Effect of Aging Treatment on Intergranular Corrosion Properties of Ultra-Low Iron 625 Alloy

The microstructures evolution of precipitations for an ultra-low iron Alloy 625 subjected to long term aging treatment at 750C was investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD). The intergranular corrosion behaviors of Alloy 625 were evaluated by using ASTM G28A. The result shows that the precipitated phase γ-Ni3Nb was mainly precipitated at the grain boundaries and twin boundaries. The number and volume fraction of γ󸀠󸀠 increased with the prolonging of aging time. The transformation of γ󸀠󸀠 to δ-Ni3Nb occurred after aging periods of 200 h.The corrosion resistance of Alloy 625 was significantly reduced during aging treatment. The decrease in intergranular corrosion resistance of Alloy 625 was attributed to the dissolution of precipitated phase and chromium depleted zone. Themass loss rate of Alloy 625 after aging treatment is related to the volume of precipitated phase and can be simulated by Johnson-Mehl-Avrami equation.


Introduction
Because of the excellent properties such as good strength, ductility and toughness, welding, and corrosion resistance, nickel-base alloy is widely used in naval architecture and ocean engineering, petroleum refining and petrochemical industries, and energy and power industry [1][2][3].As a kind of nickel-based alloy, Alloy 625 seems to be the suitable material used in the 4 th generation of nuclear power systems [4,5] owing to its attractive features.Alloy 625 has the problem of precipitate  -Ni 3 Nb and carbide [6][7][8][9] in high temperature by long term aging.The aging and creep properties of alloys are important parameters for safety operation of component made of Alloy 625.During the long term aging, the   phase can gradually transformed into needle-like -Ni 3 Nb leading to the increasing of the hardness [10] and decreasing of the ductility and toughness.Because of its high potential, preferential corrosion usually occurs on Ni 3 Nb in oxidizing media.At the same time, intergranular corrosion can also occur in the area of carbide and chromium-depleted zone at grain boundaries [11][12][13][14].However, the work concentrated on the effect of formation and transformation of precipitated phase on corrosion behaviour of nickel-based alloy during the long term aging is rarely reported.
Commercial Alloy 625 usually contains 2-5% wt Fe and Fe atoms are present in the matrix in solid solution.The Topological Close-Packed Phase such as Laves phase,  phase, and  phase [15], as shown in Table 1, will be precipitated in the long aging.These phases are plate-shaped or needle-shaped, leading to the reduction of fracture strength and plasticity [16].In addition, Si can also facilitate the precipitation of harmful phase [17] leading to the decrease of the service life of components.
The aim of present study was focused on the effect of aging treatment on precipitation behaviour and intergranular corrosion properties of a newly developed ultra-low iron Alloy 625.

Experimental
The Alloy 625 used in present work is provided by Jiangsu XinHua Alloy Electric CO., LTD., the composition of which  is listed in Table 2.The microstructure of Alloy 625 is shown in Figure 1.It can be seen that Alloy 625 has a single-phase austenite with carbide particles (black cross) distributed on the surface.The specimens of 50 × 24 × 5 (±0.3) mm for intergranular corrosion investigations were machined from the hot rolled sheets.All the specimens were solution annealed at 1140 ∘ C for 4.5 h in an argon protective atmosphere followed by water quenching.Stabilizing treatment was performed at a temperature of 1000 ∘ C for 6.5 h in an evacuated tube and cooling to room temperature in air.Aging treatment was conducted at a temperature of 1000 ∘ C 750 ∘ C for 40 h, 200 h, and 1000 h.Electrolytic etching with 10% oxalic acid solution was used to reveal the microstructure in the present study.Intergranular corrosion tests were carried out in glass container containing 236 mL H 2 SO 4 , 25g reagent grade ferric sulfate (containing about 75 % Fe 2 (SO 4 ) 3 ), and 400 mL water according to ASTM G28A-02 [18].The test was carried out in the boiling solution and the testing duration was 120 h.All the specimens for immersion tests were weighed using analytical balance with sensitivity of 0.01 mg before and after corrosion tests.The mass loss rate (mpy), with units of mm/year, was calculated as follows.
where Δ is the weight loss of specimens (g),  is the density of the stainless steels (g/mm 3 ), S is the total corrosion area (mm 2 ), and t is the time of exposure (h).
After metallographic etching or intergranular corrosion tests, the specimens were rinsed with deionized water and alcohol and then dried.The microstructural characteristics and intergranular corrosion morphologies were examined using a scanning electronic microscope (SEM, Zeiss MER-LIN) equipped with an energy dispersive spectrum analyzer.The phase analysis was performed using an X-ray diffraction (XRD, D8 Advance A25X) analyzer.

Result and Discussion
The surface morphologies of the Alloy 625 specimens after aging at 750 ∘ C for different time in ferric sulfate-sulfuric acid test for 120 h are presented in Figure 5.Only a few numbers of grain boundaries have been found to be corroded in the stabilization treated specimen; cankerous attack patterns appeared frequently along the grain boundaries, as shown in Figure 6(a).In contrast to Figure 6(a), most of the grain boundaries are completely corroded for the specimen after aging for 40 h, as envisaged from Figure 6(b).Actually, for the aging treated sample, the intergranular corrosion propagates along grain boundaries from the surface into the material interior and causes mass loss due to grain dropping, as shown in Figure 6(c).Thereafter, the corrosion rate is accelerated with the drop of the grains.Very fine cellular-like patterns could be observed on the surface (Figure 6(e)), seemingly The cross-sectional SEM images of the Alloy 625 after immersing corrosion for 120 h are shown in Figure 7. SEM imaging of cross-sections (Figure 7) further confirms the results of Figure 6.The stabilization treated sample displays a relative flat surface and only one big intergranular corrosion crack occurs on the surface, as shown in Figure 7(a).The crack originated from the area of carbide at the grain boundary and spread along the grain boundaries into the interior of the material.The aging treated specimens exhibit an obvious intergranular corrosion, as shown in Figures 7(b),  7(c), and 7(d).The comparison of cross-sections in Figure 7 effectively makes further confirmation on the deterioration of intergranular corrosion by the aging treatment method in the tested alloy.

Discussion
Cr and Mo can combine with carbon elements to form carbides which are continuously precipitated at the grain boundary during long term aging treatment.The dilution zone of Cr and Mo can form adjacent to the grain boundary because the diffusion rate of Cr and Mo is far lower than that of carbon.The widths of the depleted zone and the amount of precipitates can be calculated by using Fick's second law [19]: Initial and boundary conditions: The solution of the equation can be obtained by using Boltzmann Transformation and Gauss Error Function: where   is precipitation phase of grain boundary and alloy element concentration of substrate's interface; nonsensitized alloy's concentration of  0  is interior part; x is distance to grain boundary; D is diffusion coefficient; and t is aging time.
Alloy element of grain boundary precipitation by different sensitization time causes dilution zone's composition change, and qualitative results are shown in Figure 8.
The widths of the depleted zone,  40 =  200 / √ 5 =  1000 /5,where in this region complete passivation film cannot be formed, are proportional to the square root of the aging time; that is, the volume of the dilution area follows the parabola law.
As can be seen from Figures 6 and 7, corrosion mainly originated from the carbide and precipitated phases and then gradually expanded into the interior of the grain.When the grain loses its support, it falls off under the action of corrosion.In addition, the volume fraction of precipitated phase increased with the increasing of aging time, leading to the aggravation of corrosion.Therefore, the surface presents a glacial like corrosion appearance because a dense passivation film cannot be formed at  phase [10,20].The corrosion potential of carbides and intermetallic at the grain boundary is relatively low; therefore, preferred dissolution occurs during immersion, as shown in Figure 9.
It can be deduced from the above analysis that the mass loss rate (mpy) is related to the number of precipitated phases [21] and the diffusion of alloy elements.After aging for 200 h, because of the precipitation of needle-like  phase, the peel off grain and lead to further increases of the mass loss rate.The mass loss rate of the Alloy 625 is not proportional to   phase, grain boundary precipitator, and element dilution area near the grain boundary.The nucleation and growth of the precipitated phase can be characterized by Johnson-Mehl-Avrami (JMA) equations [22][23][24][25]: where f is volume fraction of precipitation phase; t is the ratio of the volume of the precipitated phase at a certain time to the volume of the precipitated phase at an infinite time; k is time constant associated with temperature; and n is time constant, between 1 and 4.
The volume fraction of element dilution zone and precipitated phase at the grain boundary can be obtained as follows: where   is volume fraction of precipitation phase and grain boundary element depleted zone and   is volume fraction of needle-like  phase.
In addition, corrosion rate is as follows: where a, b, c are constants.When t =0,  = 1.267 (/).
As can be seen from ( 5) and ( 6

Conclusion
The effect of aging time on intergranular corrosion behavior of a ultra-low Fe Alloy 625 was investigated by surface observation and immersion testing methods.The deterioration of intergranular corrosion resistance of the tested alloy was discussed.The results are summarized as follows.
(1) The Cr depleted zone is formed around the grain boundary during the aging process.The precipitated phase   -Ni 3 Nb was mainly precipitated at the grain boundaries and twin boundaries.The number and volume of   increased with the prolonging of aging time.The transformation of   to -Ni 3 Nb occurred after aging periods of 200 h.
(2) It can be seen that the weight loss during corrosion increases with aging time.The corrosion resistance of Alloy 625 was significantly reduced during aging treatment.The decrease in intergranular corrosion resistance of alloy 625 was attributed to the dissolution of precipitated phase and chromium depleted zone.The mass loss rate is explained by Johnson-Mehl-Avrami equation.
(3) The falling off of grains was due to the dissolution of Cr depleted zone and precipitation phase.The model of intergranular corrosion is established.

Figure 1 :
Figure 1: Microstructure of solution alloy by means of SEM.

Figure 8 :
Figure 8: The relationship between aging time and the alloy composition and the width of the depleted zone.

Table 3 :
Composition at some topical points, as obtained by EDS analysis (wt%).
3.2.Intergranular Corrosion Test.Figure5shows the mass loss of Alloy 625 specimens after the 120h ferric sulfatesulfuric acid corrosion test (i.e., ASTM-G28A-02).It can be seen that the weight loss during corrosion increases with aging time.The mass loss rate of Alloy after stabilization treatment is 1.27mm/a.After aging for 40 h, 200 h, and 1000 h, the weight loss rate is 50.86 mm/a, 64.51 mm/a, and 87.17 mm/a, respectively.Although the aging treated samples have higher corrosion rates than the stabilization treatment, they have lower corrosion rates suggesting a rapid formation of the passivation film on the sample surface.The relationship between mass loss rate and aging time can be fitted as follows: