【Related Papers】
Enhanced localized corrosion resistance of Ni-based alloy 625 processed by directed energy deposition additive manufacturing
【Related Links】
https://doi.org/10.1016/j.corsci.2024.111945
【Highlights】
• DED Alloy 625 exhibits excellent resistance to localized corrosion in 7.6 M LiCl.
• Large nitride inclusions rich in Nb, Mo initiate pits in Forge 625.
• Due to the refinement of nitride size, DED Alloy 625 inhibits pit initiation.
Abstract
We report a significant improvement in local corrosion resistance of Ni-based alloy 625 treated with directed energy deposition (DED) in 7.6 mol/L LiCl at 50°C. The differences observed in polarization behavior between DED and wrought alloy 625 were analyzed by compositional and microstructural differences. The excellent local corrosion resistance of DED alloys may be due to the refinement of the secondary phase produced by the DED process. The precipitate of the prototype is 1 μm larger than that of the forged material, and it is mainly a face-centered cubic nitride rich in Nb and Ti, in which M = Nb, Ti and Cr have a stoichiometric of MN, which may reduce the corrosion resistance as the detonation site of the pit.
Introduction
Alloy 625 is a Ni-Cr-Mo-Nb solution-strengthened alloy known for its excellent high-temperature mechanical properties, corrosion resistance, and weldability. Despite its excellent corrosion resistance due to the synergistic effect of Cr and Mo, Alloy 625 is known to experience localized corrosion at temperatures above 35 °C in the corrosive environments encountered in chemical processing and the oil and gas industry, especially in concentrated chloride media. This often requires the use of a more expensive nickel-based alloy, such as C-22, instead of Alloy 625.
In near-ambient temperatures and/or dilute chloride environments, the additive manufacturing microstructure does not appear to alter the local corrosion behavior of Alloy 625. However, for AM Alloy 625, a mild chloride environment at ambient temperature is usually of little practical significance, as the forged variant itself has high local corrosion resistance. This study examines the degree of behavioral differences between DED manufactured and commercially wrought alloy 625 in corrosive, high-concentration chloride environments and their origins.
This work provides evidence that DED alloy 625 has significantly higher local corrosion resistance at 7.6 mol/L LiCl at 50 °C compared to wrought alloy 625. We attribute this enhanced resistance to the relatively fine distribution of nitride inclusions in DED alloy 625 based on electrochemical polarization testing and multi-scale microstructure characterization. In addition, large (>1 μm diameter) nitride in wrought alloy 625 was found as a preferred pit-initiation site and may be a major source of susceptibility of wrought alloys in this environment.
Experimental
The DED specimen is made from two different batches of Alloy 625 raw material powder, labeled A and B. Table 1 gives the composition of the powder, the as-built DED material, and the hot-worked and annealed wrought alloy 625 used in this study. Equipped with a laser DED system from IPG Photonics, a YLR-12000 ytterbium fiber laser system, and a Powder Feed Dynamics Mark XV precision powder feeder for sample preparation. The 100 mm high and 13 mm wide L-shaped wall sample was built on an alloy 625 substrate. The key process parameters are shown in Table 2. The wrought alloy 625 tested in this study is alloy 625 under electroslag melting, hot working, and annealing conditions (Huntington Alloys, Huntington, West Virginia).
Results and discussion
The deposited DED alloys 625 constructs A and B exhibit the characteristics of alloys fabricated by this process (Figures 1 and 2). Both DED constructs show a dendritic structure, with the heavier elements having significant interdendritic segregation that appears as bright contrast in BSE micrographs. The SEM-EDS plots in Figures 1 and 2 show interdendritic segregation of Nb, Mo, and silicon enrichment is observed in the interdendritic region of DED-A (Figure 1) and the titanium-rich particles of DED-B (Figure 2). The inter-branch segregation of Nb and Mo is consistent with the elemental partition analysis based on the Scheil solidification calculations performed in ThermoCalc 2023b, the TCNI12 thermodynamic database shown in Figure 3, and the previous literature on DED alloy 625.
The interdendritic region in the DED construct also shows Nb-rich submicron precipitates. TEM characterization of DED-A showed that the precipitate was Nb, Mo-Rich η-nitrides, with diamond cubic structure and rod topography (Figure 4 and Table 3). On the other hand, the precipitate in DED-B was identified as a face-centered cubic nitride with stoichiometric MN where M = Nb, Ti, Cr, Nb, and Ti enrichment (Figure 5 and Table 3). The changes in nitride inclusion composition, morphology, and crystal structure between the two DED constructs are most likely driven by differences in the chemistry of the raw materials. Zuback et al. attributed MN nitride to the high Ti in DED-B materials. There is no evidence of carbides or topology-tightly packed phases in the deposited DED alloys. The secondary phase area fractions of DED constructs A and B are similar and their average dimensions (< 1 μm in diameter) are similar, as shown in Table 4.
The orientation contrast images and the EBSD reverse pole diagram (IPF) plots in Figure 6(a) and (b) show the presence of columnar grains in both DED constructs. However, the grain size and its distribution differ significantly from each other. DED-B has a grain size 10-fold that of DED-A based on the grain area constructed by repeated DED as reported by Khayat and Palmer, as shown in Table 4. While the reason behind the significant difference in grain size between the two DED constructs studied is unknown, it may be related to differences in feedstock composition (Table 1) and/or processing parameters (Table 2) and the resulting differences in the spatial distribution of the precipitate, which may fix grain boundaries. The influence of raw material composition and/or processing parameters on grain structure requires further analysis and is beyond the scope of this work. Nonetheless, the possible effects of grain size differences will be discussed in later chapters.
The microstructural characteristics of wrought alloys are very different from those of DED construction. The EBSD orientation comparison image and IPF-Z plot in Figure 7 show the near-equiaxed grain structure of partially recrystallized wrought alloy 625. Remnants of elongated particles aligned with the direction of hot rolling can also be seen. The grain size of the wrought alloy is nearly 3 orders of magnitude smaller than that constructed by DED, as shown in Table 4.
The EBSD phase plot in Figure 8, as well as the standard-based EDS plot, reveal the presence of Nb- and Ti-rich MN-type nitrides that decorate the region near the grain boundaries. Figure 8 also reveals the presence of Nb-rich carbides in the vicinity of nitride inclusions. As you can see from the EDS plot, carbon is also present in the nitride phase. However, some of the EDS spectra in Figure 8 show that the normalized counts of carbon in the nitride and matrix are similar, while the carbide is significantly rich in carbon. The elemental composition of the carbide is not reported because the interaction volume of the electron beam may be affected by the carbide and the matrix due to the small size of the carbide phase.
In addition to the nitrides observed near grain boundaries, complex Ti-rich nitrides around MgO inclusions and cubic (Nb,Ti) nitrides are also observed in wrought alloys, as shown in Figure 9. In nickel-based alloys, nitride around Mg,Al-rich oxides is commonly observed, as oxide particles can be absorbed from the refractories used in the primary melting process. It must be noted that MN nitrides rich in (Nb, Ti) around Mg,Al oxides are also found in DED-B materials, but the scale is further refined.
Figure 10 shows that at 7.6 mol/L LiCl>50, DED-A and DED-B alloy 625 are resistant to local corrosion during potentiometric polarization at 1 V compared to SCE during potentiometric polarization at 1 V, while forged materials are prone to corrosion.
Conclusions
The local corrosion resistance of DED-machined nickel-based superalloy 625 made from two different powder feedstocks was investigated and benchmarked against the behavior of the wrought alloy in an environment of 7.6 mol/L LiCl at 50°C. The following conclusions can be drawn:
• DED-625 resists localized corrosion at 7.6 mol/L LiCl at 50 °C and SCE exhibits stable passivation up to 1 V followed by transpassive dissolution. Wrought alloys, on the other hand, underwent passivation breakdown at the OCP and suffered severe localized corrosion.
• The nitride inclusions rich in (Ti,Nb) in Wrought Alloy 625 are preferred pit start sites for OCP, making the wrought alloy susceptible to localized corrosion under free corrosion conditions. At the same time, nitride inclusions are refined to the sub-micron level and evenly distributed in the DED material. The refinement of nitride inclusions may be related to the inhibition of pit initiation in DED alloys.
• The results of this study suggest that DED or a similar laser-based additive manufacturing process can enhance the corrosion resistance of Alloy 625 by inhibiting the growth of harmful secondary phases such as nitrides. However, further studies are needed to fully understand the underlying mechanisms behind nitride inclusion preference pit initiation.
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From: House of Additive Manufacturing
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