Long term aging treatment impact on ultralow iron Alloy 625 intergranular corrosion property

This paper investigates the evolution of microstructures and precipitations of an ultra-low iron alloy 625 subjected to long term aging treatment by scanning electron microscope (SEM) and X-ray diffraction(XRD). Use ASTM G28A acid Fe3(SO4)2 erosion to represent intergranular corrosion weightlessness and corrosive morphology. The result shows that alloy at 750°C by aging treatment for 40h, precipitated γ'' phase in the grain boundary. In high density area of γ'' phase, occurs γ'' phase to δ phase degeneration transformation by aging treatment for 200h and the needle-like δ phase becomes more with time prolonged. And γ'' phase degenerated to δ phase completely until treated for 1000h. The sample which has aging treatment tends to have intergranular corrosion and mainly because alloy element spreading leads to dilution area and grain boundary precipitated phase, plus interlaced δ phase’s dissolving, which makes sample grain particle fall off and this results in apparent weightlessness. The weightlessness rate(WLR) is related with precipitated volume score. With aging sensitization time change, can be described by Johnson-Mehl-Avrami equation, i.e.: = 44.32 1 − exp (− . ) + 44.62 1 − exp (− . ) + 1.267 ( / ).


Introduction
Nickle base alloy has good strengthens, ductility and toughness, welding and anticorrosive property.It is widely used in oceaneering welded structure, petrochemical engineering, energy and power evaporator [1,2] and its reinforcement theory is widely studied and reported, so we are familiar with it [3].Among this, Alloy 625 is considered as the ideal material of main parts for the 4 th generation of nuclear station [4,5].Alloy's aging and creep property is an important parameter to ensure nuclear station's safety operation.Alloy 625 can precipitate Ni3Nb and carbide [7][8][9] in high temperature by long term.γ''-Ni3Nb precipitation and prolonged heat keeping can degenerate to needlelike δ-Ni3Nb [8].In the evolution, alloy is hardened [9], ductility and toughness is decreased [7], element spread and separate out, Alloy 625 is sensitized.In oxidant transmitter, Ni3Nb with low Cr value as anode is corroded [10].In addition, carbide and dilution area cause intergranular corrosion tendency [11][12][13][14].Pay attention to alloy's mechanics and anti-corrosion property's change during aging process can increase structure's stability, which is an effective way to prolong high-temperature components' life.However, in the long term of aging and sensitization process, precipitation phase generation and conversion impact on alloy's anti-corrosion property is rarely investigated.In present study, Alloy 625 (ASTM B446) contains 2-5%wt Fe, which can solution strengthen basal body to increase strength.However, there is Topological Close-Packed Phase in long term aging, such as Laves phase, μ phase and σ phase [8,16].As shows in Table 1, it is board-like or needle-like shape, which decreases alloy's breakage strength and ductility [8,17].Besides, Si facilitates harmful phase [15].Excessive use of it in nuclear material will has hide safety risk.
The Alloy 625 used here optimizes ASTM B446's element composition, controls Fe lower than 0.05%wt.in addition, Si, P and S percentages are seriously controlled to prevent rich-iron harmful phase occurring and make a new type of ultralow iron Alloy 625.This paper makes characterization analysis of precipitation phase transformation and intergranular anti-corrosion property of ultralow iron Alloy 625 in long term aging sensitization process.

Experiment material and method
Compared with ASTM B446, Alloy 625 has strict control of Fe percentage to avoid Fe causing property becoming deterioration in the aging process.The experiment used alloy is provided by Jiangsu XinHua Alloy Electric CO., LTD..It is made by the process of vacuum melting and electroslag re-melting duplex, and forged in 1120-1030°C, then is cutted by electro-spark wire.After this, it is solid solution treated in 1140°C for 4.5h, water cooling, stabilizing treated in 1000 °C for 6.5h, air cooling.Its component shows in Table 2: Aging treatment is in 750°C, heat preservation for 1000h, then the alloy is grinded and polished.Adopt D8 Advance A25X to do XRD experiment.The polished sample is corroded in 2vol.%bromine methyl alcohol and use Zeiss MERLIN Compact field to send SEM for observing appearance.The sample size for intergranular corrosion sensibility test is 50×24×5(±0.3)mm.Re-measure the size after surface refined grinding.As per ASTM G28A-02 Standard Test Methods of Detecting Susceptibility to Intergranular Corrosion in Wrought, Nickel-Rich, Chromium-Bearing Alloy Method A-Ferric Sulfate-Sulfuric Acid Test [18] requirement, it is corroded in H2SO4+Fe2(SO4)3 solution for 120h, record the weightlessness data, and draw curve.Then observe corrosive surface by SEM after experiment.Cut sample and is packed and inlayed by epoxy resin, the cross profile of sample is eroded in 10% oxalic acid solution after grinding and polishing until the surface structure is rightly exposed.Spray metal on surface before test for better imaging quality.Compare and analyze structure impact on corrosion process.carbide particles prevent grain boundary migration.While provides nucleation locus for precipitated phase [6].

Experiment result and analysis
Observing aging treatment sample can see that small quantity and wee γ'' phase is separated out in grain boundary and twin-boundary after 40h aging treatment with the help of defected nucleation as shown in Fig 1 (b).With prolonged aging, sub-stability γ'' phase's density is increasing and coarsening, also at the same converts to stable δ phase.As shown in Fig    At750°C aging process, compared with ferrous Alloy 625, γ'' 'phase precipitation and γ'' →δ phase is delayed, which have good organizational stability [8,19,20].In the process of aging, the generation and evolution of γ'' phase, δ phase, precipitation in the grain boundary and alloy element in the grain boundary affects the performance of the alloy [10,21,22].

intergranular corrosion experiment
Alloy 625 by means of aging treatment for different time, as per ASTM G28A-02 Standard Test Methods of Detecting Susceptibility to Intergranular Corrosion in Wrought, Nickel-Rich, Chromium-Bearing Alloy Method A-Ferric Sulfate-Sulfuric Acid Test [18] requirement, it is corroded in boiled H2SO4+Fe2(SO4)3 solution for 120h and draw weightlessness curve to observe surface and cross-section feature.
Fe 3+ of boiled acid Fe2(SO4)3 solution can make the nickel substrate passivation and restrain comprehensive corrosion in H2SO4.Only can dissolve precipitation [23] in grain boundary element's dilution zone.The aging sensitized alloy material in this corrosion system occurs strong intergranular corrosion and causes grain fall off, generates obvious weightlessness.Weightlessness rate in different aging sensitization time is shown in Fig 4.

Discussion
In erosion process in H2SO4+Fe2(SO4)3 solution, Fe 3+ can prevent alloy's complete corrosion in H2SO4.Only erode dilution zone of grain boundary.Aging sensitized sample occurs strong intergranular corrosion and grain fall off in solution.Intergranular corrosion sensitivity can be evaluated by weightlessness method.
At 750°C aging sensitization process, Cr, Mo separates from the grain boundary of the solid solution.Its carbide and alloy phase precipitates along the grain boundary, which makes the continuous distribution of grain boundary precipitates.Due to the Cr's diffusion rate is far lower than C, Cr can't diffuse timely from solution and add to grain boundary.Therefore, has to consume Cr nearby grain boundary.This causes poor Cr area of grain boundary [13].Cr content value of poor Cr area is far below the limit value of passivation needed, its potential is lower than grain's internal potential, much lower than the precipitated phase of grain boundary.Poor Cr area connects closely with precipitated phase of grain boundary.Short circuit cell effect occurs when come across with corrosive medium and poor Cr area is eroded rapidly [23,24].Use Fick's second law to calculate width and precipitated amount of alloy element in dilution zone: = Determine the initial and boundary conditions: Use Boltzmann transform and Gauss error function, get the result： Among them: -precipitation phase of grain boundary and alloy element concentration of substrate's interface; non-sensitized alloy's concentration of interior part; -distance to grain boundary; D-alloy element diffusion coefficient; tsensitization time.
Alloy  , with the extension of aging time, the grain boundary space formed by the corrosion begins in surface carbide or alloy phase.It grows internally and surrounds the entire grain, which makes the grain fall off.This belongs to the typical intergranular corrosion.In addition, as the extension of sensitization time, precipitated phase forms and γ'' phase that exposed to the corrosive medium is eroded but doesn't develop to the grain interior.δ phase is dissolved because can't form passivation film and presents the glacier surface.Carbide precipitated in grain boundary and intermetallic in the corrosion process are also dissolved because electric potential is lower than the grain interior pare, as shown in Fig 8. with alloy element diffusion [13].With the extension of aging time, after aging for 200h, needle-like δ phase generates which can't form passivation film [10].It will make the grain crush and spall, WLR rises further.However, in the acidic ferric sulfate erosion experiments, WLR is not direct proportional to the square root of the sensitization time.
Especially within 200h, no needle-like δ phase affect, i.e.WLR can't only be explained by diffusion theory.Considering the gravity erosion mainly is grain fall off.when the grain boundary is completely dissolved, the grain falls off, including δ phase, grain boundary precipitation, elements' dilution zone.Elements' dilution area is associated with and accompanies grain boundary precipitation formation, which can be considered as an item.Erosion weightlessness should be proportional to the number of precipitated phase.Precipitated phase nucleation grows big can represented by Johnson-Mehl-Avrami(JMA) equations [25].This can further get relationship between erosion WLR and aging sensitization time.JMA equation [26][27][28]: ) Among them: f-precipitated phase relative volume fraction: t-time and infinite long exhalation phase volume ratio; k-independent constant related to the temperature; n-the power exponent of time, the independent constant from 1~4.
Take element dilution zone which is non-passivation film and its formed the grain boundary precipitated phase as one independent phase, due their formation is related and linked with each other, can get: Among them: -precipitated phase and grain boundary element dilution zone relative volume fraction; -needle-like δ phase relative volume fraction.
In addition, erosion weightlessness rate: = 44.32，= 44.62， It is visible that WLR weight values a is similar to b. Precipitate phase of dilution zone and needle-like phase dissolving makes similar contribution to erosion weightlessness.In addition, by： exp(−0.7)= 0.5 (7) Can get a 50% shift time ., = 7.693ℎ, ., = 229.5ℎ .Because a small amount of precipitated phase can cause the obvious change of the corrosion process, lead to fast grain spalling.Use precipitated phase to erosion weightlessness affect to represent precipitation.This increased precipitation rate excessively.Precipitated phase at this time is far from 50%.However, dilution zone and intergranular precipitate phase forms in the early aging stage.Needle-like δ phase precipitates at about 200h, which is matched with Fig 1,6.Using ASTM G28A Ferric Sulfate-Sulfuric Acid Test measurement 'can reflect the experiment used materials' intergranular corrosion property change with aging sensitization treatment time.Because precipitated phase and dilution zone in the process of erosion cause grain spalling, which can be explained by precipitation phase transformation volume fraction.

4.Conclusion
Sample alloy that is stable after solid solution, observe its structure change by different aging sensitization treatment at 750 ° C, using ASTM G28A -Ferric Sulfate-Sulfuric Acid Test to represent alloy's anti-intergranular corrosion change in aging sensitization process, and analyze and establish weightlessness model of intergranular corrosion, then get the low conclusions: 1) Use XRD and SEM to determine interior alloy aging precipitation behavior: precipitates small γ''-Ni3Nb by aging 40h at 750 ° C, generate transformation of γ''→δ-Ni3Nb by aging for 200h.By extension of long aging treatment, needle-like δ-Ni3Nb increases.In aging process, grain boundary carbides and intermetallic compound forms alloy elements dilution zone.
2) ASTM G28A erosion result shows that samples without aging sensitization treatment, only carbides of grain boundary dissolves.Its WLR is 1.267 mm/a with good corrosion resistance.In early aging sensitization, in grain boundary elements (Cr, Mo) dilution zone, intergranular precipitates dissolves, grain falls off and sample WLR increases rapidly.With formation and precipitation of needle-like δ-Ni3Nb phase that unable to form passivation film, grain crushes and spalls.Present a glacial topography in surface and alloy samples WLR raises further.
3) Element dilution zone and precipitated phase dissolving causes quickly fall off of grain.Curve of ASTM G28A erosion WLR changes with aging sensitization time can be explained by precipitation phase transformation volume fraction Johnson-Mehl-

2. 1 Fig. 1
Fig.1 Microstructure of different aging sensitization alloy by means of SEM (a) 0h aging (b)40h aging (c)200h aging (d)1000h aging 1 (c) by treated 200h, there is δ phase in grain boundary with needle-like shape and grows to intergranular from intergranular.Inside grain, γ'' phase is separated out constantly with coarsening.As shown in Fig 1 (d) by treated 1000h, δ phase is separated out continuously with bigger volume.Inside grain, short and needlelike shape precipitated phase becomes coarse γ'' phase.In addition, appears big particle of carbide in triple line boundary by aging treatment for 1000h and EDS shows (Nb,Ti)C.

Fig. 2
Fig.2 Microstructure of alloy by means of SEM and EDS (a) 0h aging sensitization (b) 1000h aging sensitization .preprints.org) | NOT PEER-REVIEWED | Posted: 29 September 2017 doi:10.20944/preprints201709.0149.v1As shown in Fig 3 at 750°C, XRD pattern of samples with different aging time, compared with Fig 1, precipitation NbC is detected in the process of stabilizing treatment.Aging treatment for 40 hours, there is tiny γ''-Ni3Nb and generates smaller diffraction intensity.γ''-Ni3Nb is being precipitated gradually and degenerated into needle-like δ-Ni3Nb by aging treatment for 200h.As shown in Fig, Ni3Nb diffraction intensity increases.Aging treatment for 1000h, there is more precipitation and precipitated phase's diffraction intensity increase further.X-ray diffraction pattern of samples with different aging time is matched with alloy's microstructure change in Fig 1, 2.

1 )
Fig 4 Alloy's weightlessness rate in different aging sensitization time

Fig. 5
Fig.5 Surface of different aging sensitization alloy corrosion by ASTM G28A (a)0h aging sensitization (b)40h (c)200h (d)1000h (e) High multiple of marked zone in (c) (f) High multiple of marked zone in (d)

Fig. 6
Fig.6 Section of different aging sensitization alloy corrosion by ASTM G28A (a)0h aging sensitization (b)40h (c)200h (d)100h (e)Grain boundary dissolves in 200h sample marked zone in (c) (f)Dissolves of δ phase in 1000h sample marked zone in (d) element of grain boundary precipitation by different sensitization time causes dilution zone's composition change, qualitative results are shown in Fig 7, among them, dilution zone width of non-generation of the passivation film after different aging sensitization time: = √ = ，which is in direct proportion to the square root of the sensitization time, i.e. the dilution zone volume obeys the rule of parabola.

Fig 7
Fig 7 Grain boundary alloy element's dilution zone content change by different sensitization time As can be seen from the Fig 5 ~ 6, with the extension of aging time, the grain boundary space formed by the corrosion begins in surface carbide or alloy phase.It grows internally and surrounds the entire grain, which makes the grain fall off.This belongs to the typical intergranular corrosion.In addition, as the extension of sensitization time, precipitated phase forms and γ'' phase that exposed to the corrosive medium is eroded but doesn't develop to the grain interior.δ phase is dissolved because can't form passivation film and presents the glacier surface.Carbide precipitated in grain boundary and intermetallic in the corrosion process are also dissolved because electric potential is lower than the grain interior pare, as shown inFig 8.

Table 3 .
Composition at some topical points, as obtained by EDS analysis (wt%) precipitate combined with element composition, which is measured by Energy Disperse Spectroscopy (EDS).After stabilizing treatment without aging treatment, grain boundary condition as shown in Fig2 (a), EDS analysis shows that the discrete irregular carbides that distribute in the grain boundary are Mo, Nb composite carbides.Granule (Nb, Ti) C exists in triple line boundary.Scanned across the grain boundary line shows that C, Nb has enrichment in grain boundary, i.e. generate NbC grain and Mo, Cr element is not found obvious enrichment.Aging treatment for 1000h, needle-like δ phase has a large amount of precipitation.It starts in the grain boundary with the same growth direction.Coarse γ'' phase still can be seen.Besides, the grain boundary precipitates are continuous, generates carbides with high content of Cr and it is distributed in triple line boundary.Meanwhile, alloy with Mo also can be discovered.Cross scanned result shows that C, Mo, Cr elements are enriched in the grain boundary.Because Nb element precipitate δ phase intergranular besides grain boundary carbides precipitation and it forms many peaks in scanning result.
Fig 3 XRD pattern of samples with different aging time at 750°C Use higher multiples microscope to observe grain boundary, and analyze

Table 4 .
Composition at some topical points, as obtained by EDS analysis (wt%)