Electrochimica Acta 50 (2005) 2685–2691

Anodic polarization behavior of low-carbon steel in concentratedsodium hydroxide and sodium nitrate solutions

Karthik Subramanian∗, John MickalonisWestinghouse Savannah River Co., Savannah River National Laboratory, Savannah River Site, 773-A, D1138, Aiken, SC 29808, USA

Received 19 August 2004; received in revised form 3 November 2004; accepted 4 November 2004Available online 15 December 2004

Abstract

High-level radioactive wastes, primarily consisting of concentrated sodium hydroxide (NaOH) and sodium nitrate (NaNO3) solutions, arestored in large underground storage tanks made of low-carbon steel. The anodic polarization behavior of low-carbon steel in concentratedsolutions of 10 M NaOH and various concentrations of NaNO3 (0.01–2.0 M) was determined in order to predict the caustic stress corrosioncracking (CSCC) susceptibility of the tanks. The active–passive transition peak exhibited during anodic polarization of low-carbon steel in1 rate.H rate in thep ere CSCC isst d cathodicr©

K steel

1

css(awiptiet

yrabletressvi-

hly

vior

s inears.g thentalteelofg is

SCCsition

0d

0 M NaOH, typically associated with CSCC, at−0.25 and−0.75 VSCE, is still present at the lower and higher concentrations of nitowever, there is a mid-range of nitrate concentrations (0.5–1 M) within which the peak is suppressed by the strongly oxidizing nitresence of oxygen, a cathodic depolarizer. Temperature also affects the magnitude of this mid-range of nitrate concentrations wheen to be electrochemically prevented. The data suggest that the oxygen solubility at the relatively low temperatures tested (<95◦C) controlshe preference of the cathodic reaction, i.e. oxygen reduction versus nitrate reduction. When oxygen reduction is the preferreeaction,Ecorr is driven more noble than the active–passive transition peak.

2004 Elsevier Ltd. All rights reserved.

eywords:Caustic stress corrosion cracking; Anodic polarization; High-level waste tank corrosion; Nitrate stress corrosion cracking; Low-carbon

. Introduction

High-level radioactive wastes generated through the pro-essing of nuclear materials are stored in large undergroundtorage tanks made of low-carbon steel. The wastes con-ist primarily of concentrated solutions of sodium nitrateNaNO3) and sodium hydroxide (NaOH). The temperaturesnd concentrations of sodium nitrate and sodium hydroxideithin the wastes are carefully maintained to preclude nitrate-

nduced intergranular stress corrosion cracking (IGSCC), therimary mechanism of failure for the storage tanks[1]. In

his case, hydroxide levels are maintained to prevent nitrate-nduced IGSCC. These controls are based upon a series ofxperiments performed on wedge-opening loaded specimenshat identify an envelope within which the potential for stress

∗ Corresponding author. Tel.: +1 803 725 8528; fax: +1 803 725 7369.E-mail address:karthik.subramanian@srs.gov (K. Subramanian).

corrosion cracking is minimized[2]. The target chemistrcontrol envelope intends to keep the pH above the vulneregime and also inhibit the cathodic reaction for nitrate scorrosion cracking[3]. However, there is contradictory edence as to the synergistic effects of NaNO3 and NaOH onthe SCC behavior of low-carbon steel, particularly in higconcentrated sodium hydroxide solutions (>8 M)[4]. Addi-tionally, much of the literature focuses on the SCC behaat boiling temperatures.

The stress corrosion cracking of low-carbon steelstrongly alkaline solutions has been studied for over 50 ySeveral review papers have been published summarizineffects of various metallurgical factors and environmefactors that contribute to SCC susceptibility of mild sin alkaline solutions[5,6]. The minimum concentrationhydroxide required to produce stress corrosion crackinreported to be 5% NaOH. The greatest susceptibility tohas been correlated with potentials observed for the tran

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.

oi:10.1016/j.electacta.2004.11.013

2686 K. Subramanian, J. Mickalonis / Electrochimica Acta 50 (2005) 2685–2691

from a passive to active condition. In 10 M NaOH, the tran-sition is reported to begin at approximately−1.0 VSCE andis manifested as an anodic peak on an anodic polarizationcurve [5]. However, the critical potentials at which crack-ing is most evident is reported to be dependent upon steelcomposition, most notably carbon composition[7]. Addi-tionally, the current densities correlating to critical poten-tials that are observed are subject to change under strain[8].

The use of inhibitory additions to prevent caustic stresscorrosion cracking has also been the focus of much research.There is contradictory evidence on the efficacy of nitrate asan inhibitor that has led to it being deemed an “unsafe in-hibitor” [4]. Small additions of sodium nitrate to hydroxidesolution have been reported to cause cracking, where pureNaOH did not produce failure[9]. Additionally, it has beenreported that bubbling oxygen through a boiling NaOH so-lution prevented cracking, as did nitrogen. These dissolvedgases are proposed to affect the formation of the passive film,and consequently the onset of stress corrosion cracking. Thepresence of oxygen may support the formation of Fe3O4 fromFe(OH)2, whereas the presence of nitrogen may have a par-tially inhibitive effect on passive film formation[4]. All ofthese results were determined at boiling temperatures sincethe application was to boilers that exhibited stress corrosionc turesb

ow-ct riedf bil-i and9 stes

2

eel,A olu-t O( 70l rep -t

TN

E

C

MPSS

t

Table 2Parametric test matrix variables

NaOH (M) NaNO3 (M) Temperature (◦C)

10 0.01 600.1 750.5 9512

The anodic polarization scans were performed using acomputer-controlled potentiostat (EG&G PAR Model 273A).The tests were performed in a three-electrode 1-l cell madeof polytetrafluoroethylene (PTFE), with a hot plate to controltemperature (±2◦C). Approximately 750 ml of stirred solu-tion was used for each test. Deaerated tests were conductedby bubbling nitrogen through the solution an hour prior totesting. The working electrode was encapsulated in a metal-lurgical epoxy mount and prepared to an 800-grit finish. Astainless steel mesh mounted on the inner circumference ofthe container was used as the counterelectrode. An Hg–HgOelectrode encased in a PTFE body was used as the referenceelectrode. All reported potentials are given versus a saturatedcalomel electrode to facilitate interpretation and comparison.The Hg–HgO (10 M NaOH) electrode was measured to be0.142 VSCE, and calculated to be 0.3432 VNHE. The anodicpolarization scans were run at a scan rate of 0.5 mV/s, over apotential range, versus the open-circuit, of−0.05–1.0 V. Theworking electrode was allowed to stabilize for approximately1 h prior to polarization. The ending potential was within thetranspassive range.

3. Results

per-a

3

cteda -c tionw eris-t( n-s ntd r-r of6 n-t tyo inga -s lutedp

thea

racking. However, few results are available for temperaelow boiling.

This study focuses on the polarization behavior of larbon steel at temperatures between 65 and 95◦C. The ni-rate concentration within a 10 M NaOH solution was varom 0.01 to 2.0 M, through the range of expected “instaty” as an inhibitor. The testing was performed at 60, 755◦C which are lower than boiling, but relevant to the watorage conditions.

. Experimental

The anodic polarization behavior of low-carbon stSTM A516-70, was determined in concentrated s

ions of 10 M NaOH and various concentrations of NaN30.01–2.0 M). The nominal composition of ASTM A516-ow-carbon steel is shown inTable 1. The experiments weerformed at temperatures of 60, 75 and 95◦C. The test ma

rix variables are summarized inTable 2.

able 1ominal composition of ASTM A516-70 steel

lements Nominal (wt.%)max

0.27 fort≤ 0.5 in.0.28 for 0.5 in. <t≤ 2 in.

n 0.85–1.20.0350.035

i 0.15–0.4

= thickness of plate.

The anodic polarization scans for each of the test temtures are presented in the following sections.

.1. Anodic polarization behavior at 60◦C

The anodic polarization curves for the tests condut 60◦C are shown inFig. 1 as a function of nitrate conentration. The scan performed in the 10 M NaOH soluithout any additions exhibits several distinct charact

ics: (i) a short active dissolution regime between−0.84Ecorr) to −0.78 VSCE, (ii) a broad active to passive traition peak starting at−0.78 with a corresponding curreensity of 10−4 A/cm2, (iii) a second transition peak occuing at−0.29 VSCE with a corresponding current density.3× 10−5 A/cm2, (iv) a broad humped region in the pote

ial range of 0.21–0.61 VSCEwith a maximum current densif 7.9× 10−5 A/cm2 and (v) a transpassive regime startt a potential of approximately 0.62 VSCE. The broad tranition peak appeared to be composed of several convoeaks.

The polarization curves distinctively changed withddition of sodium nitrate. TheEcorr shifted slightly to

K. Subramanian, J. Mickalonis / Electrochimica Acta 50 (2005) 2685–2691 2687

Fig. 1. Anodic polarization curves for tests conducted at 60◦C.

a more positive potential and the transition peaks under-went various changes at low concentrations of sodium ni-trate (0.01 M NaNO3). TheEcorr shifted positive by greaterthan 100 mV and the current density plateau in the poten-tial range of 0.21–0.61 VSCE was reduced with the addi-tion of intermediate amounts of sodium nitrate (0.1–1.0 MNaNO3). The second transition peak displayed in the 10 MNaOH polarization curve was suppressed becauseEcorr wasat or more positive than the potential of the second tran-sition peak. TheEcorr shifted back to a more negativepotential than the initial transition peak potential at thehighest sodium nitrate concentration (2.0 M NaNO3). Thecurve characteristics, therefore, were similar to those ata concentration of 0.01 M NaNO3, although passive cur-rent densities in the range of 0.21–0.61 VSCE were re-duced.

3.2. Anodic polarization behavior at 75◦C

The anodic polarization curves for the tests conducted at75◦C are shown inFig. 2. The characteristics of the anodicpolarization scans performed at 75◦C were similar to thoseat 60◦C. The characteristics of the polarization curves for the10 M NaOH solution without any additions exhibited char-acteristics as follows: (i) a short active dissolution regimebetween−0.83 (Ecorr) and−0.78 VSCE, (ii) an initial tran-sition peak initiating at−0.78 VSCE with a correspondingcurrent density of 3.2× 10−4 A/cm2, (iii) a second transitionpeak at−0.29 VSCE with a corresponding current densityof 6.3× 10−5 A/cm2, (iv) a broad humped potential regionbetween potentials of 0.21 and 0.61 VSCE at a passive cur-rent density of 7.9× 10−5 A/cm2, and (v) the initiation of thetranspassive regime at a potential of 0.62 VSCE.

curve

Fig. 2. Anodic polarization s for tests conducted at 75◦C.

2688 K. Subramanian, J. Mickalonis / Electrochimica Acta 50 (2005) 2685–2691

Fig. 3. Anodic polarization curves for tests conducted at 95◦C.

The polarization curves displayed nearly identical be-haviors to the 10 M NaOH solution at low additions ofsodium nitrate (0.01–0.1 M). The open-circuit potentialswere 50–100 mV more positive than the 10 M NaOH poten-tial, but with similar remaining characteristics. The additionof intermediate amounts of sodium nitrate (0.5–1 M NaNO3)shifted theEcorr to more positive potentials than the first tran-sition peak. The second transition peak was also suppressed atthese more positive potentials, similar to the 60◦C tests. Thepotential range of the humped region was similar to that ofthe 10 M NaOH solution although the current density peakedat approximately 10−4 A/cm2 rather than being constant overthe potential range. The anodic polarization scans at high ni-trate additions (2 M NaNO3) once again exhibited the returnof the Ecorr to more negative values and consequently thereturn of the initial transition peak.

3.3. Anodic polarization behavior at 95◦C

The anodic polarization curves for the tests conductedat 95◦C are shown inFig. 3. The scan performed in 10 MNaOH solution without any additions exhibited several dis-tinct characteristics, similar to the testing at the other temper-atures: (i) an active dissolution regime between−0.83 (Ecorr)and−0.78 VSCE, (ii) a broad active–passive transition start-i of6w(bop mpedc pera-t nsityi

teda ifted

more positive by only 200 mV for concentrations up to 1 MNaNO3. These smaller shifts did not polarize the sample suf-ficiently to suppress the second transition peak as occurredat the lower temperatures. The only concentration of sodiumnitrate at which the second transition peak was suppressedwas 1 M NaNO3.

4. Discussion

The results revealed several key characteristics of theanodic polarization behavior of low-carbon steel in mixedsodium hydroxide and sodium nitrate environments. Low-carbon steel undergoes several oxidation stages during an-odic polarization in highly concentrated sodium hydroxidesolution. These oxidation changes are integral in determin-ing the presence or suppression of active passive transitionpeaks that correlate to potential regimes where stress corro-sion cracking may occur. The presence of nitrate, a strongoxidizer, greatly affects these oxidation stages, the stabilityof the films formed and consequently the SCC response.

The active–passive transition peaks, or anodic currentmaxima, have been identified through X-ray diffraction tech-niques by Salkind and Venuto[10]. The first active–passivetransition peak seen at−0.78 to−0.4 VSCE corresponds tot − -t m.I -s i-t ssivet -tT andC con-c ations per-f m

ng at −0.78 VSCE with a corresponding current density.3× 10−4 A/cm2, (iii) a second transition peak at−0.3 VSCEith a corresponding current density of 2.5× 10−4 A/cm2,

iv) a passive plateau at a current density of 7.9× 10−5 A/cm2

etween potentials of 0.1 and 0.6 VSCE, and (v) the initiationf the transpassive regime at a potential of 0.61 VSCE. Theassive plateau was not stable and did not show the huharacteristic as the scans obtained for the other temures. The instability in the so-called passive current des suspected to be due to the high temperatures.

The effect of sodium nitrate additions is relatively mut the higher temperature. The open-circuit potentials sh

he dissolution of iron as HFeO2 and the ultimate formaion of a Fe3O4 film, as indicated by the Pourbaix diagrantermediate steps of the formation of Fe(OH)2 and the subequent breakdown to form Fe3O4 in the first passive condion have been identified. The second, small, active–paransition peak seen at−0.3VSCE for the 10 M NaOH soluion corresponds to the further oxidation of Fe3O4 of Fe2O3.hese results are in accordance with the results of Zouhin who determined the behavior of low-carbon steel inentrated NaOH solutions and compared anodic polarizcans with the theoretical thermodynamic calculationsormed by Sriram[11–13]. Zou and Chin concluded fro

K. Subramanian, J. Mickalonis / Electrochimica Acta 50 (2005) 2685–2691 2689

these comparisons that the hydroxyl ion plays the key role byaccelerating iron dissolution at active potentials and promot-ing dissolution of Fe2O3 film.

The stability of oxide films in hydroxide versus nitratesolutions plays a key role in the stress corrosion crackingsusceptibility of low-carbon steel. Instability of an adherentoxide film by straining the material can promote the anodicdissolution processes typical of nitrate corrosion or causticcracking[14]. Considerable evidence has been presented tosupport stress corrosion cracking growth via localized passivefilm rupture exposing bare metal[15]. However, the specificoxide film reported to be stable in concentrated solutionsof hydroxide and nitrate is contradictory. The Fe2O3 filmis thought to be more ductile than Fe3O4, thereby requiringhigher stress intensities for rupture to maintain an exposedcrack tip[16]. On the other hand, Fe3O4 is reported to be moreprotective against nitrate-induced IGSCC by protecting grainboundaries from preferential dissolution[17].

The testing revealed that with various additions of asodium nitrate, a strong oxidizer, the active–passive tran-sition peaks may be suppressed. The cathodic depolariza-tion by nitrate shifts theEcorr to more passive values thanin the presence of oxygen alone, thereby preventing SCC.The suppression of the active peaks results from an increasein the cathodic polarization current corresponding to the de-p ers.T c po-l singt

theE ks isdl dti nals ten-t -t than

F OH.

Fig. 5. Comparison of aerated/deaerated anodic polarization behavior at60◦C.

at higher temperatures. A similar effect of temperature wasnot revealed in plain 10 M NaOH solution and solutions withminimal additions of sodium nitrate addition.

The temperature effect was hypothesized to be associatedwith oxygen presence, since the oxygen solubility at 60◦Cis greater than that of 95◦C. Testing at 60◦C with a sodiumnitrate concentration of 1 M under deaerated conditions wasdone to confirm this hypothesis. The results, shown inFig. 5,show the return ofEcorr below the first active–passive tran-sition peak potential for the deaerated solution. The firstactive–passive transition peak was broad while the secondpeak was just detectable, which was unlike the sharp peaksat 95◦C. These results confirm that the temperature effect inthe presence of nitrate is associated with oxygen availability.

There appears to be a competitive mechanism betweenoxygen and nitrate reduction for the cathodic reaction. In10 M NaOH solution, without any additions of sodium ni-trate, oxygen is the only available reductant, regardless oftemperature. Therefore, temperature (and consequently dis-solved oxygen content) does not greatly affect the anodicpolarization behavior. Cathodic polarization scans were runto determine the preferred reduction reaction at select condi-tions where the active–passive transition peak was bypassed.The cathodic polarization scans were performed at the follow-ing conditions: (1) 0.5 M NaNO3 (60◦C), (2) 0.5 M NaNO3( ◦ ◦N firmt sultso

a dr tionr -t rateds ter lar-i tion( igh-t tent.

olarization of the cathodic reaction by the strong oxidizherefore the cathodic reaction intersects with the anodi

arization curve at a more noble potential, thereby bypashe active passive transition peak.

The range of sodium nitrate concentrations over whichcorr is driven beyond the active passive transition peaependent upon temperature, as shown inFig. 4. At lower

evels of nitrate additions,Ecorr is not sufficiently affecteo suppress the initial active passive transition peak.Ecorrs sufficiently driven beyond the initial peak with additioodium nitrate additions, but returns to more noble poials at a concentration of 2 M NaNO3. At lower temperaures, the effect of nitrate additions is relatively greater

ig. 4. Ecorr as a function of temperature and nitrate additions to 10 M Na

60 C deaerated), (3) 0.5 M NaNO3 (95 C), and (4) 2 MaNO3 (60◦C). These conditions were chosen to con

he combinatorial effects of nitrate and oxygen. The ref the cathodic polarization scans are shown inFig. 6.

The cathodic polarization scan performed at 60◦C withn addition of 0.5 M NaNO3 shows that the initial preferreeduction reaction is oxygen, with the nitrate reduceaction taking place at potentials below−0.6 VSCE. The cahodic polarization scan of the same condition but deaehowed the return ofEcorr to more active values since nitraeduction is the only cathodic reaction. The cathodic pozation scan performed at high sodium nitrate concentra2 M) shows similar results to those of the deaerated or hemperature solutions with low dissolved oxygen con

2690 K. Subramanian, J. Mickalonis / Electrochimica Acta 50 (2005) 2685–2691

Fig. 6. Cathodic polarization scans at select conditions.

The data suggest that the ionic strength of the solution hasbecome too high to support a sufficient amount of dissolvedoxygen to support the oxygen reduction reaction. Fromthe data, nitrate concentration, therefore, affects oxygenreduction occurring on the steel surface. At very low concen-trations, the combined reductive current polarizes the steel tomore positive potentials, but not sufficient for a change in theoxide state. At slightly higher concentration, the polarizationis large enough that stable oxides are present. At high con-centrations, nitrate replaces the oxygen at the surface leadingto a shift to active potentials and regimes of unstable oxides.

The effect of nitrate was also observed on the humpedregion for the 10 M sodium hydroxide anodic polarizationscans. In the presence of nitrate especially at the lower tem-peratures, the humped characteristic became flatter with thecurrent density being more constant throughout the potentialregime. Zou and Chin had attributed the hump shape to twolimiting steps, an ohmic resistance of Fe(OH)3 at the lowerpotentials and a concentration profile for FeO2

− at the higherpotentials. The nitrate anion appears to influence the concen-tration profile since at higher potentials the current densitywas constant as opposed to decreasing for simple sodiumhydroxide solutions.

5

eel,A en-t O( sivet rbonsa n-c tratec em-

perature affects the magnitude of this mid-range of nitrateconcentrations where CSCC is seen to be electrochemicallyprevented. At the lower test temperature of 60◦C, the peak isnot present at nitrate concentrations of greater than 0.1 M, anddoes not reappear at least till 1 M nitrate concentration. At anintermediate test temperature of 75◦C, the active transitionpeak is suppressed at 0.5 M nitrate concentration, whereasthe peak is suppressed at 1 M concentration at 95◦C. In thesesolutions, the cathodic reaction can be oxygen reduction ornitrate reduction. In the presence of both the nitrate speciesand oxygen, it was confirmed that when nitrate reduction isthe controlling factor, the active–passive transition peaks arenot suppressed, and when oxygen is the controlling cathodicreaction, the corrosion potential is driven beyond those asso-ciated with the active–passive transition peak.

Acknowledgements

The authors gratefully acknowledge K.R. Hicks for per-forming the experiments. The authors also thank Dr. M.R.Louthan Jr., Dr. P.E. Zapp, Dr. B.J. Wiersma for their valuedinput.

R

erf.

. 44

orro-

J.E.rittle-

. Conclusions

The anodic polarization behavior of low-carbon stSTM A516-70, was determined in solutions of conc

rated 10 M NaOH and various concentrations of NaN30.01–2.0 M). The results indicate that the active–pasransition peak seen during anodic polarization of low-cateel in 10 M NaOH, typically associated with CSCC at−0.25nd−0.75 VSCE is still present at the lower and higher coentrations of nitrate. However, there is a mid-range of nioncentrations within which the peak is suppressed. T

eferences

[1] M.L. Holzworth, R.M. Girdler, L.P. Costas, W.C. Rion, Mater. P7 (1968).

[2] R.S. Ondrejcin, S.P. Rideout, J.A. Donovan, Nucl. Technol(1979) 297.

[3] K.H. Subramanian, P.E. Zapp, B.J. Wiersma, Paper #04420, Csion 2004. NACE, Houston, 2004.

[4] M.J. Humphries, R.N. Parkins, Corr. Sci. 7 (1967) 747.[5] J.E. Reihnohl, W.E. Berry, Corrosion 28 (1972) 151.[6] R.N. Parkins, in: R.W. Staehle, J. Hochmann, R.D. McCright,

Slater (Eds.), Stress Corrosion Cracking and Hydrogen Embment of Iron Base Alloys, NACE, Houston, 1977, p. 601.

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[7] K. Bohkenkamp, Proceedings of the Conference on Fund. Asp. StressCorrosion Cracking, NACE, Columbus, 1967, p. 374.

[8] T.P. Hoar, R.W. Jones, Corr. Sci. 13 (1973) 725.[9] W.C. Schroder, A.A. Berk, R.A. O’Brien, Met. Alloys 8 (1937) 320.

[10] A.J. Salkind, C.J. Venuto, J. Electrochem. Soc. 111 (1964) 493.[11] J.-Y. Zou, D.-T. Chin, Electrochim. Acta 33 (4) (1988) 477.

[12] R. Sriram, D. Tromans, Corr. Sci. 25 (1985) 79.[13] H.E. Townsend, Corr. Sci. 10 (1970) 343.[14] T.P. Hoar, J.R. Galvele, Corr. Sci 10 (1970) 211.[15] R.N. Parkins, Corr. Sci. 20 (1980) 147.[16] R.B. Diegle, D.A. Vermilyea, J. Electrochem. Soc. 122 (1975) 180.[17] J. Flis, Corr. Sci. 15 (1975) 553.