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UCRL-ID-127355

Electrochemical and MetaHographicEvaluation of Alloys C-22 and 625

A.K. RoyDL. Fleming

B.Y. Lum

May 1997

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Electrochemical and Metallographic Evaluation of AUoys C-22 and 625

Ajit K. RoyDennis L. Fleming

Beverly Y. Lum

May 1997

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Electrochemical and Metallographic Evaluationof ~lOyS C-22 and 625

Ajit K. Roy Dennis L. FlemingFramatome Cogema Fuels Beverly Y. Lumc/o LLNL (On Assignment) Lawrenee Livermorep. O.Box 808, M/S: L-369 National LaboratoryLivermore, CA 94551 P.O.Box 808

Livermore, CA 94551

Abstract

Electrochemical cyclic potentiodynamic polarization(CPP) experiments were performed on Alloys C-22 and625 to evaluate their susceptibility to localized corrosionin acidic brines of various salt content at 90”C. Thernicrostructures of both tested and untested speeimenswere evaluated by optical microscopy. This paperpresents the results showing the effect of chloride ion(Cl”)concentration on the pitting and creviee corrosionbehavior of these alloys, and the relationship of theobserved microstructure to the resulting surhcedegradation modes.

Background

The results{’) of a recent ekctroehemical corrosion studyperformed on several candidate inner container wastepackage materials at the Lawrenee L~vexmoreNationalLaboratory (LLNL) have shown that nickel-ehromium-molybdenum (lQi-Cr-Mo) Alloy C-22 was immune tolocalized attack in brines of various pHs at temperaturesup to 90”C. More recently, Alloy 625, also a Ni-Cr-Moalloy, was included in the electroehernieal corrosiontesting program to compare its localized corrosionresistance to that of Alloy C-22. This paper presents theresults of CPP tests involving both these alloys incomparable environments. Furthermore, the results ofmetallographic evaluation of both tested and untestedspecimens are presented.

Materials and Environments

Materials tested include Ni-Cr-Mo Alloys C-22 and 625.Their chemieal compositions are given in Table 1. Testspecimens were t%bneated finm annealed materials by anoutside vendor. Additional thermal treatments were notgiven to these speeimens prior to their exposure to thetest environments.

Tests were conducted in deaerated acidic brinescontaining 1 to 10 weight percent (wt%) NaCl at 90°C.Acidifkation was done by adding stdfhric acid to thebrine. The pH of these solutions ranged between 2 and 3.

Experimental Procedure

A three-electrode technique was used to perform CPPexperiments in a Pyrex cell using a cylindrical testspeeimen as the working eleetrode, two graphite counterelectrodes, and a Luggin capillary eomeeted to aAg/AgC1 reference electrode. The test specimens werepolished with 600-grit paper and cleaned with distilledwater, acetone and ethanol prior to their exposure to thetest environments.

Electrochemical potential was applied to the testspecimens at a scan rate of 0.17 mV/sec using an EG&GModel 273 or 283 potentiosta~ controlled by an IBM-compatible PC with EG&G corrosion soflware. Forwardpotential scans were done in the noble direetioncommencing at the stable corrosion potential (E-), andcontinuing to a potential sufficiently noble to either causepitting or indicate pit-like behavior prior to reversing thepotential scan at the same rate.

For alloys susceptible to pitting eorrosio~ a clockwisehysteresis loop is traced during reverse scan. Two pittingparameters, namely the critical pitting potential ~,and the repassivation or protection potential ~ em bedetermined fkom these tests. The value of ~,, denotes thepotential at which current increases abruptly on theforward ~ indicating the possibility of pit initiation.~ indicates the potential at which eurmnt returns topassive values during the reverse scan, indicatingrepassivation of pits.

The pH of the test solution was measured at roomtemperature prior to initiation of eaeh experiment. Atthe conclusion of eaeh t@ the specimen was cleanedwith distilled water, acetone and ethanol. The cleanedspeeimen was visually e- followed by an opticalmicroscopic evaluation to detect the presence or absencxof pitting. Furthermom, metallographic evaluation ofboth tested and untested speeimens was pedormed usingconventional techniques to characterize and compare themetallurgical mierostruetures of both alloys. Thespecimens were electrolytically etched in 4% nital (amixture of nitric acid and ethanol) prior to theirevaluation.

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Resultsand Discussion

Electroehernical Study

Results indicate that Alloy C-22 was immune to pittingcorrosion in all salt solutions tested. Furthermore, thisalloy showed very little or no crevice corrosion tendency.In general, the specimen surface was unattached.Contray to this observation Alloy 625 exhibited atendency to surface degradation including crevicecorrosiom pitting corrosio~ and cracking. A comparisonof specimen surface appearance of both alloys tested inacidified brine containing 10 wt!%NaCl is illustmted inFigure 1.

The initiation of pitting in susceptible alloys is the resultof Iodized breakdown of their passive films due to thepresence of aggressive anions such as Cl-. Two theories

‘v) to explain the concept of l+ii, athave been proposedor above which passive metals and alloys may undergopitting in the presence of halide ions. One of thesetheories, proposed by Hoar et al(2), suggests that pittingcorrosion occurs as a result of adsorption of aggressiveanions such as Cl-on an oxide film, followed bypenetration of this film under the influence of anelectrostatic field. Pits initiate when the field across thefilm-solution interface reaches a critical value.

h alternate model, proposed by Leckie and Uhlig(3), isbased on reversible competitive adsorption of Cl- into theoxiddliquid interface (double layer) with oxygen for siteson the alloy surface. Oxygen normally has higherafhity than Cl-for an adsorption site, but as theelectrochemical potential of the alloy is driven in thenoble direetiou more Cl- ions move into the double layer.At a sufficiently high concentration corresponding to ~Cl- ions succeed at favored sites in destroying passivity bydisplacing adsorbed oxygen ions.

The relationship behveen ~k and Cl- eoncxmtration forAlloys C-22 and 625 in acidic brines is illustrated inFigure 2. It appears that for Alloy 625, ~ wasgradually shifted to more active valuea with increasingCl- concentration. A similar effeet of Cl- concentration

‘+’) for alloyson ~,~ has been reported elsewheresusceptible to pitting eormsion. For AIloy C-22, eventhough %,, was shifted to a more active value due to anincrease in NaCl concentration from 1 to 5 wt%, above 5to 7 v#Y. NaCl ~fi was shifted to more noble values.

Except in 1 w&! NaCl solutiom a clockwise hysteresisloop was traced for Alloy 625 during reverse potentialscan in all environments test~ identifying ~it. But~o, could not be determined ffom these CPP dia-

sinee the return polarization curve did not intersect theforward polarization eume at the passive current density(Figure 3), thus, showing no repassivation phenomenonin these teats.

For Alloy C-22, which did not exhibit any pittingsusceptibility, the return polarization cume followed aeounter+loekwise path upon reversal of the potentialscan. The critical potential beeame stilciently noble tooverlap the transpassive region, as shown in Figure 4. Ithas been Suggested(*”g)that when the electrochemicalpotential of a passivated alloy is raised sufficiently, thepotential drop across the double layer may ofien becomehigh enough for the oxidation of entities in the solution -often that of water, but especially in concentratedsolutions. This may result in the formation of oxides onthe alloy surface as anodic products, and uniformcorrosion of the alloy may then occur with little or nolocalized attack. Such a phenomenon may possiblyamount for the absence of pitting in Alloy C-22.

Metallographic Evaluation

The results of metallographic evaluation are illustrated inFigures 5 through 8. Figures 5 and 6 show themierostructures of untested and polarized Alloy 625Speeimens, respectively. An examination of thesemicrostruetures reveals precipitation in one direetion,possibly the direction of working. Fu~ermore, thepolarized speeimen shows grain boundary attack uponexposure to the test solution.

Itis well known thatmost high nickel chromium bearingalloys ean be made susceptible to intergramdar corrosionin certain corrosive environments as a results of improperheat treatment following fabrication. If these austeniticalloys are heated in the temperature range of 500 to850”C (932 to 15620F) or am allowed to cool slowlythrough this temperature range, the carbon in the vicinityof the grain boundaries will difhse toward theboundaries and there combine with chromium to formchromium earbides. These earbides bind eonaiderablequantities of chromi~ thereby leaving the grainboundaries depleted of Cr and therefore susceptible topreferential attack by a cording medium. Thisphenomenon is emnmonly known as sensitization.

One way of reducing the tendency to sensitization is tomduee the carbon content of the alloy (as in Types 304Land 316L stainless steels). The other way is to stabilizethe alloy with Ti or Nb (as in Types 321 and 347stainless steels). These elements have a greater a!hityfor carbon than Cr, binding the carbon as titanium orniobium carbides and suppressing the formation of

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chromium carbides. But the avoidance of sensitizationeffects in Cr bearing alloys of higher Ni content is moredifficult than with the 300 series stainless steels. Carbonvolubility is considerably less in the higher Ni alloys sothat reduction in carbon content or addition of stabilizingelements such as Nb or Ti is less effective.

As mentioned earlier, all specimens were fabricated by aqualified outside vendor from annealed materials. Theprecise annealing temperature and time at temperaturefor either alloy are not available from this vendor. Buttheir chemical composition and mechanical properties,provided by the vendor, meet the requirements of therespective ASTM Designation. No attempt has beenmade to analyze the precipitates obsenwd in Figures 5and 6. However, there are indications in the openIiterature(’o’]’)that for standard solution-annealed Alloy625, these prtxipitates could be chromium andmolybdenum-rich cadides of the type Mx~ and WC,which deplete the y matrix in these elements in thevicinity of the grain boundaries.

The metallurgical microstructure of untested andpolaxized specimens of Alloy C-22 are shown in Figures7 and 8, respectively. Scattered precipitates are observedalong grain boundaries and within grains. It has beensuggested(’o)that tildes of the WC lype can berandomly distributed throughout the austenitic matrix inAlloy C-22 regardless of heat treatment and with noapparent effect on corrosion resistance. Thus, it ispossible that the absence of pitting in this alloy cmdd bedue to the lack of continuous precipitation along thegrain boundaries.

Summary and Conclusions

Ni-Cr-Mo Alloys C-22 and 625 were studied forevaluating their localizd corrosion behavior in 90”Cacidic brines containing 1 to 10 wt’%NaCl using CPPtechnique. The effkct of Cl- concentration on ~ wasinvestigated. The metallurgical microstmctures ofuntested and polarized specimens were also evaluated.The conclusions drawn from this study are the following

. Alloy 625 became susceptible to pitting corrosion,crevice corrosiom and surface attack.

● Alloy C-22 was immune to localized attack under allenvironmental conditions studi~ thus demonstratingits improved corrosion resistance.

active values with increasing Cl- concentrationcoti:rming observations by other investigators. Butfor Alloy C-22, above 5 to 7 WY. NaCl, %,, wasshifted to more noble values.

. Alloy 625 showed a conventional hysteresis loopupon reversal of the potential scan. On the contrary,a counter-clockwise path was observed with Alloy C-22 during the reverse potential scan, indicatingabsence of localized attack possibly due to theformation of oxides on the alloy surface.

● The metallurgical microstructure of polarized Alloy625 specimens revealed intergranular corrosion. incontrast, random precipitation was observed alonggrain boundaries and within grains of Alloy C-22.

Acknowledgments

This work was suppofied by the U.S. Department ofEnergy, Ofike of Civilian Radioactive WasteManagement Yucca Mountain Site CharacterizationOfiice, Las Vegas, NV, and performed under the auspicesof the U.S. Department of Energy by the LawrenceLivermore National Laboratory under contract numberW-7405-ENG48 and by TRW Environmental SafetySystems Inc. under contract number DE-ACOl -RWO0134.

Acknowledgment is also made to Robert Kershaw for hisassistance in metailographic study.

References

1. A. K. Roy, D. L. Fleming and B. Y. Lure, “Effect ofEnvironmental Variables on Localiti Corrosion ofHigh-Performance Container Materials,” LLNLUCRL-JC-125329 January 1997, to be presented atthe Fifth International Conference on NuclearEngineering, Nice. France, May 26-30, 1997

2. T. P. Hoar, D. C. Mears and G. P. Rothwell, “TheRelationships Between Anodic Passivity, Brighteningand Pitting,” Corrosion Science Vol. 5, pp 279-289(1965)

3. H. P. Leckie and H. H. Uhlig, “EnvironmentalFactors Afkct.ing the Critical Potential for Pitting in18-8 Stainless Steel,” J. Ekct.rochem. Sot. Vol. 113,No. 12, pp 1262-1267 (1966)

. For Alloy 625, & was gradually shifkd to more

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6.

7.

8.

9.

A. U. hkd~ P. C. M. Kutty, A. N. Siddiqi, I. N.Andijani and S. Ahmed, “The Influence of pH andChloride Coneentmtion on the Corrosion Behavior ofAISI 316L Steel in Aquems Solutions,” CorrosionScience Vol. 33, No. 11, pp 1809-1827 (1992)

J-H. Wang, C. C. Su and Z. Sz&rska-Smialowsb“Effects of Cl- Concentration and Temperature onPitting of AJSI 304 Stainless Steel.” CorrosionVol. 44, No. 10, pp 732-737 (1988)

R. Ambat and E. S. Dwarakdasa, “Studies on theInfluence of Chloride lon and pH on theElectrochemical Behavior of Aluminum Alloys 8090and 2014,” J. Appl. Electroehem. Vol. 24, No. 9,pp 911-916 (1994)

N. Sridhar and G. A. Cragnolino, “Applicability ofRepassivation Potential for Long-Term Prediction ofLocalized Corrosion of Alloy 825 and Type316LStainless Steel,” Corrosion Vol. 4, No. i I,pp 885-894 (1993)

J. Horvath and H. H. Uhlig, “Critical Potentials forPitting Corrosion of Ni, Cr-Ni, Cr-Fe, and RelatedStainless Steels,” J. Electrochem. Sot. Vol. 115,No. 8, pp 791-795 (1968)

T. P. Hoar, “The Production and Breakdown of thePassivity of Metals; Corrosion Scienee Vol. 7,pp 341-355 (1967)

10.M. H. Brown and R. W. Kirchner, “Sensitization ofWrought High Nickel Alloys; Corrosion Vol. 29,No. 12 (1973)

11. R Cozar, M. Rouby, B. Mayonobe and C. MorizoL“Mechanical Propties, Corrosion Resistance andMicrostructure of both Regular and TitaniumHardened 625 Alloys; Proc. Int. Symp. Metal]. Appl.Superalloys 718,625 Var. Deriv. Ed(s): E. A. Loria,Pubs: Miner. Met. Mater. SW.. Warrendale, P%pp 423-436 (1991)

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Table 1

Chemical Composition of Materials Tested (w%)

Material Heat No Q ~~ ~ g ~ Q& & ~ & Qy Others

~]Oy C-22 J244 0.004 0.21 0.004 <0.001 0.03 Bal 20.65 14.08 2.61 -- -- 0.11 3.29 v 0.02

Ail]oyC-22 L380 0.004 0.15 0.010 0.007 0.027 Bal 21.40 13.41 4.90 -- -- 1.50 3.10 v 0.12

Al]oy 625 L704 0.020 0.08 0.006 0.001 0.08 Bal 21.67 8.90 3.86 0.27 0.29 0.10 -- Nb+Ta: 3.45

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Alloy C-22 Alloy 625

Figure 1. Surface Appearance (Vertical Mode) of Alloys C-22 and 625in Acidic Brines Containing 10 wt% NaCl at 90”C

1000 ‘ , , , I1

1800

600

400

I200 ! , , , I I , , , , 1 !-

0123455 7E91O%1

Chloride Concentration (Wt. %)

Figure 2. Critical Pitting Potential vs Chloride Concentrationin Acidic Brines at 90”C

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Figure 3. CPP Diagram of Alloy 625 in Acidic BrineContaining 5 wt% NaCl at 90”C

1 I 111111 i I 8 81111 1 I 1 11881 8 I 841111 1 * 1111111 I ( 1 11111 I I 81

I I 111111 # 1 ! tlltt t I I tttllt ! Itttat I 4 t Illlt 1 I I 14111 I I 1’

-@ -7 -6 -s -4 -3 -2v- We=-a 10”

Figure 4. CPP Diagram of Alloy C-22 in Acidic BrineContaining 5 wt% NaCl at 90”C

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Figure 5. Microstmcttue of UntestedAlloy625 Specimen.200X. Etchant 4% Nitsl Ekctrolytic.

F@ure6. Microstructure ofPolsrized Al@ 625 Specimen.200X. Etchsnt 4% Nital Electrolytic,

Figure 7. Microstructwe ofUntestedAlloy C-22 Specimen.200X. Etcimnt 4% N]talElectmlydc.

Figure 8. MicrostrucmreofPolarizedAlloy C-22 Specimen,200X. Etchant: 4% Nltal Electroljzic,

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