.2. S. Ylasaari and M. Mertsola, "On the corrosion of stainless steel in white water

systems," Pulp & Paper Industry Corrosion Problems, vol. 4 (Stockholm, Swe-den: Swedish Corr. mat., 1983), pp. 103-107.

3. C.C. Nathan and A.J. Piluso, "Pulp & Paper Industry Corrosion Problems," vol. 1(Houston, TX: NACE, 1977), pp.126-132.

4. D.F. Bowers, CORROSION/86, paper no. 144 (Houston, TX: NACE, 1986).5. R. Yeske, Proc. 1987 Engg. Conf. (Atlanta, GA: TAPPI Press, 1987), pp. 17-25.6. M. Smialowska et al., Corr. Science 9(1969): pp. 123-125.7. G. Eklund, J. Electrochem. Soc. 121(1974): pp. 467-473.8. Z. Szklarska-Smialowska, Pitting Corrosion of Metals (Houston, TX: NACE, 1986)

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10. P. Hronsky and D.J. Duquette, Corrosion 38(1982): pp. 63-69.11. J.B. Lumsden and P.J. Stocker, Corrosion 37(1981): pp. 60-62.12. H.D. Salomon and T.M. Devine, Duplex Stainless Steel, ed. R.A. Luie ( Metals

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