Introduction1)

Molten salt technology has been widely applied in theindustrial world because of its physical and chemicalcharacteristics, especially its high electrical conductivity,high processing rate, fluid features, etc. And recently, ithas attracted much attention in the fields of jet engines,fuel cells, catalysts, solar energy, and a metal refinement.Therefore, studies on the corrosion of the equipment andthe structure materials for handling high temperaturemolten salts have also been continuously carried out. Forexample, the corrosion of a gas turbine by Na2SO4 depos-it [1-3] and the corrosion related to a molten carbonatefuel cell [4,5] have been extensively investigated. Alsothere are many reports on an accelerated oxidation inchloride molten salts [6-10]. However, they are mainlyshort time electrochemical experiments and few reportsare found on long term experiments at a high temperaturefor the corrosion of commercial alloys in a chloride mol-ten salt by considering their corrosion products, rate andbehaviors. Because of a chloride molten salt withoutoxygen and a low solubility of oxygen in a molten salt[6], the corrosion rate measured by the electrochemical

†To whom all correspondence should be addressed.

(e-mail: nshcho1@kaeri.re.kr.)

methods in a LiCl-KCl mixed molten salt was propor-tional to the concentration of the oxidative impurities,NO3

-. Also a corrosion was not observed at a very low

impurity level in a molten salt. It has also been reportedthat an oxidation reaction was a primary corrosion mech-anism in a chloride molten salt [7,8].The electrochemical reduction process for a spent oxidenuclear fuel is carried out in a LiCl-Li2O molten salt at650

oC. The liberation of oxygen on an anode and the

high temperature molten salts of the process cause achemically aggressive environment that is too corrosivefor ordinary structural materials. Therefore, for an im-plementation of the electrochemical reduction technol-ogy, corrosion resistance materials should be developed.However, few reports are found on the corrosion resist-ance of the structural materials for handling a hot moltensalt. Guided by their excellent mechanical property andcorrosion resistance at a high temperature atmosphere,we selected Haynes 263, Inconel 718, Nimonic 80 A, andIncoloy 800 H as the candidate materials and inves-tigated their corrosion behaviors under simulated electro-chemical reduction conditions.

Experimental

The chemical compositions of the superalloys used are

Soo-Haeng Cho†, Chung-Seok Seo, Ji-Sup Yoon, Hyun-Soo Park, and Seong-Won Park

Korea Atomic Energy Research Institute, 305-353 Daejeon, Korea

Received December 6, 2006; Accepted July 16, 2007

Abstract:The electrolytic reduction of a spent oxide fuel involves a liberation of the oxygen in a molten LiCl

electrolyte, which is a chemically aggressive environment that is too corrosive for typical structural materials.

So, it is essential to choose the optimum material for the process equipment for handling a molten salt. In this

study, corrosion behaviors of Haynes 263, Inconel 718, Nimonic 80 A and Incoloy 800 H in a molten LiCl-Li2O

salt under an oxidizing atmosphere were investigated at 650oC for 72 216 h. Haynes 263 alloy showed the∼

highest corrosion resistance among the examined alloys. Corrosion products of Haynes 263 were Li(Ni,Co)O2

and Li((Cr,Al)TiO4), and those of Inconel 718 were Cr2O3, NiFe2O4, and CrNbO4, while Cr2O3, LiFeO2,

(Cr,Ti)2O3, and Li2Ni8O10 were identified as the corrosion products of Nimonic 80 A. Incoloy 800 H showed

Cr2O3 and FeCr2O4 as its corrosion products. Haynes 263 showed a localized corrosion behavior while Inconel

718, Nimonic 80 A and Incoloy 800 H showed a uniform corrosion behavior.

Keywords: Hot corrosion, molten salt, electrolytic reduction, structural material

Soo-Haeng Cho, Chung-Seok Seo, Ji-Sup Yoon, Hyun-Soo Park, and Seong-Won Park730

Table 1. The Chemical Compositions of the Tested Alloys (wt%)

Alloy Ni Cr Fe C Si Mn Mo Nb Al Co Ti

Haynes 263 51.35 20.0 0.5 0.05 0.13 0.1 5.8 - 0.45 19.2 2.39

Inconel 718 54.12 17.88 17.22 0.02 0.11 0.10 2.97 5.42 0.51 0.22 0.99

Nimonic 80 A 74.90 19.24 1.14 0.06 0.12 0.03 - - 1.68 - 2.40

Incoloy 800 H 34.0 21.0 43.7 0.03 0.40 0.50 - - 0.16 0.15 -

Figure 1. Schematic diagram of the apparatus for the corrosiontest.

shown in Table 1. After the specimens were heat treatedat 950 1,200∼

oC for 1 h and water cooled to be homo-

genized, they were prepared with dimensions of 70 × 15× 2 mm

3. Before the corrosion test, The specimens were

ground with SiC paper, polished with diamond paste andcleaned in deionized water and acetone.The experimental apparatus is shown in Figure 1. A

LiCl-Li2O molten salt was introduced into a high-density

MgO crucible and then heated at 300oC for 3 h in an ar-

gon atmosphere to remove any possible moisture. After

reaching the set conditions, the specimens and alumina

tube were immersed into the molten salt, and mixed gas

(Ar-10 % O2) was supplied through an alumina tube. The

corrosion tests were carried out at 650oC and for 72∼

216 h. The Li2O concentration in LiCl was 3 wt%.

Following the corrosion test, the specimens were with-

drawn from the salt and kept under an argon gas atmos-

phere while the furnace was cooled to room temperature.

All the specimens were visually inspected after the corro-

sion test and the salt attached to the test specimens was

removed by washing them with deionized water in an ul-

trasonic cleaner. The insoluble part of the corrosion scale

removed from the specimens was analyzed by using

X-ray diffraction (XRD). All the clean specimens were

later checked for changes in their weight and sub-

sequently examined by an optical and scanning-electron

microscopy (SEM). Selected spots in the corroded speci-

mens were analyzed by an energy-dispersive spectrome-

try (EDS).

Figure 2. Weight loss of the alloys corroded at 650o

C, as a

function of time.

Results and Discussion

Corrosion Rate

The weight losses of Haynes 263, Inconel 718, Nimonic

80 A, and Incoloy 800 H after the corrosion tests in a

LiCl-3 % Li2O molten salt at 650oC as a function of the

time are shown in Figure 2. Haynes 263 showed the low-

est corrosion rate while Incoloy 800 H showed the high-

est corrosion rate. Under a high temperature oxidative

molten salt environment, the reason for the weight loss of

the specimens with time was attributed to a cracking of

the protective layer which led to its spallation from the

base metal surface. Therefore, it was concluded that the

weight variations of a specimen would be greatly af-

fected by the adhesive strength between a corrosion layer

and a base metal by considering the formation, suste-

nance, and spallation of a corrosion layer and, also the

stability of a corrosion layer’s growth

Corrosion Products

Figure 3 shows the XRD patterns of the corrosion prod-

ucts after the corrosion tests in a LiCl-3 % Li2O molten

salt at 650oC for 72 and 216 h. For Haynes 263, cor-

roded for 216 h, peaks attributed to Li((Cr,Al)TiO4) and

Li(Ni,Co)O2 were observed (Figure 3(a)). The physical

properties of Co, such as its crystal structure at a high

temperature and atomic diameter, density and melting

point are also very similar to those of Ni. Therefore Co

Corrosion Behavior of Superalloys in a Hot Lithium Molten Salt 731

Figure 3. XRD patterns of the corrosion products of (a) Haynes 263, (b) Nimonic 80 A, (c) Inconel 718 and (d) Incoloy 800 H cor-

roded at 650o

C for 72 216 h.∼

Figure 4. Cross-sectional SEM images of (a) Haynes 263, (b)

Nimonic 80 A, (c) Inconel 718 and (d) Incoloy 800 H corroded

at 650o

C for 72 h.

and Ni will show similar diffusion behaviors resulting in

the co-existence of them in the surface oxide layer. The

Ti content in Haynes 263 is about 2.4 %. And the ex-

ternal diffusion rate of the more oxidative Ti at a high

temperature is faster than that of the Cr element [11].

Therefore Ti was diffused through the Cr2O3 and existed

in the surface oxide layer mixed with Cr and Al. From a

thermodynamic point of view, Cr2O3 is the most stable

oxide in the Ni-Cr-Fe based alloys [12], therefore, for

Nimonic 80 A, Cr2O3 was initially formed as a corrosion

product. Also the diffusion of the Fe ion and its reaction

with a Li based molten salt led to the formation of

LiFeO2. And then for the 216 h conditions, the formation

of Li2Ni8O10 and (Cr,Ti)2O3 was observed (Figure 3(b)).

Figure 3(c) shows the corrosion products of Inconel 718,

Cr2O3, NiFe2O4 and CrNbO4 for 72 and 216 h. The corro-

sion products of Incoloy 800 H were FeCr2O4 and Cr2O3

for 72 and 216 h (Figure 3(d)). Consequently, the for-

mation of the corrosion products in a oxidative Li based

molten salt depends on the thermodynamical stability of

an oxide formation and the diffusion of the consitituent

elements and the Li ion.

Corrosion Behaviors

The cross-sectional SEM images of the superallys cor-

roded for 72 h at 650oC in a LiCl-3 % Li2O molten salt

were taken to investigate the morphology of the inner

corrosion layer after the spallation of the outer corrosion

layer (Figure 4). Haynes 263 showed a localized corro-

sion behavior while Inconel 718, Nimonic 80 A, and

Incoloy 800 H showed a uniform corrosion behavior.

Figure 5 shows the cross-sectional SEM image and ele-

mental maps of Haynes 263 corroded for 72 h. The cor-

rosion layer was continuous, dense and adherent. The

Soo-Haeng Cho, Chung-Seok Seo, Ji-Sup Yoon, Hyun-Soo Park, and Seong-Won Park732

Figure 5. Cross-sectional SEM image and maps of Cr, Ni, O, Co, Ti, and Al of the Haynes 263 corroded at 650o

C for 72 h.

Figure 6. Cross-sectional SEM image and maps of O, Fe, Cr, Ti, Al, and Ni of the Nimonic 80 A corroded at 650o

C for 72 h.

corrosion layer consisted of Cr, Al, Ti based, and Ni, Co

based complex oxides. These results are in coincident

with the analysis of the corrosion products (Figure 3(a)).

Consequently, the outstanding corrosion resistance of

Haynes 263 can be explained by the retarded Cr deple-

tion and also the prevention of a crack formation due to

the Co effects which are a decrease of the stacking fault

energy and an increase of the high temperature stability

[13]. Ti, an oxygen active element, can prevent an oxida-

tion of austenite stainless steels by its oxide formation

[14]. This surface Ti oxide can also contribute to the cor-

rosion resistance of Haynes 263. On the surface of the in-

ner corrosion layer of Haynes 263, a Ni rich region was

observed and a diffusion of oxygen can be restrained in

this region which can lead to the corrosion resistance of

Haynes 263.

Figure 6 shows the cross-sectional SEM image and ele-

mental maps of Nimonic 80 A corroded for 72 h. The

dense outer layer mainly consisted of Cr, Fe-oxides. Due

to the high diffusion coefficient of the Fe ion in the ox-

ides [11], the diffusion of the Fe ion through the initially

formed Cr2O3 layer and its reaction with a molten salt re-

sulted in LiFeO2. The preferential oxidation of Al and Ti

beneath the outer corrosion layer and its participation in

the corrosion layer formation were also observed [15].

The cross-sectional SEM image and elemental maps of

Inconel 718 corroded for 72 h are shown in Figure 7. The

analysis was carried out for a specific region where a

spallation of the outer corrosion layer of Inconel 718 had

not occurred. The outer corrosion layer consisted of Ni

and Fe based oxides as shown in Figure 7. Generally, the

most stable oxide in Ni-Cr-Fe based alloys is expected to

be a Cr2O3 [12]. Because the diffusion coefficients of

metal ions in oxides are in the order of Fe3+> Fe

2+> Ni

2+

> Cr3+

[11], the Ni and Fe based oxides appeared to be

formed by an external diffusion of the Fe and Ni ion

through the initially formed Cr based oxide layer.

Concurrently, NiFe2O4, a spinel oxide was formed by a

solid state reaction with the surface generated Fe2O3 and

NiO [11]. This result is consistent with the analysis on

the corrosion products shown in Figure 3(c). Especially,

Inconel 718 contains relatively high amounts of Fe ( 17∼

%) and thus the Fe-rich oxides formed at the periphery of

an outer corrosion layer are attributed to the easy Fe ion

diffusion due to it containing more cation defects than

the Ni-rich oxides [16]. Also, the resultant voids caused a

plastic deformation of the Fe oxide layer resulting in the

formation of micro channels [17]. It has been reported

Corrosion Behavior of Superalloys in a Hot Lithium Molten Salt 733

Figure 7. Cross-sectional SEM image and maps of Cr, Fe, O, Mo, Nb, and Ni of the Inconel 718 corroded at 650o

C for 72 h.

Figure 8. Cross-sectional SEM image and maps of Cr, Fe, O, and Ni of the Incoloy 800 H corroded at 650o

C for 72 h.

that the oxygen ion penetrates through the micro chan-

nels and accelerates the internal oxidation [18]. From

these results and corrosion rate measurements shown in

Figure 2, the greater dependancy of a corrosion resist-

ance on the Fe content can be easily deduced. The accu-

mulations of Nb and Mo with Cr were observed in the in-

terface between an outer and an inner corrosion layer.

When alloys containing small amounts of oxygen active

elements are oxidized at high temperatures, the oxygen

active elements can affect the growth rates of the oxides

depending on the kinds of oxides and the adhesive

strength between an oxide and a base metal [19,20].

Especially, Mo has been reported to reinforce the adhe-

sive strength between a base metal and an oxide in stain-

less steel and also prevent the migration of ions across

the boundary between a passive film and a base metal

[21,22]. In this study, the accumulation of Mo in an in-

terface of the corrosion layers was observed. Also corro-

sion products containing Mo were not identified (Figure

3(c)). These results confirm Mo’s role of reinforcing the

adhesive strength between base metals and oxides.

Figure 8 shows the cross-sectional SEM image and ele-

mental maps of Incoloy 800 H corroded for 72 h. The

analysis was carried out for a specific region where a

spallation of the outer corrosion layer of Incoloy 800 H

had not occurred. The surface of the outer corrosion layer

mainly consisted of Cr and Fe based oxides. The reason

can be explained by the faster outer diffusion rate of the

Fe ion than the Cr ion [11] and its diffusion through the

initially formed Cr based oxide layer and its reaction

with the oxygen ion. Concurrently, FeCr2O4, a spinel ox-

ide was formed by a solid state reaction with the surface

generated FeO and Cr2O3 [11]. This result is consistent

with the analysis on the corrosion products shown in

Figure 3(d). An accumulation of the Fe content in the

corrosion layer was observed. The fast outer diffusion of

Fe in the base metal caused the formation of Fe oxide

during the oxidation reaction resulting in a less dense

outer corrosion layer when compared to Inconel 718 and

a spallation of the outer corrosion layer. Fe based oxides

are less dense than Ni based and Cr based oxides which

can lead to many cation defects [15], a void formation,

an internal stress and a consequent spallation in the inter-

face of an oxide layer/oxide layer or the interface of an

oxide layer/base metal. The corrosion layer in Figure 8

shows the depletion of Cr. The depletion of Cr was at-

Soo-Haeng Cho, Chung-Seok Seo, Ji-Sup Yoon, Hyun-Soo Park, and Seong-Won Park734

tributed to a spallation of the outer corrosion layer due to

its surface diffusion and preferential oxidation for the

formation of an outer corrosion layer. Ling and cow-

orkers reported on the depletion of Cr beneath the outer

Cr2O3 corrosion layer from their experiments on a high

temperature oxidation of Ni-Cr-Fe based alloys [23].

Their result was explained by the consumption of Cr for

the oxides formation in the surface. Therefore, with the

progress of an oxidation, the Cr depletion region will be

expanded into the inside of an alloy due to a diffusion of

the Cr element from a base metal for a supplementation

of the consumed Cr. A depletion of Cr can also occur by

a spallation and regeneration cycle of oxide films due to

a thermal stress and growth stress [12]. The difference of

the thermal expansion coefficients between a base metal

and a oxide film is the reason for the thermal stress at the

interface. And the growth of the oxide films generates

the growth stress.

Conclusion

The corrosion behaviors of superalloys in an oxidative

LiCl-Li2O molten salt were investigated in association

with their electrochemical reduction process. The corro-

sion rate was in the order of Haynes 263 < Nimonic 80 A

< Inconel 718 < Incoloy 800 H. Corrosion products of

Haynes 263 were Li(Ni,Co)O2 and Li((Cr,Al)TiO4), and

those of Inconel 718 were Cr2O3, NiFe2O4, and CrNbO4,

while Cr2O3, LiFeO2, (Cr,Ti)2O3, and Li2Ni8O10 were

identified as the corrosion products of Nimonic 80 A.

Incoloy 800 H showed Cr2O3 and FeCr2O4 as its corro-

sion products. Haynes 263 showed a localized corrosion

behavior while Inconel 718, Nimonic 80 A, and Incoloy

800 H showed a uniform corrosion behavior. Ni which

had accumulated around the inner oxides delayed the

corrosion rate. Fe-rich oxides contained many cation de-

fects and they were not dense. Also the oxygen ions

which had penetrated through the oxides accelerated their

internal oxidation. Therefore, an excess Fe appeared to

be deleterious to their corrosion resistance. Al and Ti,

oxygen active elements, formed oxides by their prefer-

ential oxidation through their outward diffusion, and pre-

vented an internal oxidation by their accumulation in an

interface between an oxide and a base metal.

Acknowledgments

This work was funded by the National Mid- and

Long-term Atomic Energy R&D Program supported by

the Ministry of Science and Technology of Korea.

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