Effect of Aging Treatment on Intergranular Corrosion ...downloads.hindawi.com/journals/ijc/2019/9506401.pdf · Effect of Aging Treatment on Intergranular Corrosion Properties of Ultra-Low

Research ArticleEffect of Aging Treatment on Intergranular CorrosionProperties of Ultra-Low Iron 625 Alloy

Zhi-yuan Zhu 1 Yi Sui1 An-lun Dai1 Yuan-fei Cai1 Ling-Li Xu 1 Ze-xinWang1

Hong-mei Chen1 Xing-ming Shao2 andWei Liu2

1School of Materials Science and Engineering Jiangsu University of Science and Technology Zhenjiang Jiangsu 212003 China2Jiangsu Xinhua Alloy Electric Co Ltd Xinghua Jiangsu 225722 China

Correspondence should be addressed to Zhi-yuan Zhu salanganezhu163com

Received 3 August 2018 Accepted 6 December 2018 Published 2 January 2019

Guest Editor Xiang Li

Copyright copy 2019 Zhi-yuan Zhu et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The microstructures evolution of precipitations for an ultra-low iron Alloy 625 subjected to long term aging treatment at 750∘Cwas investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD) The intergranular corrosion behaviorsof Alloy 625 were evaluated by using ASTM G28A The result shows that the precipitated phase 12057410158401015840-Ni3Nb was mainly precipitatedat the grain boundaries and twin boundaries The number and volume fraction of 12057410158401015840 increased with the prolonging of aging timeThe transformation of 12057410158401015840 to 120575-Ni3Nb occurred after aging periods of 200 hThe corrosion resistance of Alloy 625 was significantlyreduced during aging treatment The decrease in intergranular corrosion resistance of Alloy 625 was attributed to the dissolutionof precipitated phase and chromium depleted zone Themass loss rate of Alloy 625 after aging treatment is related to the volume ofprecipitated phase and can be simulated by Johnson-Mehl-Avrami equation

1 Introduction

Because of the excellent properties such as good strengthductility and toughness welding and corrosion resistancenickel-base alloy is widely used in naval architecture andocean engineering petroleum refining and petrochemicalindustries and energy and power industry [1ndash3] As a kind ofnickel-based alloy Alloy 625 seems to be the suitable materialused in the 4th generation of nuclear power systems [4 5]owing to its attractive features Alloy 625 has the problem ofprecipitate12057410158401015840-Ni3Nb and carbide [6ndash9] in high temperatureby long term aging The aging and creep properties of alloysare important parameters for safety operation of componentmade of Alloy 625 During the long term aging the 12057410158401015840phase can gradually transformed into needle-like 120575-Ni3Nbleading to the increasing of the hardness [10] and decreasingof the ductility and toughness Because of its high potentialpreferential corrosion usually occurs on Ni3Nb in oxidizingmedia At the same time intergranular corrosion can alsooccur in the area of carbide and chromium-depleted zoneat grain boundaries [11ndash14] However the work concentrated

on the effect of formation and transformation of precipitatedphase on corrosion behaviour of nickel-based alloy duringthe long term aging is rarely reported

Commercial Alloy 625 usually contains 2-5wt Fe and Featoms are present in the matrix in solid solution The Topo-logical Close-Packed Phase such as Laves phase 120583 phase and120590 phase [15] as shown in Table 1 will be precipitated in thelong aging These phases are plate-shaped or needle-shapedleading to the reduction of fracture strength and plasticity[16] In addition Si can also facilitate the precipitation ofharmful phase [17] leading to the decrease of the service lifeof components

The aim of present study was focused on the effect ofaging treatment on precipitation behaviour and intergranularcorrosion properties of a newly developed ultra-low ironAlloy 625

2 Experimental

The Alloy 625 used in present work is provided by JiangsuXinHua Alloy Electric CO LTD the composition of which

HindawiInternational Journal of CorrosionVolume 2019 Article ID 9506401 9 pageshttpsdoiorg10115520199506401

2 International Journal of Corrosion

Table 1Themain phases occurring in long-term aging nickel-basedsuperalloy

Phase Structure Typical Composition

12057410158401015840 body-centeredtetragonal Ni3(Nbgt05Tilt05Allt05Molt05)

120575 orthorhombic Ni3NbMC cubic (NbTi)CM6C cubic (Crgt02Mogt02Fe)CM23C6 cubic (Crgt08FeMo)CLaves MgZn2-Types(AB2) TiFe2 NbFe2 MoFe2 etc120583 Hexagonal(B7A6) Fe7Mo6 Fe7Nb6 Fe7Mo6 etc

120590 Tetragonal(BA) (CrMo)x(NiFe)y FeCr MoFeNbFe etc

Table 2 Chemical composition of Alloy 625 (wt)

C Cr Mo Co Nb+Ta Fe Al Ti Si P S Ni005 224 95 003 366 001 024 023 004 0002 0001 631

is listed in Table 2 The microstructure of Alloy 625 is shownin Figure 1 It can be seen that Alloy 625 has a single-phaseaustenite with carbide particles (black cross) distributed onthe surfaceThe specimens of 50times 24times 5 (plusmn03)mm for inter-granular corrosion investigations were machined from thehot rolled sheets All the specimens were solution annealed at1140∘C for 45 h in an argon protective atmosphere followedby water quenching Stabilizing treatment was performed ata temperature of 1000∘C for 65 h in an evacuated tube andcooling to room temperature in air Aging treatment wasconducted at a temperature of 1000∘C 750∘C for 40 h 200 hand 1000 h Electrolytic etching with 10 oxalic acid solutionwas used to reveal the microstructure in the present study

Intergranular corrosion tests were carried out in glasscontainer containing 236mLH2SO4 25g reagent grade ferricsulfate (containing about 75 Fe2(SO4)3) and 400mL wateraccording to ASTM G28A-02 [18]The test was carried out inthe boiling solution and the testing durationwas 120 h All thespecimens for immersion tests were weighed using analyticalbalance with sensitivity of 001 mg before and after corrosiontests The mass loss rate (mpy) with units of mmyear wascalculated as follows

mpy = Δ119882120588119878119905 times 24 times 365 (1)

whereΔ119882 is the weight loss of specimens (g) 120588 is the densityof the stainless steels (gmm3) S is the total corrosion area(mm2) and t is the time of exposure (h)

After metallographic etching or intergranular corrosiontests the specimens were rinsed with deionized water andalcohol and then dried The microstructural characteristicsand intergranular corrosion morphologies were examinedusing a scanning electronic microscope (SEM Zeiss MER-LIN) equipped with an energy dispersive spectrum analyzerThe phase analysis was performed using an X-ray diffraction(XRD D8 Advance A25X) analyzer

Wt At

Mo 968 344Nb 6433 2363Ti 047 034C 2552 7258

Figure 1 Microstructure of solution alloy by means of SEM

3 Result and Discussion

31 Microstructure Characterization The composition of theprecipitated phase in Figure 2(b) was analyzed using theenergy dispersive X-ray (EDX) Figures 2(a) and 2(b) showthe morphology of Alloy 625 after stabilized treatment Itcan be seen that after the stabilized treatment the carbideis preferentially precipitated at the grain boundary and twinboundary Discontinuous and irregular precipitated phasedistributed in grain boundary is (Mo Nb)CThe precipitatedphase at the triple line boundary is (NbTi)C In additiona localized enrichment of C and Nb was observed at thegrain boundaries in Figures 2(b) 3(a) 2(h) and 3(b) In thesubsequent aging process these carbide particles can preventthe migration of grain boundary However there is also thenucleation site for the precipitated phase After 40 h agingtreatment granular like 12057410158401015840-Ni3Nb phases are present at thegrain boundary and twin boundary as shown in Figures2(c) and 2(d) With the increasing of aging time the densityof metastable 12057410158401015840 phase increases the coarsening of whichoccurred The metastable 12057410158401015840 phase gradually transforms tothe stable 120575 phase After 200 h aging treatment the needle-like 120575 is preferentially precipitated at the grain boundaryand then grown into the inside of the grain as shown inFigures 2(c) and 2(d) Also 12057410158401015840 is continuously precipitated inthe grain and coursing The number of needle-like 120575 phasesincreased significantly after 1000 h aging treatment as shownin Figure 2(g) Also 12057410158401015840 phase can be observed inside thegrains

The precipitated phase of Alloy 625 during aging sensi-tization at 750∘C for different time was determined by X-ray diffraction (XRD) the composition of which is listedin Table 3 The XRD profiles of Alloy 625 after aging sen-sitization for different time are shown in Figure 4 FromFigure 4 it can be seen that the peak appeared at 350∘ and407∘ corresponding to the diffraction peak of NbC afterstabilization After 40h of aging the peak appeared at 423∘corresponding to the diffraction peak of 12057410158401015840-Ni3Nb Becausethe amount of precipitation was small the peak intensity

International Journal of Corrosion 3

(a)

+ 1

+ 2

+ 3

4

(b)

(c)

-NＣ3Nb

(d)

(e)

-NＣ3Nb-NＣ3Nb

(f)

(g)

8

+ 6

+ 7

+ 5

(h)

Figure 2 Microstructure of Alloy 625 after aging sensitization for different times (a)(b) 0h (c)(d)40h (e) (f)200h (g)(h)1000h


0 1 2 3 4 52468

101210

20

3010203040506010

20

30

400

10

20

0 1 2 3 4 5

Ni

Cr

Mo

Nb

C

Distance (m)

(a)

0 1 2 3 4 52468

101210

20

3010203040506010

20

30

400

10

20

0 1 2 3 4 5

Ni

Cr

Mo

Nb

C

Distance (m)

(b)

Figure 3 EDS analysis of the area marked in Figure 2 (a) 0h aging sensitization (b)1000h aging sensitization

Table 3 Composition at some topical points as obtained by EDSanalysis (wt)

Position Ni Cr Mo Nb Ti C1 5703 1357 833 1189 170 7472 5850 1525 884 1000 218 5223 5579 1575 1301 760 - 7854 6711 1791 778 357 - 3625 4142 3935 883 - 231 8096 6559 754 856 997 256 5777 4165 1600 3022 242 - 9718 6751 1743 764 237 - 504

is relatively weak The intensity for the diffraction peak ofNi3Nb increases with the increases of aging time The resultsshown in Figure 4 are consistent with the results obtained inFigure 2

32 Intergranular Corrosion Test Figure 5 shows the massloss of Alloy 625 specimens after the 120h ferric sulfate-sulfuric acid corrosion test (ie ASTM- G28A-02) It canbe seen that the weight loss during corrosion increases withaging time The mass loss rate of Alloy after stabilizationtreatment is 127mma After aging for 40 h 200 h and 1000h the weight loss rate is 5086 mma 6451 mma and 8717

mma respectively Although the aging treated samples havehigher corrosion rates than the stabilization treatment theyhave lower corrosion rates suggesting a rapid formation ofthe passivation film on the sample surface The relationshipbetween mass loss rate and aging time can be fitted as follows

119898119901119910 = 9020 minus 4432 exp (minus 1199051099)

minus 4452 exp (minus 1199053278)

(2)

The surface morphologies of the Alloy 625 specimensafter aging at 750∘C for different time in ferric sulfate-sulfuricacid test for 120 h are presented in Figure 5 Only a few num-bers of grain boundaries have been found to be corroded inthe stabilization treated specimen cankerous attack patternsappeared frequently along the grain boundaries as shownin Figure 6(a) In contrast to Figure 6(a) most of the grainboundaries are completely corroded for the specimen afteraging for 40 h as envisaged fromFigure 6(b) Actually for theaging treated sample the intergranular corrosion propagatesalong grain boundaries from the surface into the materialinterior and causes mass loss due to grain dropping as shownin Figure 6(c) Thereafter the corrosion rate is acceleratedwith the drop of the grains Very fine cellular-like patternscould be observed on the surface (Figure 6(e)) seemingly


20 30 40 50 60 70 80 90 100

Aging 1000h

Aging 200h

Aging 40h

34 35 36 37 36 38 40 42 44 46 48 50

NbC

Ni-Base

Inte

nsity

Non Aging

2 Theta (∘)

Ｃ3＜

Figure 4 XRD profiles of Alloy 625 after aging sensitization for different time

0 100 200 300 400 500 600 700 800 900 1000

0

20

40

60

80

100

Aging Time (h)

mpyfitted line

Wei

ght L

oss R

ate (

mmmiddot

minus1)

Figure 5 Mass loss rate of Alloy 625 as a function of aging time

suggesting a preferential dissolution of Ni3Nb With theincrease of aging time the surface grains have completelydropped as shown in Figure 6(d) and correspondingly thecorrosion rate rapidly increases Note that the Alloy 625specimen exhibits two types of precipitation phase one isthe detached needle like 12057410158401015840-Ni3Nb from the surface and theother is the detached lamellar like 120575-Ni3Nb as indicated bythe red and yellow arrows in Figure 6(f) respectively

The cross-sectional SEM images of the Alloy 625 afterimmersing corrosion for 120 h are shown in Figure 7 SEMimaging of cross-sections (Figure 7) further confirms theresults of Figure 6The stabilization treated sample displays arelative flat surface and only one big intergranular corrosioncrack occurs on the surface as shown in Figure 7(a) Thecrack originated from the area of carbide at the grainboundary and spread along the grain boundaries into theinterior of the material The aging treated specimens exhibitan obvious intergranular corrosion as shown in Figures 7(b)7(c) and 7(d) The comparison of cross-sections in Figure 7effectively makes further confirmation on the deteriorationof intergranular corrosion by the aging treatment method inthe tested alloy

4 Discussion

Cr and Mo can combine with carbon elements to formcarbides which are continuously precipitated at the grainboundary during long term aging treatment The dilutionzone of Cr and Mo can form adjacent to the grain boundarybecause the diffusion rate of Cr and Mo is far lower than thatof carbon The widths of the depleted zone and the amountof precipitates can be calculated by using Ficks second law[19]

120597C120597t = 119863

1205972C1205971199092 (3)


(a) (b)

(c) (d)

(e) (f)

-NiNb Dissolved

-NiNb Dissolved

Figure 6 SEM morphologies showing the microstructure of Alloy 625 after aging for (a) 0h (b) 40h (c)(e) 200h and (d)(f) 1000h

Initial and boundary conditions

119905 = 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 gt 0

119905 gt 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 = +infin

(4)

The solution of the equation can be obtained by usingBoltzmann Transformation and Gauss Error Function

C (x t) = 119862119875 + (1198620119866 minus 119862119875) ∙ 2radic120587 int

120573

0exp (minus1205732) 119889120573

(120573 = 1199092radic119863119905)

(5)

C (x t) = 119862119875 + (1198620119866 minus 119862119875) erf ( 1199092radic119863119905) (6)

where 119862119875 is precipitation phase of grain boundary and alloyelement concentration of substratersquos interface nonsensitized


(a) (b)

(c) (d)

Figure 7 SEM images of the cross-sections in the Alloy 625 after for (a) 0h (b) 40h (c) (e) 200h and (d)(f) 1000h after 120 h ferric sulfate-sulfuric acid corrosion test

alloyrsquos concentration of 1198620119866 is interior part x is distance tograin boundary D is diffusion coefficient and t is aging time

Alloy element of grain boundary precipitation by dif-ferent sensitization time causes dilution zonersquos compositionchange and qualitative results are shown in Figure 8

The widths of the depleted zone 11990940 = 119909200radic5 =11990910005where in this region complete passivation film cannotbe formed are proportional to the square root of the agingtime that is the volume of the dilution area follows theparabola law

As can be seen from Figures 6 and 7 corrosion mainlyoriginated from the carbide and precipitated phases and thengradually expanded into the interior of the grain Whenthe grain loses its support it falls off under the action ofcorrosion In addition the volume fraction of precipitatedphase increased with the increasing of aging time leading tothe aggravation of corrosionTherefore the surface presents aglacial like corrosion appearance because a dense passivationfilm cannot be formed at 120575 phase [10 20] The corrosionpotential of carbides and intermetallic at the grain boundaryis relatively low therefore preferred dissolution occurs dur-ing immersion as shown in Figure 9

It can be deduced from the above analysis that the massloss rate (mpy) is related to the number of precipitated phases[21] and the diffusion of alloy elements After aging for 200 hbecause of the precipitation of needle-like 120575 phase the peeloff grain and lead to further increases of the mass loss rateThe mass loss rate of the Alloy 625 is not proportional to

Depleted-Zone

x

Non SensitizationSensitization 40hSensitization 200hSensitization 1000h

Allo

y El

emen

t Con

cent

ratio

nC(xt) Concentration of Forming

Passivation Film

0

０

x40 x200 x1000

Figure 8 The relationship between aging time and the alloycomposition and the width of the depleted zone

the square root of aging time especially within 200 hourswhich means that the mass loss rate cannot be explainedby diffusion theory alone When the grain boundary iscompletely dissolved the grain loses its support and falls offIn the process of corrosion the mass loss mainly consists of


Depleted-ZoneIntergranular Precipitation

++ +

++ +

+

Get electrons

[M]

+

Lost electrons

Ferric SulfateSulfuric Acid

Non-exposure

+

Dissolution

Material

Internal

Dissolved

+ + +

n

ndashNiNb

ndashNiNb

minus

minus

minus

minus

minus

minusFe+

lowastM Ni Cr Mo Nb

of ndashNiNbndashNiNb

Fe+

Mn+

Figure 9 Intergranular corrosion mechanism of Inconel 625

120575 phase grain boundary precipitator and element dilutionarea near the grain boundary The nucleation and growth ofthe precipitated phase can be characterized by Johnson-Mehl-Avrami (JMA) equations [22ndash25]

119891 = 1 minus exp (minus119896119905119899) (7)

where f is volume fraction of precipitation phase t is the ratioof the volume of the precipitated phase at a certain time to thevolume of the precipitated phase at an infinite time k is timeconstant associated with temperature and n is time constantbetween 1 and 4

The volume fraction of element dilution zone and precip-itated phase at the grain boundary can be obtained as follows

119891119894119901 = 1 minus exp (minus1198961198941199011199051198991)119891120575 = 1 minus exp (minus1198961205751199051198992)

(8)

where 119891119894119901 is volume fraction of precipitation phase and grainboundary element depleted zone and 119891120575 is volume fraction ofneedle-like 120575 phase

In addition corrosion rate is as follows

119898119901119910 = 119886119891119894119901 + 119887119891120575 + 119888 (119898119898119886) (9)

where a b c are constantsWhen t =0119898119901119910 = 1267 (119898119898119886)As can be seen from (5) and (6)

119898119901119910 = 4432 [1 minus exp (minus 1199051099)]

+ 4462 [1 minus exp (minus 1199053278)]

+ 1267 (119898119898119886)

(10)

119886 = 4432 119887 = 4462 119896119894119901 = 11099 119896120575 = 13278 1198991 = 1and 1198992 = 1

5 Conclusion

The effect of aging time on intergranular corrosion behaviorof a ultra-low Fe Alloy 625 was investigated by surface obser-vation and immersion testing methods The deteriorationof intergranular corrosion resistance of the tested alloy wasdiscussed The results are summarized as follows

(1) The Cr depleted zone is formed around the grainboundary during the aging process The precipitated phase12057410158401015840-Ni3Nb was mainly precipitated at the grain bound-aries and twin boundaries The number and volume of 12057410158401015840increased with the prolonging of aging timeThe transforma-tion of 12057410158401015840 to 120575-Ni3Nb occurred after aging periods of 200 h

(2) It can be seen that the weight loss during corrosionincreases with aging time The corrosion resistance of Alloy625 was significantly reduced during aging treatment Thedecrease in intergranular corrosion resistance of alloy 625was attributed to the dissolution of precipitated phase andchromium depleted zone The mass loss rate is explained byJohnson-Mehl-Avrami equation

(3) The falling off of grains was due to the dissolutionof Cr depleted zone and precipitation phase The model ofintergranular corrosion is established

Data Availability

All the data in the manuscript is original All data is onlyaccessible in the manuscript The data in the text is originaland anyone cannot modify it at will

Conflicts of Interest

The authors declare that they have no conflicts of interest

Acknowledgments

The authors are grateful for the financial support by Sci-ence and Technology Program of Jiangsu Province (NosBE2015144 and BE2015145)


References

[1] S A Al-Fozan and A U Malik ldquoEffect of seawater level oncorrosion behavior of different alloysrdquo Desalination vol 228no 1ndash3 pp 61ndash67 2008

[2] Z Yin W Zhao W Lai and X Zhao ldquoElectrochemicalbehaviour ofNi-base alloys exposed under oilgas field environ-mentsrdquo Corrosion Science vol 51 no 8 pp 1702ndash1706 2009

[3] TM Smith BD Esser NAntolin et al ldquoPhase transformationstrengthening of high-temperature superalloysrdquo Nature Com-munications vol 7 no 1 2016

[4] R Dutta ldquoCorrosion aspects of NindashCrndashFe based and NindashCubased steam generator tube materialsrdquo Journal of NuclearMaterials vol 393 no 2 pp 343ndash349 2009

[5] X Ren K Sridharan and T R Allen ldquoCorrosion Behavior ofAlloys 625 and 718 in Supercritical Waterrdquo Corrosion vol 63no 7 pp 603ndash612 2007

[6] V Shankar K Bhanu Sankara Rao and S Mannan ldquoMicro-structure and mechanical properties of Inconel 625 superalloyrdquoJournal of NuclearMaterials vol 288 no 2-3 pp 222ndash232 2001

[7] L M Suave D Bertheau J Cormier P Villechaise A SoulaZ Hervier et al ldquoImpact of Thermomechanical Aging onAlloy 625 High Temperature Mechanical Propertiesrdquo in 8thInternational Symposium on Superalloy 718 and Derivatives pp317ndash331 John Wiley amp Sons Pittsburgh PA USA 2014

[8] L M Suave J Cormier P Villechaise et al ldquoMicrostructuralEvolutions During Thermal Aging of Alloy 625 Impact ofTemperature and Forming ProcessrdquoMetallurgical andMaterialsTransactions A Physical Metallurgy and Materials Science vol45 no 7 pp 2963ndash2982 2014

[9] M Shaikh M Ahmad K Shoaib J Akhter and M Iqballdquo Precipitation hardening in Inconel rdquo Materials Science andTechnology vol 16 no 2 pp 129ndash132 2013

[10] H M Tawancy I M Allam and NM Abbas ldquoEffect of Ni3Nbprecipitation on the corrosion resistance of Inconel alloy 625rdquoJournal of Materials Science Letters vol 9 no 3 pp 343ndash3471990

[11] K Kaneko T Fukunaga K Yamada et al ldquoFormation ofM23C6-type precipitates and chromium-depleted zones inaustenite stainless steelrdquo Scripta Materialia vol 65 no 6 pp509ndash512 2011

[12] Y - Pan D S Dunn G A Cragnolino and N Sridhar ldquoGrain-boundary chemistry and intergranular corrosion in alloy 825rdquoMetallurgical andMaterials Transactions A Physical Metallurgyand Materials Science vol 31 no 4 pp 1163ndash1173 2000

[13] H Sahlaoui H Sidhom and J Philibert ldquoPrediction ofchromium depleted-zone evolution during aging of Ni-Cr-Fealloysrdquo Acta Materialia vol 50 no 6 pp 1383ndash1392 2002

[14] S K B B S Prasad V Kain and J Reddy ldquoMethods formakingalloy 600 resistant to sensitization and intergranular corrosionrdquoCorrosion Science vol 70 pp 55ndash61 2013

[15] L Yuan RHu T Zhang YHan X Xue and J Li ldquoPrecipitationBehavior of 120590-FeCr Phases in Hastelloy C-2000 SuperalloyUnder Plastic Deformation and Aging Treatmentrdquo Journal ofMaterials Engineering and Performance vol 24 no 2 pp 565ndash571 2015

[16] CM Kuo Y T Yang H Y Bor C NWei and C C Tai ldquoAgingeffects on the microstructure and creep behavior of inconel718 superalloyrdquoMaterials Science and Engineering A StructuralMaterials PropertiesMicrostructure and Processing vol 510-511pp 289ndash294 2009

[17] K ke-hsin ldquoPhases in high alloy steels and superalloysrdquo ActaMetallurgica Sinica (English Letters) vol 14 pp 73ndash95 1978httpwwwcnkicomcnArticleCJFDTotal-JSXB197801008htm7395

[18] Anon ldquoStandard test methods of detecting susceptibilityto intergranular attack in wrought nickel-rich chromium-bearing alloysrdquo in Proceedings of the Laboratory Corrosion Testsand Standards pp 527ndash533

[19] A Fick ldquoUeber diffusionrdquoAnnalen der Physik vol 170 no 1 pp59ndash86 1855

[20] W Boesch and H Canada ldquoPrecipitation Reactions and Sta-bility of Ni3Cb in Inconel Alloy 718rdquo in Proceedings of theSuperalloys pp 579ndash596 September 1968

[21] M Prohaska H Kanduth G Mori R Grill and G TischlerldquoOn the substitution of conventional corrosion tests by anelectrochemical potentiokinetic reactivation testrdquo CorrosionScience vol 52 no 5 pp 1582ndash1592 2010

[22] X Di X Liu C Chen B Wang and X Guo ldquoEffect of Post-weld Heat Treatment on the Microstructure and CorrosionResistance of Deposited Metal of a High-Chromium Nickel-Based Alloyrdquo Acta Metallurgica Sinica (English Letters) vol 29no 12 pp 1136ndash1143 2016

[23] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939

[24] M Avrami ldquoKinetics of phase change 2rdquo The Journal ofChemical Physics vol 7 no 12 pp 1103ndash1112 1939

[25] W A Johnson and R F Mehl ldquoReaction Kinetics in Process ofNucleation and Growthrdquo Journal of Chemical Physics vol 8 pp212ndash1112 1940

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Table 1Themain phases occurring in long-term aging nickel-basedsuperalloy

Phase Structure Typical Composition

12057410158401015840 body-centeredtetragonal Ni3(Nbgt05Tilt05Allt05Molt05)

120575 orthorhombic Ni3NbMC cubic (NbTi)CM6C cubic (Crgt02Mogt02Fe)CM23C6 cubic (Crgt08FeMo)CLaves MgZn2-Types(AB2) TiFe2 NbFe2 MoFe2 etc120583 Hexagonal(B7A6) Fe7Mo6 Fe7Nb6 Fe7Mo6 etc

120590 Tetragonal(BA) (CrMo)x(NiFe)y FeCr MoFeNbFe etc

Table 2 Chemical composition of Alloy 625 (wt)

C Cr Mo Co Nb+Ta Fe Al Ti Si P S Ni005 224 95 003 366 001 024 023 004 0002 0001 631

is listed in Table 2 The microstructure of Alloy 625 is shownin Figure 1 It can be seen that Alloy 625 has a single-phaseaustenite with carbide particles (black cross) distributed onthe surfaceThe specimens of 50times 24times 5 (plusmn03)mm for inter-granular corrosion investigations were machined from thehot rolled sheets All the specimens were solution annealed at1140∘C for 45 h in an argon protective atmosphere followedby water quenching Stabilizing treatment was performed ata temperature of 1000∘C for 65 h in an evacuated tube andcooling to room temperature in air Aging treatment wasconducted at a temperature of 1000∘C 750∘C for 40 h 200 hand 1000 h Electrolytic etching with 10 oxalic acid solutionwas used to reveal the microstructure in the present study

Intergranular corrosion tests were carried out in glasscontainer containing 236mLH2SO4 25g reagent grade ferricsulfate (containing about 75 Fe2(SO4)3) and 400mL wateraccording to ASTM G28A-02 [18]The test was carried out inthe boiling solution and the testing durationwas 120 h All thespecimens for immersion tests were weighed using analyticalbalance with sensitivity of 001 mg before and after corrosiontests The mass loss rate (mpy) with units of mmyear wascalculated as follows

mpy = Δ119882120588119878119905 times 24 times 365 (1)

whereΔ119882 is the weight loss of specimens (g) 120588 is the densityof the stainless steels (gmm3) S is the total corrosion area(mm2) and t is the time of exposure (h)

After metallographic etching or intergranular corrosiontests the specimens were rinsed with deionized water andalcohol and then dried The microstructural characteristicsand intergranular corrosion morphologies were examinedusing a scanning electronic microscope (SEM Zeiss MER-LIN) equipped with an energy dispersive spectrum analyzerThe phase analysis was performed using an X-ray diffraction(XRD D8 Advance A25X) analyzer

Wt At

Mo 968 344Nb 6433 2363Ti 047 034C 2552 7258

Figure 1 Microstructure of solution alloy by means of SEM

3 Result and Discussion

31 Microstructure Characterization The composition of theprecipitated phase in Figure 2(b) was analyzed using theenergy dispersive X-ray (EDX) Figures 2(a) and 2(b) showthe morphology of Alloy 625 after stabilized treatment Itcan be seen that after the stabilized treatment the carbideis preferentially precipitated at the grain boundary and twinboundary Discontinuous and irregular precipitated phasedistributed in grain boundary is (Mo Nb)CThe precipitatedphase at the triple line boundary is (NbTi)C In additiona localized enrichment of C and Nb was observed at thegrain boundaries in Figures 2(b) 3(a) 2(h) and 3(b) In thesubsequent aging process these carbide particles can preventthe migration of grain boundary However there is also thenucleation site for the precipitated phase After 40 h agingtreatment granular like 12057410158401015840-Ni3Nb phases are present at thegrain boundary and twin boundary as shown in Figures2(c) and 2(d) With the increasing of aging time the densityof metastable 12057410158401015840 phase increases the coarsening of whichoccurred The metastable 12057410158401015840 phase gradually transforms tothe stable 120575 phase After 200 h aging treatment the needle-like 120575 is preferentially precipitated at the grain boundaryand then grown into the inside of the grain as shown inFigures 2(c) and 2(d) Also 12057410158401015840 is continuously precipitated inthe grain and coursing The number of needle-like 120575 phasesincreased significantly after 1000 h aging treatment as shownin Figure 2(g) Also 12057410158401015840 phase can be observed inside thegrains

The precipitated phase of Alloy 625 during aging sensi-tization at 750∘C for different time was determined by X-ray diffraction (XRD) the composition of which is listedin Table 3 The XRD profiles of Alloy 625 after aging sen-sitization for different time are shown in Figure 4 FromFigure 4 it can be seen that the peak appeared at 350∘ and407∘ corresponding to the diffraction peak of NbC afterstabilization After 40h of aging the peak appeared at 423∘corresponding to the diffraction peak of 12057410158401015840-Ni3Nb Becausethe amount of precipitation was small the peak intensity


(a)

+ 1

+ 2

+ 3

4

(b)

(c)

-NＣ3Nb

(d)

(e)

-NＣ3Nb-NＣ3Nb

(f)

(g)

8

+ 6

+ 7

+ 5

(h)



0 1 2 3 4 52468

101210

20

3010203040506010

20

30

400

10

20

0 1 2 3 4 5

Ni

Cr

Mo

Nb

C

Distance (m)

(a)

0 1 2 3 4 52468

101210

20

3010203040506010

20

30

400

10

20

0 1 2 3 4 5

Ni

Cr

Mo

Nb

C

Distance (m)

(b)







119898119901119910 = 9020 minus 4432 exp (minus 1199051099)

minus 4452 exp (minus 1199053278)

(2)



20 30 40 50 60 70 80 90 100

Aging 1000h

Aging 200h

Aging 40h

34 35 36 37 36 38 40 42 44 46 48 50

NbC

Ni-Base

Inte

nsity

Non Aging

2 Theta (∘)

Ｃ3＜


0 100 200 300 400 500 600 700 800 900 1000

0

20

40

60

80

100

Aging Time (h)

mpyfitted line

Wei

ght L

oss R

ate (

mmmiddot

minus1)




4 Discussion


120597C120597t = 119863

1205972C1205971199092 (3)


(a) (b)

(c) (d)

(e) (f)

-NiNb Dissolved

-NiNb Dissolved



119905 = 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 gt 0

119905 gt 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 = +infin

(4)



120573

0exp (minus1205732) 119889120573

(120573 = 1199092radic119863119905)

(5)




(a) (b)

(c) (d)







Depleted-Zone

x


Allo

y El

emen

t Con

cent

ratio


Passivation Film

0

０

x40 x200 x1000





++ +

++ +

+

Get electrons

[M]

+

Lost electrons


Non-exposure

+

Dissolution

Material

Internal

Dissolved

+ + +

n

ndashNiNb

ndashNiNb

minus

minus

minus

minus

minus

minusFe+

lowastM Ni Cr Mo Nb


Fe+

Mn+



119891 = 1 minus exp (minus119896119905119899) (7)




(8)



119898119901119910 = 119886119891119894119901 + 119887119891120575 + 119888 (119898119898119886) (9)


119898119901119910 = 4432 [1 minus exp (minus 1199051099)]

+ 4462 [1 minus exp (minus 1199053278)]

+ 1267 (119898119898119886)

(10)

119886 = 4432 119887 = 4462 119896119894119901 = 11099 119896120575 = 13278 1198991 = 1and 1198992 = 1

5 Conclusion





Data Availability




Acknowledgments



References




























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





(a)

+ 1

+ 2

+ 3

4

(b)

(c)

-NＣ3Nb

(d)

(e)

-NＣ3Nb-NＣ3Nb

(f)

(g)

8

+ 6

+ 7

+ 5

(h)



0 1 2 3 4 52468

101210

20

3010203040506010

20

30

400

10

20

0 1 2 3 4 5

Ni

Cr

Mo

Nb

C

Distance (m)

(a)

0 1 2 3 4 52468

101210

20

3010203040506010

20

30

400

10

20

0 1 2 3 4 5

Ni

Cr

Mo

Nb

C

Distance (m)

(b)







119898119901119910 = 9020 minus 4432 exp (minus 1199051099)

minus 4452 exp (minus 1199053278)

(2)



20 30 40 50 60 70 80 90 100

Aging 1000h

Aging 200h

Aging 40h

34 35 36 37 36 38 40 42 44 46 48 50

NbC

Ni-Base

Inte

nsity

Non Aging

2 Theta (∘)

Ｃ3＜


0 100 200 300 400 500 600 700 800 900 1000

0

20

40

60

80

100

Aging Time (h)

mpyfitted line

Wei

ght L

oss R

ate (

mmmiddot

minus1)




4 Discussion


120597C120597t = 119863

1205972C1205971199092 (3)


(a) (b)

(c) (d)

(e) (f)

-NiNb Dissolved

-NiNb Dissolved



119905 = 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 gt 0

119905 gt 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 = +infin

(4)



120573

0exp (minus1205732) 119889120573

(120573 = 1199092radic119863119905)

(5)




(a) (b)

(c) (d)







Depleted-Zone

x


Allo

y El

emen

t Con

cent

ratio


Passivation Film

0

０

x40 x200 x1000





++ +

++ +

+

Get electrons

[M]

+

Lost electrons


Non-exposure

+

Dissolution

Material

Internal

Dissolved

+ + +

n

ndashNiNb

ndashNiNb

minus

minus

minus

minus

minus

minusFe+

lowastM Ni Cr Mo Nb


Fe+

Mn+



119891 = 1 minus exp (minus119896119905119899) (7)




(8)



119898119901119910 = 119886119891119894119901 + 119887119891120575 + 119888 (119898119898119886) (9)


119898119901119910 = 4432 [1 minus exp (minus 1199051099)]

+ 4462 [1 minus exp (minus 1199053278)]

+ 1267 (119898119898119886)

(10)

119886 = 4432 119887 = 4462 119896119894119901 = 11099 119896120575 = 13278 1198991 = 1and 1198992 = 1

5 Conclusion





Data Availability




Acknowledgments



References




























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





0 1 2 3 4 52468

101210

20

3010203040506010

20

30

400

10

20

0 1 2 3 4 5

Ni

Cr

Mo

Nb

C

Distance (m)

(a)

0 1 2 3 4 52468

101210

20

3010203040506010

20

30

400

10

20

0 1 2 3 4 5

Ni

Cr

Mo

Nb

C

Distance (m)

(b)







119898119901119910 = 9020 minus 4432 exp (minus 1199051099)

minus 4452 exp (minus 1199053278)

(2)



20 30 40 50 60 70 80 90 100

Aging 1000h

Aging 200h

Aging 40h

34 35 36 37 36 38 40 42 44 46 48 50

NbC

Ni-Base

Inte

nsity

Non Aging

2 Theta (∘)

Ｃ3＜


0 100 200 300 400 500 600 700 800 900 1000

0

20

40

60

80

100

Aging Time (h)

mpyfitted line

Wei

ght L

oss R

ate (

mmmiddot

minus1)




4 Discussion


120597C120597t = 119863

1205972C1205971199092 (3)


(a) (b)

(c) (d)

(e) (f)

-NiNb Dissolved

-NiNb Dissolved



119905 = 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 gt 0

119905 gt 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 = +infin

(4)



120573

0exp (minus1205732) 119889120573

(120573 = 1199092radic119863119905)

(5)




(a) (b)

(c) (d)







Depleted-Zone

x


Allo

y El

emen

t Con

cent

ratio


Passivation Film

0

０

x40 x200 x1000





++ +

++ +

+

Get electrons

[M]

+

Lost electrons


Non-exposure

+

Dissolution

Material

Internal

Dissolved

+ + +

n

ndashNiNb

ndashNiNb

minus

minus

minus

minus

minus

minusFe+

lowastM Ni Cr Mo Nb


Fe+

Mn+



119891 = 1 minus exp (minus119896119905119899) (7)




(8)



119898119901119910 = 119886119891119894119901 + 119887119891120575 + 119888 (119898119898119886) (9)


119898119901119910 = 4432 [1 minus exp (minus 1199051099)]

+ 4462 [1 minus exp (minus 1199053278)]

+ 1267 (119898119898119886)

(10)

119886 = 4432 119887 = 4462 119896119894119901 = 11099 119896120575 = 13278 1198991 = 1and 1198992 = 1

5 Conclusion





Data Availability




Acknowledgments



References




























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





20 30 40 50 60 70 80 90 100

Aging 1000h

Aging 200h

Aging 40h

34 35 36 37 36 38 40 42 44 46 48 50

NbC

Ni-Base

Inte

nsity

Non Aging

2 Theta (∘)

Ｃ3＜


0 100 200 300 400 500 600 700 800 900 1000

0

20

40

60

80

100

Aging Time (h)

mpyfitted line

Wei

ght L

oss R

ate (

mmmiddot

minus1)




4 Discussion


120597C120597t = 119863

1205972C1205971199092 (3)


(a) (b)

(c) (d)

(e) (f)

-NiNb Dissolved

-NiNb Dissolved



119905 = 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 gt 0

119905 gt 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 = +infin

(4)



120573

0exp (minus1205732) 119889120573

(120573 = 1199092radic119863119905)

(5)




(a) (b)

(c) (d)







Depleted-Zone

x


Allo

y El

emen

t Con

cent

ratio


Passivation Film

0

０

x40 x200 x1000





++ +

++ +

+

Get electrons

[M]

+

Lost electrons


Non-exposure

+

Dissolution

Material

Internal

Dissolved

+ + +

n

ndashNiNb

ndashNiNb

minus

minus

minus

minus

minus

minusFe+

lowastM Ni Cr Mo Nb


Fe+

Mn+



119891 = 1 minus exp (minus119896119905119899) (7)




(8)



119898119901119910 = 119886119891119894119901 + 119887119891120575 + 119888 (119898119898119886) (9)


119898119901119910 = 4432 [1 minus exp (minus 1199051099)]

+ 4462 [1 minus exp (minus 1199053278)]

+ 1267 (119898119898119886)

(10)

119886 = 4432 119887 = 4462 119896119894119901 = 11099 119896120575 = 13278 1198991 = 1and 1198992 = 1

5 Conclusion





Data Availability




Acknowledgments



References




























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





(a) (b)

(c) (d)

(e) (f)

-NiNb Dissolved

-NiNb Dissolved



119905 = 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 gt 0

119905 gt 0119862 = 119862119875 119909 = 0119862 = 1198620119866 119909 = +infin

(4)



120573

0exp (minus1205732) 119889120573

(120573 = 1199092radic119863119905)

(5)




(a) (b)

(c) (d)







Depleted-Zone

x


Allo

y El

emen

t Con

cent

ratio


Passivation Film

0

０

x40 x200 x1000





++ +

++ +

+

Get electrons

[M]

+

Lost electrons


Non-exposure

+

Dissolution

Material

Internal

Dissolved

+ + +

n

ndashNiNb

ndashNiNb

minus

minus

minus

minus

minus

minusFe+

lowastM Ni Cr Mo Nb


Fe+

Mn+



119891 = 1 minus exp (minus119896119905119899) (7)




(8)



119898119901119910 = 119886119891119894119901 + 119887119891120575 + 119888 (119898119898119886) (9)


119898119901119910 = 4432 [1 minus exp (minus 1199051099)]

+ 4462 [1 minus exp (minus 1199053278)]

+ 1267 (119898119898119886)

(10)

119886 = 4432 119887 = 4462 119896119894119901 = 11099 119896120575 = 13278 1198991 = 1and 1198992 = 1

5 Conclusion





Data Availability




Acknowledgments



References




























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





(a) (b)

(c) (d)







Depleted-Zone

x


Allo

y El

emen

t Con

cent

ratio


Passivation Film

0

０

x40 x200 x1000





++ +

++ +

+

Get electrons

[M]

+

Lost electrons


Non-exposure

+

Dissolution

Material

Internal

Dissolved

+ + +

n

ndashNiNb

ndashNiNb

minus

minus

minus

minus

minus

minusFe+

lowastM Ni Cr Mo Nb


Fe+

Mn+



119891 = 1 minus exp (minus119896119905119899) (7)




(8)



119898119901119910 = 119886119891119894119901 + 119887119891120575 + 119888 (119898119898119886) (9)


119898119901119910 = 4432 [1 minus exp (minus 1199051099)]

+ 4462 [1 minus exp (minus 1199053278)]

+ 1267 (119898119898119886)

(10)

119886 = 4432 119887 = 4462 119896119894119901 = 11099 119896120575 = 13278 1198991 = 1and 1198992 = 1

5 Conclusion





Data Availability




Acknowledgments



References




























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






++ +

++ +

+

Get electrons

[M]

+

Lost electrons


Non-exposure

+

Dissolution

Material

Internal

Dissolved

+ + +

n

ndashNiNb

ndashNiNb

minus

minus

minus

minus

minus

minusFe+

lowastM Ni Cr Mo Nb


Fe+

Mn+



119891 = 1 minus exp (minus119896119905119899) (7)




(8)



119898119901119910 = 119886119891119894119901 + 119887119891120575 + 119888 (119898119898119886) (9)


119898119901119910 = 4432 [1 minus exp (minus 1199051099)]

+ 4462 [1 minus exp (minus 1199053278)]

+ 1267 (119898119898119886)

(10)

119886 = 4432 119887 = 4462 119896119894119901 = 11099 119896120575 = 13278 1198991 = 1and 1198992 = 1

5 Conclusion





Data Availability




Acknowledgments



References




























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





References




























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls




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