The corrosion mechanism of Fe-based superalloy in molten NaCl-52 wt-%MgCl 2 at 520°C

Abstract

NaCl-52 wt-%MgCl₂ is a good thermal storage medium at medium–high temperature. But the corrosion of chlorate on metal is serious and the mechanism is unclear. In this paper, the corrosion kinetics curves of Fe and three kinds of Fe-based superalloys were measured by the immersion salt corrosion method at 520°C. The microstructure and composition on the surface and cross-section were characterised by a scanning electron microscopy with EDS analysis and X-ray diffraction. The results show that corrosion kinetics curves obey linear law, and the average mass loss rate of Fe is the lowest of all. After corrosion for 20 h, the main composition on Fe surface was mainly magnesia (MgO) and Fe. Shell structure appeared on the surface of three alloys, the composition of shell was MgO, while matrix had Fe, Ni and its compounds. After corrosion for 160 h, the surfaces of four samples became loose, and they generated different corrosion products. The cross-sectional morphology and line scanning analysis results show that Fe corrosion is relatively mild, while Fe-based alloys presented obvious corrosion layer, and the content of Fe and Cr near corrosion layer decreased. The corrosion mechanism of Fe-based superalloys mainly involves oxidation–chlorination.

Keywords

Corrosion superalloy molten salt corrosion mechanism

Introduction

As an alternative to fossil fuels, solar energy is one of the renewable energies, which is used in two forms: photovoltaic and thermal. In order to solve the intermittent problem of solar energy, a lot of studies have been carried out. Among them, the inorganic salt phase change thermal storage device at high temperature can strengthen the stability of the solar thermal power generation device when sunlight radiation is insufficient [1,2]. Inorganic salt encapsulated in cavity melts and stores heat when sunshine radiation is rich, while it releases heat to supply the equipment when sunshine radiation is weak [3]. Compared to widely used nitrate, carbonate and other oxygen acid salt, chlorate is characterised by high melting point, high temperature decomposition and stable physicochemical properties and so on. So, it is recommended as solar energy high temperature phase change thermal storage medium [4]. In contrast to more than 50 kinds of mixed chlorate thermal storage media, as Kenisarin found, NaCl-52 wt-% MgCl₂ presented high latent heat per unit mass and melting point, which is about 430 J g⁻¹ and 520°C, respectively. So, it is generally believed that NaCl-52 wt-% MgCl₂ has a very good development potential as medium–high temperature phase change thermal storage medium [5]. However, molten chlorate corrodes the thermal storage container material seriously. The corrosion mechanism is unclear for lack of related corrosion data. Therefore, for the purpose of revealing molten salt corrosion mechanism, the corrosion behaviours of Fe and three kinds of Fe-based superalloys (GH1140, GH1015 and GH1035) were studied by the immersion salt corrosion method at 520°C in the atmospheric environment, and the corrosion kinetics curve, microstructure and composition were analysed.

Experiment

The samples were fabricated under the vacuum condition by the power frequency induction furnace SP-25TC, and their chemical compositions are shown in Table 1. Smelted alloy samples need to be annealed, and the annealing processing was as follows: the annealing temperature of GH1140, GH1015 and GH1035 was 1070, 1160 and 1130°C, respectively. Their heat preservation time was 10 min, 10 min and 1.5 h, respectively. All samples were air-cooled. Then they were cut to a dimension of 10 mm × 10 mm × 5 mm with an electrical spark line cutting machine, and ground using a silicon carbide paper down to 600 grit followed by polishing with 1 μm diamond powder and acetone ultrasonic cleaning, afterwards dried and the surface area and weight were measured. Before preparing the salt-mixture, analytically pure sodium chloride (NaCl) was dried in a drying oven for 1.5 h, and then mixed with magnesium chloride (MgCl₂) in the proportion of 12:13 (MgCl₂ was not dried because of water absorption), then the salt-mixture was stored in 50 mL corundum crucibles. Next, the crucibles were placed in a muffle furnace and heated to 520°C with a heating rate of 5°C min⁻¹. After the melting of mixed salt, four parallel samples of each kind were put in one crucible (one of them was marked and taken out after 20 h to observe surface morphology and other samples were used for drawing corrosion kinetics curve).

Table 1

Fe-based alloy nominal chemical compositions (in wt-%).

Name	Fe	Ni	Cr	Mo	W	Al	Ti	Nb
GH1015	Balance	34.0∼39.0	19.0∼22.0	2.5∼3.2	4.8∼5.8	–	–	1.1∼1.6
GH1035	Balance	35.0∼40.0	20.0∼23.0	–	2.5∼3.5	–	0.7∼1.2	1.2∼1.7
GH1140	Balance	35.0∼40.0	20.0∼23.0	2.0∼2.5	1.4∼1.8	0.2∼0.6	0.7∼1.2	–

After corrosion for 20 h, these samples were taken out from the muffle furnace and air-cooled (to prevent from air oxidation, quickly put into the same composition of molten salt to be coated with salt shell after cooling), then the marked sample was collected to analyse the composition of sample surface and residual salt by X-ray diffraction (XRD), as well as by a scanning electron microscope with EDS-analysis to observe the corrosion microstructure. Other samples were cleaned for 10 min by ultrasonic wave in deioniser water and alcohol successively. After drying with cool wind, these samples were weighed with an electronic balance (0.1 mg) and measured with vernier caliper to calculate the average mass loss rate and draw the corrosion kinetics curve. Finally, after repeating this process eight times, the corrosion time was 160 h. When it is ended, take the following steps: the first one was managed as the marked one, the second one was grounded the cross-section to analyse the element distribution near the corrosion layer by means of line scanning, the third one was applied in reserve.

Experiment results

Average mass loss rate and corrosion kinetics characteristic

Figure 1 shows the average mass loss rate of four samples. It indicates that the average mass loss rate of Fe is the lowest of all, the value is 2.0 × 10⁻³ mg mm⁻². GH1015, GH1035 and GH1140 presented the similar rate: 14.4 × 10⁻³, 11.3 × 10⁻³ and 13.9 × 10⁻³ mg mm⁻², respectively. Figure 2 shows the corrosion kinetics curves, which obey linear law. Therefore, it is demonstrated that the corrosion resistance of Fe is better than those of other Fe-based alloys, which corresponds well with the result of average mass loss rate.

Morphology and composition of sample surface

The morphology of four kinds of samples after corrosion for 20 h is shown in Figure 3, and the EDS results of designated area are given on the upper-right. The morphology after corrosion for 160 h is shown in Figure 4. XRD results of samples surface after corrosion for 20 and 160 h are shown in Figure 5. It can be seen from Figure 3(a) that there were plenty of polyhedron particles on the surface of Fe after corrosion for 20 h. The analysis of EDS and XRD results shows that the main components are MgO and Fe. Compared to Figure 4(a), after corrosion for 160 h, a lot of spherical particles appeared on the surface of Fe, and also there are many holes between particles and inside particles. Combining with the phase analysis of Figure 5(a), the spherical particles comprised still MgO and Fe, as detected by XRD. In addition, the amount of MgO increased after corrosion for 160 h. With regard to MgO on the surface of Fe, it may be speculated that there are three sources: First, MgCl₂ absorbed moisture during preparing and mixing salt, MgCl₂ after absorption of moisture can be expressed as MgCl₂·6H₂O, which will decompose thermally and lead to dehydration, then MgO will be generated at 520°C as follows: (1) (2) Second, there are abundant chloride ions (Cl⁻) in molten salt, which will react with ferric oxide (Fe₂O₃) from oxide film on the sample surface. Ferric chloride (FeCl₃) that has a low boiling point will be produced, and then escapes system [6]. Oxygen ions (O²⁻) formed during this process will react with magnesium ions (Mg²⁺) existing in molten salt, and also MgO is generated, possible chemical reaction equations are as follows: (3) (4) Decomposing of magnesium chloride with water (MgCl₂·6H₂O) is dominant between the two sources of MgO as mentioned above. Because MgCl₂ is extremely hygroscopic, it is very difficult to achieve total dehydration [7]. Under the experimental temperature of 520°C, MgCl₂·6H₂O with two molecules water will hydrolyse easily and generate MgO, such as the reaction (2).

It can be seen from Figure 3(b–d) that there was a shell structure appearing on the surface of GH1035, GH1015 and GH1140 after corrosion for 20 h. Combining with the results of EDS and XRD from Figure 5(b–d), the main composition of shell can be proved to be MgO, and the substrates are Ni, Fe, Ni₃Fe, Fe₃Ni. Compared to Figure 4(b–d), after corrosion for 160 h, microstructure turned to be loose. Three different corrosion products were generated. The main compositions were Ni, Fe, Ni₃Fe, Fe₃Ni and MgO. For the reason for the formation of the shell structure, it lies in chromium (Cr) existing in these three Fe-based alloys, which can be oxidised more easily than Fe and Ni according to relation between T and ΔG ₀, hence trace amounts of oxygen in the system will give priority to Cr to form chrome oxide (Cr₂O₃). Figure 6 [8,9] shows common metal solubility in molten NaCl–KCl. The solubility of Cr₂O₃ in the chlorine salt-mixture is larger than those of ferric oxide (Fe₂O₃) and nickel oxide (NiO). As a result, Cr₂O₃ formed on the sample surface was partly dissolved into the molten salt, then the integrity of MgO shell was destroyed. Because Cr content is similar to GH1015, GH1035 and GH1140, the dissolution of Cr led to the consequence that the corrosion rate of GH1015, GH1035 and GH1140 was higher than that of Fe.

The cross-sectional morphology and EDS analysis near corrosion layer after corrosion for 160 h are shown in Figure 7. Figure 7(a) shows that corrosion on cross-section of Fe was very mild, and there was no clear interface between substrate and corrosion layer. The XRD result of residual salt after Fe being corrosion for 20 h is shown in Figure 8. The results reveal that the main compositions were NaCl, MgO, Na₂MgCl₄, NaMgCl₃ and MgCl₂·6H₂O. Among them, Na₂MgCl₄ and NaMgCl₃ are two new phases after the salt-mixture (NaCl-52 wt-% MgCl₂) melting. MgCl₂·6H₂O is the hydrate of MgCl₂, and MgO comes from decomposition of MgCl₂·6H₂O and combination of Mg²⁺ and O²⁻, as shown in reaction (1)–(4). In the process of corrosion, MgO partly peeled was attached to residual salt. The emergence of NaCl in the residual salt is due to the fact that eutectic salt composition point shifts to the left when MgCl₂ absorbs moisture. It can be seen from Figure 7(b–d) that there was a clear corrosion layer on the cross-section of three kinds of Fe-based alloy samples, and the layer turned looser than that of Fe. The result of line scanning reveals that the content of Cr and Fe decreases in the corrosion layer, while that of nickel (Ni) increases.

Corrosion mechanism

The corrosion resistance of Fe immersed in NaCl-52 wt-% MgCl₂ is better than those of other samples, which were corroded badly as average mass loss rate, corrosion kinetic curves and morphology shown. Existing researches show that chlorine (Cl₂) and hydrogen chloride (HCl) are the principal factors that caused high temperature chlorination corrosion [10–12]. The reason for high temperature corrosion under chloric atmosphere is that chloride in corrosion products has lower melting point and boiling point compared with those of metal oxide [13,14]. Therefore, the corrosion mechanism of three kinds of Fe-based alloys involves oxidation–chlorination: Gibbs free energy (ΔG) of Cr and Fe oxidation is more negative than that of Ni, which indicates that Cr and Fe tend to be oxidised more easily than Ni, so Cr and Fe are oxidised first, and then the oxide film dissolves in the oxide film/molten salt interface, and metal chloride generates [15,16]. Possible chemical reaction equations are as follows: (5) (6) Part of the generated chloride volatilises and escapes the system, and part of them is oxidised by oxygen (O₂) dissolved in molten salt during the process of diffusing outward, and the possible chemical reaction equations are as follows: (7) (8) In addition, Cl₂ is also formed from HCl coming from reaction (2) by the following reaction: (9) Generated Cl₂ reaches oxide film/matrix interface through the oxide layer, and reacts with matrix metal M (Fe and Cr). Therefore, volatile metal chloride is produced as follows: (10) (11) These metal chlorides constantly volatilise at experimental temperature, and part of them will be oxidised and release Cl₂ in the regions of higher oxygen partial pressure. Cl₂ spreads to substrate and corrodes the matrix metal by reactions (10) and (11). Oxidation–chlorination cycles constantly result in Fe and Cr depletion in the matrix consequently, and it is calculated that depletion of Fe and Cr is the main reason for the corrosion of Fe-based alloys. A model is developed for corrosion mechanism analysis, as shown in Figure 9.

Conclusions

Corrosion kinetics curves of Fe, GH1015, GH1035 and GH1140 obey linear law, and the average mass loss rate per 20 h is 2.0 × 10⁻³, 14.4 × 10⁻³, 11.3 × 10⁻³ and 13.9 × 10⁻³ mg mm⁻², respectively. The value of three kinds of Fe-based alloys is five to seven times that of Fe, and the alloys were corroded badly.

After corrosion for 20 h, the main compositions on Fe surface were MgO and Fe. MgO comes from decomposition of hygroscopic MgCl₂ and a combination of Mg²⁺ and O²⁻ which came from the reaction of Fe₂O₃ on the sample surface and Cl⁻ in the molten salt. Shell structure appeared on the surface of three alloys, and the composition of shell was MgO, while the matrix was Fe, Ni and its compounds. After corrosion for 160 h, surfaces of four samples became loose, and generated different corrosion products.

After corrosion for 160 h, the cross-sectional morphology and results of the line scanning analysis show that corrosion of Fe is relatively mild, while three kinds of Fe-based alloys presented obvious corrosion layer, and the content of Fe and Cr near corrosion layer decreased.

The corrosion mechanism of Fe-based superalloys mainly involves oxidation–chlorination. The oxide film on alloy surface was dissolved in the oxide film/molten salt interface, then metal chloride generated, part of which volatilised and escaped the system, and part of which was oxidised by O₂ dissolved in molten salt during the process of diffusing outward. At the same time, Cl₂ was released. Cl₂ reached oxide film/metal interface through the oxide layer, and reacted with matrix metal. Therefore, volatile metal chloride would be oxidised and chlorinated during the process of evaporation. So, oxidation–chlorination cycled constantly and resulted in sample corrosion consequently.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

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