4.1 IGC mechanism of GH3535 alloy in a molten salt environment
As different grains on the GBs have different particle arrangements and orientations, the GB structure has a more pronounced surface effect than the grain density. In addition, the energy of the solute atoms in the grain is higher than that at the GBs, resulting in the spontaneous gravitation of the solute atoms toward the GBs. The calculation of the Gibbs free energy of the carbides formed by the main elements in the GH3535 alloy at 700°C yielded a positive Gibbs free energy for the Ni and Fe carbides, whereas that of the Cr and Mo carbides was negative [33]. Hence, the formation of Ni3C and Fe3C is unfavourable, whereas that of Mo3C and Cr3C is favourable and stable. Therefore, the GB carbides observed in Fig. 10 are more likely to be Mo3C and Cr3C (Table 3). The GB precipitates have an adverse effect on the plasticity and fracture strain of the Ni-based alloy under loading due to the weak interface between the carbide and matrix, which may cause damage during loading [44]. Particularly, triple GB precipitation, which results in stress concentration, generates the main failure area in the creep process. Under stress application, GB sliding produces holes or wedge cracks [45, 46]. In addition, carbide precipitation consumes carbon in the adjacent matrix, resulting in the softening of the adjacent material [47].
Ni-based alloys can withstand extreme service conditions with the addition of alloy elements Mo and Cr, which imparts the necessary high-temperature stability and corrosion resistance of the alloy in different media [48, 49]. Cr is a basic element of stainless steel and Ni-based alloys that aids in maintaining corrosion resistance. Particularly, the combination of Cr and O2 can produce a corrosion-resistant Cr2O3 film. In addition, an increase in Cr content can improve the repairability of a passive film of steel. As GH3535 alloy is solution-strengthened with 16 wt.% Mo, which stabilises the Ni2Mo4(C,Si) and Ni3Mo3(C,Si) primary precipitates in the alloys [46, 50], it exhibits excellent creep resistance. In addition, colossal supersaturation with C continuously induces the diffusion of Cr and Mo to the GBs owing to their high carbon affinity [33], thereby resulting in the easy formation of carbides without saturation.
The Gibbs free energies of the molten salt constituents (LiF, NaF, and KF) are more negative than those of the metallic fluorides in pure FLiNaK molten salt [34, 39]. The corrosion in molten salt is an impurity-driven process [51–53], and typical impurities include H2O and SO42−. The impurities in the salt provide pathways for corrosion by forming relatively unstable fluorides, such as HF, which tend to corrode the alloy materials by a reaction such as
$$3 \text{H}{F}_{\left(d\right)}+{M}_{\left(c\right)}\to {MF}_{3\left(d\right)}{+1.5 H}_{2\left(g\right)},$$
4
where subscripts (d), (c), and (g) denote dissolved, crystalline, and gaseous species, respectively.
Owing to the high affinity between Cr, molten salt, and molten salt impurities, Cr continues to diffuse along the GBs to the matrix surface. For example, the diffusion coefficient of Cr along the GBs is at least one order of magnitude higher than that in the grains: DCr,Grain < 1E-13 cm/s < DCr,GBs [54]. Accordingly, the outward diffusion of the alloying elements leads to GB cavitation, thereby diffusing molten salt and molten salt impurities inward along the GBs and accelerating the corrosion dissolution of the GBs. However, although Mo has a notable affinity for C, the Gibbs free energy of generating MoF3, calculated by Eq. (4), is more positive than that of generating CrF3 (\(\varDelta {{G}^{0}}_{\text{M}\text{o}{\text{F}}_{3}, 700 \text{℃}}=-27.288 kJ/mol\), \(\varDelta {{G}^{0}}_{\text{C}\text{r}{\text{F}}_{3},700 \text{℃}}=-277.105 kJ/mol\)) [41], which implies the lower tendency of Mo to diffuse toward the GBs and react with molten salt impurities to form MoF2. This makes it difficult for Mo to dissolved into the molten salt and diffuse along the GBs to the substrate surface, resulting in the abundance of Mo in the GBs (Fig. 6d and 7d). However, we still detected a significant increase in the Mo content by the ICP-OES tests of the molten salt after the immersion corrosion test of the loaded sample (Table 2). This can be attributed to the Mo6C melt into the molten salt during the expansion of the IGC into the grain.
Therefore, from the above analysis, we determined that carbide precipitation and selective dissolution of the impurity elements lead to Cr loss in the GB region of the GH3535 alloy. Further, these are the main factors for the formation of IGC cracks in the GH3535 alloy in a molten salt environment.
4.2 Effect of stress on IGC of the GH3535 alloy in a molten salt environment
In Ni-based alloys, carbides often precipitate and/or coarsen at the GBs under high temperatures and stresses. GB carbide coarsening rates have been reported to be three orders of magnitude greater than the stress- and/or strain rates in creep tests [55, 56]. The tensile stress of solid materials can be relieved by dislocation slip movement, dislocation creep, diffusion flow, or cracks perpendicular to the direction of tensile stress. When the molten salt test temperature exceeds 0.5Tm (Tm represents the melting point of the alloy), dislocation creep and vacancy flow have important effects on the diffusion of the alloy elements. The loading stress can trigger the dislocation-assisted alloying element migration mechanism and is also closely related to the alloying element flux. When the crystal is subjected to tensile stress, the dislocation and vacancy concentrations will be higher than those in the thermal equilibrium state. In contrast, under compressive stress, the dislocation and vacancy concentrations will be lower than those in the thermal equilibrium state.
According to the Nabarro–Herring creep model, under thermal equilibrium, the concentration of vacancies, C, in the tensile region is [57]
$$C={C}_{0}exp\left(\frac{\sigma {a}^{3}}{kT}\right)$$
5
,
where \(\sigma\)a3 is the work done by the stress for removing an atom, C0 is the thermal equilibrium concentration of the vacancies in the stress-free region, T is the temperature in K, and k is the Boltzmann constant. In other words, after stress is applied, the vacancy concentration in the matrix significantly increases. Accordingly, a greater probability of element diffusion can be achieved through vacancy migration and diffusion.
The vacancy concentration gradient is given by [57]:
$$\frac{dC}{dx}=\frac{{C}_{0}}{L}\left[exp\left(\frac{\sigma {a}^{3}}{kT}\right)-1\right]$$
6
,
where L is the distance between the stress and stress-free zones.
The diffusion flow of the alloying elements per unit time is
$${J}_{v}={D}_{v}\frac{dC}{dx}$$
7
,
where Dv is the coefficient of surface diffusion of the alloying elements. Among the main elements of the GH3535 alloy, Cr has the highest affinity for F− and second-highest affinity for C [33, 34], resulting in the fastest rate of dissipation of Cr along the GBs, which is equivalent to the highest surface diffusion coefficient at the GBs. With the high mobility of the GBs, Cr has the maximum gradient between the GBs and interior of the matrix and the largest flux from the grains to the GBs. Therefore, stress has the most notable effect on the accelerated diffusion of Cr. Consequently, the rate of carbide formation on the GBs of the GH3535 alloy under stress increases, leading to a continuous network distribution (Fig. 10f). Moreover, during the stress-assisted immersion corrosion process, an increased amount of Cr diffuses into the molten salt through the GBs. Under the application of stress, the range of the GB degradation area is widened (Fig. 7); further, IGC cracks arise in this area and their width increases (Fig. 5d).
In addition, under all stress conditions, large carbides are formed, resulting in the enhanced galvanic corrosion in the area near the carbides and accelerated corrosion of the matrix around the carbides. Further, this results in the propagation of the IGC cracks into the grains. The enhanced galvanic corrosion also results in the loosening of the carbides and their release from the alloy surface, resulting in an increase in the corrosion crack size (Fig. 5c and 5d). Accordingly, the scale of the diffusion channel of the elements to the substrate surface increases, further enhancing the corrosion rate of the entire substrate.