97191 applicability of the competition concept in determining the stress corrosion cracking behavior...

7/27/2019 97191 Applicability of the Competition Concept in Determining the Stress Corrosion Cracking Behavior of Austenitic Stain (51300-97191-Sg)

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Paper No.191 CORROSION07

Peter Che-Sheng Chen, Tadashi ShinoharA Shigco TsujikawaDepartment of Metallurgy, school of Engineering, The University of Tokyo

7-3-1 Hongo, Butrkyo-ku

Tokyo, JAPAN, 113

ABSTRACT

The competition concept states that stress corrosion cracking (SCC) can only occur when thecrack growth rate exceeds the dissolution ratq moreover, based upon this concept, there also exist acritical temperature for SCC of austenitic stainless steels in chloride solutions where SCC can notoccur at or below this temperature. The objectives of this study are to explain both the observed SCC@ential region and the SCC critical temperature obtained for 18Cr-14Ni steels by referencing to thecompetition concept. The SCC ~tential range was found to be the potential range between thecrevice repsssivation potential and the potential where the crack growth rate equals the dissolution


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INTRODUCTIONChloride stress corrosion cracking (SCC) has been a problem for austenitic stainless steel in

aqueous environments containing chlorides, p~icularly in the chemical industry 1. Previousinvestigations have shown that SCC can only initiates from a dissolving surfaces 2; and, moreover, itinitiates more readily from crevice than pits 3,4. To systematic~ly study the SCC behavior thatinitiates from a crevice, a new testing method using spot welded specimens was developed ] 5.Empirical data of the studies using this testing method reveaf that SCC can only occurs at ptentirdsmore noble than the crevice repassivation ptential, ER I I; an~ if SCC doe s n ot occu r a t a p t en t ia l

6 Mor eove r, fo r s t ee ls t h at s uffe r SCC, it w~ju st a t r oveER, SCC will n ot occ ur for a s pec ific s te el .fou nd t ha t a po t en t ia l e xis t s above wh ich t h e s t ee l will n ot expe r ie n ce SCC, t h us , SCC can on lyoccurs at potentials within this potential region 6,7. Empirical studies also indicate the existence of aSCC critical temperature,c, belowwhichsteelsillnotsuffer SCC 7. This Tc was demonstratedto be an effective indicator for evaluating the resistance of steels to SCC, for instance, a high SCCresistance steel would have a higher Tc than a low SCC resistance steel 8. Thus, the alloying effectsof va rio us e lem e n t s o n SCC s u sc ep t ibilit y of s t ee ls c an b e s ys t em a t i ca llye va fu a t ed in t e rm s o f t h isSCC c r it i c al t empe r a t u r e8 .

Tsujikawa et af. have postulated a competition concept which states that SCC can only occurwhen the crack growth rate, C, exceeds the dissolution rate, V, at the dissolving surfacq thus,making the condition V c C the prerequisite for SCC initiation S%lo. The ~nceptud representationof this competition concept is exhibited in Figure 1. Since crevice corrosion can only develop at a

11 scc cm therefore only occurred ata potentialorenoblethanh.otentialorenoblethanER ,By arbitrarily defining a ptential Ev as the potential where C equals to V, the competition conceptpostulates that the potential range where SCC can occur is limited to potentials between E~ and EvStudies have shown that C is mostly influenced by temperature and far less by electrode ptential and


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bulk Cl- concentration 12,13 whereas V depends strongly on the electrode ptential and ordy slightlyon temperature. From these, the V < C criteria proposed by the competition concept can be satisfiedby raising C through the increase temperature. The competition concept suggests that the observedTc is the temperature where the Arrhenius plots of C and V interswt. Therefore, the steel will notsubject to SCC at temperatures below Tc because V is greater than C. However, as the temperatureis increased, C will eventually exceeds V when the temperature is raised above Tc, satisfying thecriteria advocated by the competition concept, and consequently, the steels will suffer SCC.

Moire technique has been applied to study the dissolution behavior of crevice corrosion withgreat success a) . Previous studies has shown that the in-depth development of a typical crevicecorrosion location can be categorized into three stages when monitored over time Stage I, wherecrevice corrosion initiates but does not reach the critical depth; a transient stage where crevicedissolution is impeded or even stopped, and the final steady growth stage, stage II, where thepenetration steadily grows 4. The dissolution rate in stage II, Vn, was reported to strongly dependon the potential and steel composition, while the dissolution rate in stage I, VI, does not exhibit thisstrong dependency . Previous studies of rdloying effects on dissolution behavior of crevicecorrosion also reveal a strong positive correlation between Tc and %[[, indicatinghata relationshipexists between the two . To further explore this observed correlation and examine the applicabilityof the competition concept in determining the SCC behavior in chloride solutions, VII was chosen asthe dissolution rate referred to in the competition concept.

The objectives of this study is three-fold. The first is to compare the measured SCC potentialrange obtained using spot-welded specimens with the SCC ptential range estimated by plotting VIIagainst C as postulated by the competition concept for different concentrations of NaCl solutions.This paper then investigate the observed Tc by comparing measured Tc values with those estimatedby intersecting measured C and V of steels. This study concludes by examining the effects of 300


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EXPERIMENTAL PROCEDURE

SteelssedHigh-purity 18%Cr-14%Ni austenitic stainless steels sheets were heat-treated at 12004C for 30min before water quenching. The chemical compositions of the three steels used are shown in Table

1.

MeasurementfDissolutionateviaMoireTechniqueThe 3 mm thick steel sheets were machined to a dimension of 20 mm X 5 mm before being wet

polished to #1200 finish and used as the specimens. The metal/glass-crevice was formed by pressingthe specimen against a 0.5 mm thick optical glass on top of the insitu cell. The cell was t h e n p la c e don t op of a Z axis m ovin g s t age powered by a h igh p rec is ion s t epp in g m o to r an d t he c ircu la t ions ys t emwa s t u rn e d on t o c ir cu la t e t h e NaCl solution at the desired temperature. The specimen was setto the desired potential with reference to a SCE (saturated ealomel electrode) at room temperature.Theentirepecimenasmeasurediththe100pm X 100pm/pixelesolutionhusthetotalumberofpixelsas 10,000.The developmentfmetalfglassrevice corrosion was monitored in-situ viathe Moire system at 30 minute intervals for 25-50 hours depending on the potential. Previousstudies have shown that the dissolution rate in stage II, VII, should be selected as the dissolution rateto compare with the crack growth rate in determining whether SCC will initiate from a dissolvingcrevice. The dissolution rate of pixels entering the steady growth stage was calculated andstatistically analyzed, the mean was used as the dissolution rate in the growth stage, VII. Thedissolution rate at a potential immediately above the repassivation potential, V*II, Was fin~lydetermined from the electrode ptential dependency of VII.

RESULTS AND DISCUSSION


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SCE for 3%, 0.3% and 0.03% NaCl solutions, respectively. As observed in the figure, the electrodepotential dependency of VII was lowered when the chloride concentration was decreased. The figureshowed that for ptentials more noble than -220 mV vs. SCE, V,, decreased as the solution wasdiluted but for ptentials less noble than -220 mV vs. SCE, VII instead increased as the solution wasdiluted. Thus, this potential of -220 mV vs. SCE seemed to act as a transitional ptential for 4M-1steels because VII is the same regardless of solution concentration. The V*II value were determinedfrom the electrode ptential dependency of V,, and were found to increase as the chlorideconcentration in solution was reduced.

TheresultsfmessoredVIIforthe4M-3 steelnboth3% and0.3%NaClsolutionst80CareexhibitedFigure.ThemeasuredR for 31%and 0.370 NaCl solutions were -310 and -220 mV vs.SCE respectively; it is shown again that E~ has the tendency to move toward the noble directionwhen the chloride concentration in solution was decreased. Nevertheless, unlike 4M- 1 steels, themeasured VII value all markedly decreased as the solution concentration was reduced particularly innoble region. The electrode potential dependency of VII was also lower but not significantly lowerthan that of 4M-1 steels. The V*IIvalues were then determined and it was found that V*ll decremedwhen solution concentration was decreased.

Themoke measurementsfVII for the 4M-13 steel in both 3% and 0.03% NaCl solutions at80C are presented in Figure 4. The ~ was observed to move from -410 mV vs. SCE for 3% NaClsolution to -330 mV vs. SCE for o.l)s~o NaC1. Again, decreasing the solution concentration wasfound to have the effect of moving ~ toward the noble region. The observed behavior of VII for4M-13 was similar to that of 4M-3. Tfre VII above E~ decreased as the solution was diluted from 3%to 0.03% NaCl . The electrode potential dependency of VII afao lower th~ that of 4M-1 steels butthe decrement was not significant. Whereas V[l values were basically the ssme in boths% Nacl Srtd


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growth rates measured in IN H#04 + 0.5M NaCl solution at 80C using the pterrtisl method wereused as C to apply the competition concept 2. Thus, measured C for the 4M-1 steel 7was plotted inFigure 2 together with the results of VII for the 4M-1 steel. As shown in the figure, the VII value forbo t h 0 .3 9t an d 0 .0 39 0 Na Cl so lu t ion s were h igh er t h an C a t d ] po t en t ia ls above E~, an d t h us , t h ecompetition concept would suggest no SCC patential region for these two solutions because thepotential Ev where C equals V must be less noble than ER The results of the SCC tests conductedusing a spot-welded specimens were afso plotted in Figure 27. No SCC were observed with the spot-welded specimens for all measured poterrtiafs for both 0.370 and 0.03% NaC1. This observationcorrelated precisely with that suggested by the competition concept as V exceeds C for all potentials.The Ev for the steel immersed in 3% NaCl snlutions was estimated as the potential where C equals toV, an~ equal to -345 mV vs. SCE. Thus, the competition concept would said the SCC potentialregion for the 4M-1 steel immersed in 3~o NaCl snIution at 80C lies between -380 and -345 mV vs.SCE. The spot-welded specimens showed no occurrences of SCC at -330 and -280 mV vs. SCE, butSCC at -370 mV vs. SCE which is written in the range suggested by the competition concept. Thus,this cross examination confirms the applicability of the competition concept in determining the SCCptential region.

Ilr e C fo r the 4M-3 s t ee lm e as u re d in IN H2S01 + 0.5M NaCl solution at 80C is shown in theFigure 3 together with the measured results of VII 12. The Ev valueserecalculatedobe-215and-40 mV vs. SCE for 3% and 0.3% NaCl solutions respectively. A@ thus, the SCC ptential regionwas determined to be between -310 and -215 mV vs. SCE for s~o NaCl solution and between -220and -40 mV vs. SCE for ().q~o NaCl solution. Since the electrode potential dependency of V,l waslowered for 0.3%NaCl solution, the competition concept would have predicted the SCC potentialregion to expand for 0.3% NaCl solution an~ indee~ the estimated potential region expended byalmost 100% when solution concentration was decreased from 3% NaCl to 0.3% NaCL The resultsof the spot-welded specimens are afso plotted in the same figure 7. For 3% NaCl solution, SCC was


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these measured SCC results to the estimated SCC potential region, which is between -220 and -40mV vs. SCE, it is demonstrated that both results coincided perfectly. These comparisons not onlyverified the correctness of the SCC potential range predicted by the competition concept but alsostrongly support the vsfidity of the competition concept in determining SCC behavior for steelsimmersed in chloride environments.

Finally, the C for the 4M-13 steel in IN H$04 + 0.5M NaCl solution at 80C is shown inFigure 4 with the measured vafues of VII 2. Because V,, values for the 4M-13 steel in both 3% ~d0.03% FJaCl solutions were higher than C at rdl ~tentisls above ER, the SCC criteria, C > V, aspostulated by t h e c om p et it ion c on c ep t c an n ot be s at is fie d a n d t h u s n o SCC r egion for 4M-13 steelscan be identified at 80C. By cross examination of the results of spot-welded specimens 7, it can beconcluded that no SCC occurred at afl measured potentisfs as predicted by the competition concept.This is an additional evidence supporting the applicability of the competition concept in determiningthe SCC potential region for steels immersed in chloride environments.

Effects of Temperature on Dissolution Rate of Crevice Corrosion

The measured VII vsfues of 4M-1 steels in 3% NaCl solution at 40, 50, 60 and 80C aredisplayed in Figure 5. The E~ were found to be displaced in the noble direction as temperaturedecreased, from -390, -380, -360 to -310 mV vs. SCE for 80, 60, 50 and 40C respectively. Asobserved in the figure, the electrode potential dependency of VII remains practiall y mtsff@ed by thechanges in temperature. However, as the temperature decreased, VII also decreased ~d themagnitude of the decrease in VII was greater when the temperature wss reduced from 50 to 40C.The V*I1vsfues were then determined from the electrode ptential dependency of VII. Afthough VIImarkedly decreased with decreasing temperature, the effect of temperature on V*II was not so


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80Q thus, the E~ remained basically unaffected by the increase in temperature. As thetemperature increased, the VII also increased and this increment increased as the electrode potentialmove towards the noble region. Consequently, the electrode @ential dependency of VII increasedwith increasing temperature. The V*U, as determined from the dependency of electrode ~tential,were calculated to be 1.4 pm/hr for 40C, 1.4 ~m/hr for 60C and 1.5 ~m/hr for 80C. Despite of asharp increment observed in both V,, and its electrode potential dependency when the temperaturewas raised, the temperature dependency of V*lI was found to be almost negligible.

The VII values of 4M-13 steels for 3% NaCl solution at 40, 60 and 80C were measured and theresults are presented in Figure 7. The ~ was observed to increase from -410 mV vs. SCE for 80Cto -380 mV vs. SCE for 60C and -310 mV vs. SCE for 40C. The behavior of VII for 4M-13 steelswas basically similar to that of the 4M- 1 steels except that the magnitude of the change in VII withtemperature changes was greater in the 4M-13 steel. While increasing the temperature have an effecton VII, it had no effect on the electrode ptential dependency of VIP The V*lI v~ues were alsodetermined from the electrode potential dependency and estimated to be 2. 3pmihr for 80C, 2.2pmilrr for 60C and 2.1 pmh for 40C. Although V,, is increme in temperate, the effect oftemperature is not very significant.

Applicability of TC as Measurement of SCC Susceptibility

TheVu values determinedboveandtheC valueseasuredor the 4M-1 steel in lN HZS04 +0.5M NaCl solution at both 25C and 80C from previous resesrch7 are plotted in Figure 8 in anArrhenius plot format. As shown in the figure, atthough a strong temperature dependency wasobserved for C, the temperature dependency for V*I1was insignificant. The Tc was evaluated as the


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Figure 9 shows the V[[ values reported above and the C value for both the 4M-3 and 4M-13steels in IN H2S04 + 0.5M NaCl solution7 at both 25C and 80C in the Arrhertius plot against theinverse of temperature. Similar to that observed for the 4M-1 steel as shown in Figure 8, a strongtemperature dependency is only observed for C and not for V*II, which is unaffected by temperature.The Tc was again evaluated by determining the temperature where C and V*II intersects in theArrhenius plot shown in Figure 9. The Tc was calculated to be 61C for 4M-3 steel and 112C for4M-13 steel. The vafues measured in the spot-welded specimens from a previous study7 were 50Cfor 4M-3 steel and 130C for 4M-13 steel, as listed in Table 2. The fact that these two measured Tcvalues were relatively close to those calculated for both steels clearly demonstrated the validity of theprediction put forth by the competition concept that Tc is the temperature where C and V intersects.Thus, as stated by the competition concept, when the crack growth rate is accelerated by increasingthe temperature, the dissolution rate was less affected by this increase of temperature and thereforethere will exist a temperature, Tc, where the crack growth rate will exceed the dissolution rate andconsequently will lead to the occurrence of SCC. Baaed upon these findings, Tc can be used as aneffective indicator for the SCC susceptibility of afloyed stainless steels.

Effects of P and Cu on Improving the Resistances of SCCPhosphorus are inevitably introduced in as impurity into steels during production. It is well

known that phosphoms cart increase the susceptibility of steels to SCC 7. Since a eontent of 300ppm P is commonly introduced into Type 304 steels, the effect of 300 ppm P was therefore studiedin this paper. Because ample literature is available detailing the effect of phosphorus on SCC, thisstudy focused on investigating the effect of 300 ppm P on both C and VII to examine the ~mpetitionconcept in relation to the susceptibility to SCC. Figure 10 displays the C value measured in aprevious investigation 12 ~d the V*II v~ue5 c~culated in lftis s tu dy for t h e 1 8%Cr-1 4%Ni b~e s t ee l


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phosphorus. In summary, an increase in phosphorus content is detrimental to SCC resistanceinaccordance with existing knowledge.

Previous research haa revealed that alloying with 1% copper can significantly increase the SCCresistance of stainless steel 16. The alloying effect of l% copper on both C and V*II in 3%NaCl at80C is afso shown in Figure 10. The figure showed that copper alloying has the opposite effect ofphosphorus addition. Copper was found to increase V*II considerably, from 1.4 pmhr to 2.9 pm/hr,wh ile d ecrea s in g C, from 1 .7 p ,m lu t o 1 .4 pm/br. Again, based upen the competition concept, theafloying of copper will make it more difficult to satisfy the SCC susceptibility criterion, C > V, andtherefore copper is an effective alloying element in improving the SCC resistance. The beneficialeffect of copper is also in agreement with reported results 7.

Finally, the effects of phosphorus and copper on both C and V*I1 for 3%NaCl at varioustemperaturesreshowninFigure1.Datanotmeasuredetween25cto140Cweredeterminedyextrapolatinghemeasureddatato thespecificemperatureeforeplottedlsoin the figure.Similarly to the data shown in Figure 10, the addition of phosphorus was found to increase C anddecrease V whereas copper afloying was found to decrease C and increase V for all observedtemperatures. From the plotted figure, the temperature where C equals V can be estimated for thethree steels. These temperatures were 74C for the base steel, 42C for the 300 ppm P alloyed steeland 123C for the l% copper afloyed steels, respectively. Thus, based upon the competition concept,it is again confirmed that an increase of P is detrimental to SCC resistance but alloying of Cuimproves SCC resistance. Moreover, a good agreement was found by comparing these temperaturesto the Tc measured using the spot-welded specimen listed in Table 2, which were 70C, 50C and1 30 C, re sp ec t ive ly . Th is e xa m in in at ion of t h e t em p era tu re sin dic at ed t h at Tc k t h e t em p era tu rewhere C and V intersects in an Arrhenius plot and as a consequence the competition concept isindeed applicable for determining the SCC behavior of austenitic stainless steels in chloride


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ptentiaf range an d SCC c r it i c a l t empe r a t u r efo r a u s t e n i t ics t a i n le s ss t e e ls i n c lu d in g t h o s e co n t a i n in gP a n d Cu a d di t io n s .Th e SCC p ot e n t i a lr an ge i s in d e e d t h e r an ge be twe e n t h e c r evic e r e pa s siv ad onpo t e n t i a l,ER and t h e p o t e n t i a lwhere the crack growth rate equals to the dissolution rate, Ev TheSCC critical t em p er at u r e,Tc , k t h e t em p er at u rewhere the crack growth rate equals the dissolutionrate in the Arrhenius plot against temperature.

ACKNOWLEDGMENTS

This research was partially supported by NIDI (Nickel Development Institute). We also wouldlike to express our gratitude to H. Abe, of Nippon Steel Co. for preparation of the steel specimens.

1.2.3.

4.

5.

6.

7.

T. Ta k ek aws j Bo sh o ku Oiju t s u 3 1 , 9 (1 9 82 ) p .5 3 7 .T. Sh in oh a r% S. Ts u jik awa , Y. Hk am a t su , Bo sh o ku Giju t s u 3 4 , 5 (1 9 85 ): p .2 8 3 .S. TsujikawA Roles of Localized Corrosion on Initiation of Stress Corrosion Cracks forAustenitic Stainless Steels in Chloride Environment, Stainless Steels 91, page. no.48,(Tokyo, Japan: ISIJ, 1991).S. Tsujikawa, T. Shinohsra, Y. Hisamatsu, The Role of Crevices in Comparison to Pits inInitiating Stress Corrosion Cracks of Type 310S Steel in Different Concentrations of MgC12Solutions at 80C, Corrosion Cracking 85, page. no.35, (Metafs Park, Ohio, USA ASM,1986).K. Tarnski, S. Tsujikawa and Y. Hisarnatsu, Development of a New Test Method forChloride Stress Corrosion Crackirw of Stainless Steels in Dilute NaCI Solutions, Proc. 2ndIntern. Conf. on Localized corrosi~n, page. no.207, (Houston, Texas: NACE, 1987).S. Tsujikawa, T. Shinohsra, L. Wen, Spot-welded specimen maintained above the crevice-repaasivation potential to evaluate stress corrosion cracking susceptibility of stainless steel inNaCl solutions, Proc. of the ASTM Symp. on Application of Accelerated Corrosion Tests toService Life Prediction of Materiafs, ASTM STP 1194, page. no.340, (Philadelphia PAUSA: ASTM, 1994).C. Liarrg, T. Sinohara, S. Tsujikawa, Boshoku Gijutsu 39, 5(1990): p.238.


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1 4. T. Sh in oh ara ,S . Ts ujik awa , N. Ma su ko , Bos hok u Giju t su 3 9, 5 (1 99 0): p .2 38 .1 5. P. Ch en , T . Sh in oh ar%S. Ts ujik aw% Za iryo-t o-Ka nk yo 4 5, 7 (1 99 6) p .4 20 .1 6 . P. Ch e n , T . Sh in oh a ra ,S . Ts ujik awa , Aflo yin g Effe ct s on Dis solu t ion Ra t e o f Cr evic e

Co rr os io n F or Au s t e n it i c S t a in le a s St e e ls i n 3% NaCl S olu t i on a t 8 0C , P ro c. o f Co rr os io n9 6 , p ap er n o .4 3 6, (Ho us t on , USA NACE , 1 99 6 ).

1 7. H. J . Grabk e, St ee l R es . 5 8, 1 0(1 98 7): p .4 77 .1 8 . N. Ma t su u r% N~p po n St a in le ss Te ch . R ep ., 2 3 , 1 (1 98 8 ): p .1 0 9

Table.1 Composition of Steels Used

0,019


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1) Crevice Corrosion2) No Localized Corrosion

+ cv,,

Scc No SCC

4 *(2) (1)

Electrode PotentiaiFigme.1 Conceptual Representation of the Competition Concept

20 I

10 4M-I; 80t

) c

E

s 39w4aclq 0.3%NaClA 0.03%NaCl

vSCC Test ,x ) o 3%NaCl


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20

4M-310 80t

z :x ~2 A A,.g33 v.-; xx ( 0

SCCTes t xxx Xo 0.3%NaClR0.1 1 1 t 1 1

-400 -300 -200 -100 0 100 200

20

10

1

Electrode Potential (mV vs. SCE)Figuzw.3 SCC Electrcde Potential Range for 4M-3 Steel.

4M-133%NaCl, 8&C

cns 39tNaClA 0.03%NaCl

$CCTest 00 0 3%NaCl


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20 (

1M-1

10 3% NaCl

0.1L*llat80T60ER-500 -400 18WCq 6LTCA 5crco 4rYc+ +, J I-300 -20a -100 0Electrode Potential (mV vs. SCE)

Figure.5 Dissolution Rate for 4M-1 Steel in 3%NaCL20 I

11at80T60T4

t

4M-310 3% NaCl

WCq 60C


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204M-13

10

0.1-500 434 .3W .200 -100

Electrode Potential (mV vs. SCE)

Figure.7 Dissolution Rate for 4M-13 Steel in 3%NaCL

Temperature (~)

0-

0


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Temperature (~)10, 150 120 100 80 &l 40 20I I 1 1 I 1 1 1 I I I 1 1 1 I

3% NaCl~*.& \. .18Cr- 14N1 . . . \2.2 2.4 2.6 2.8 3.0 3.2 3.4 3,6

l~X1000 (K -1)

Figure.9 Determination of T, for both 4M-3 and 4M-13 Steels.
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Figure.11 Effect of P and Cu on C and V*II in 3% NFICI.

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97191 applicability of the competition concept in determining the stress corrosion cracking behavior...

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