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Sarvghad Moghaddam, M. Sarvghad, Steinberg, Ted, & Will, Geoffrey(2018)Corrosion of stainless steel 316 in eutectic molten salts for thermal energystorage.Solar Energy, 172, pp. 198-203.

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https://doi.org/10.1016/j.solener.2018.03.053

https://eprints.qut.edu.au/view/person/Sarvghad,_Madjid.htmlhttps://eprints.qut.edu.au/view/person/Steinberg,_Ted.htmlhttps://eprints.qut.edu.au/view/person/Will,_Geoffrey.htmlhttps://eprints.qut.edu.au/119350/https://doi.org/10.1016/j.solener.2018.03.053

1

Corrosion of stainless steel 316 in eutectic molten salts for thermal energy storage

Madjid Sarvghad*, Theodore A. Steinberg, Geoffrey Will

Science and Engineering Faculty, Queensland University of Technology (QUT), Queensland

4001, Australia

Cite this article as:

Madjid Sarvghad, Theodore A. Steinberg and Geoffrey Will, Corrosion of stainless steel 316 in

eutectic molten salts for thermal energy storage, Solar Energy, 2018

https://doi.org/10.1016/j.solener.2018.03.053

Abstract

Stainless steel 316 was examined for compatibility with the eutectic mixtures of NaCl + Na2CO3 and

NaCl + Na2SO4 at 700 °C and Li2CO3 + K2CO3 + Na2CO3 at 450 °C in air for thermal energy storage.

Electrochemical measurements combined with advanced microscopy and microanalysis techniques

were employed. NaCl + Na2CO3 was found as the most aggressive salt at 700 °C. The attack

morphology on the surface was uniform corrosion with no localized degradation at 450 ºC. Microscopy

observations showed grain boundary oxidative attack as the primary corrosion mechanism at 700 °C

with depletion of alloying elements from grain boundaries.

Keywords

Steel alloy; Molten Salt; Corrosion; Microscopy

* Corresponding author; Email: Sarvghad_madjid@hotmail.com; madjid.sarvghadmoghaddam@hdr.qut.edu.au

https://doi.org/10.1016/j.solener.2018.03.053mailto:Sarvghad_madjid@hotmail.commailto:madjid.sarvghadmoghaddam@hdr.qut.edu.au

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1. Introduction

Thermal Energy Storage (TES) is a critical component in Concentrated Solar thermal Power (CSP)

plants through providing dispatchability and increasing the capacity factor of the plant [1-4]. To fulfil

the recent interests in raising the working temperature of these plants, considerable improvement in

material compatibility between the containment material (tank) and storage medium is a prerequisite

[5-8]. Some TES systems employ eutectic mixtures of molten salts with high thermal capacity as Phase

Change Materials (PCM) to store the thermal energy [9-11].

Steel alloys, as economic candidates for containment materials, are subject to hot corrosion and

oxidation from the molten media in TES systems [3-7, 9-22]. Molten eutectic mixtures of carbonate,

chloride-carbonate, and chloride-sulfate salts are also considered as PCM candidates that provide high

heat capacity and energy density [3, 23]. The solubility of corrosion products and oxidation potential

of the alloy are key factors that affect compatibility between the containment material and molten

medium [24]. In steel alloys, the development of protective oxides on the material surface promotes

resistance against corrosion where the material chemistry, temperature and atmosphere determine the

scaling rate [25, 26]. However, in molten salts, protective layers consisting of components like

chromium oxide are often dissolved into the salt mixture by fluxing. Once the oxide film is removed,

the least noble constituent of the exposed metal will be attacked [24, 27, 28]. For example, the corrosion

of Fe-based alloys at 450 °C in ZnCl2–KCl was shown to be due to the separation and spallation of the

oxide films [29].

Intergranular corrosion of steel alloys in contact with molten chloride and chloride-carbonate salts has

been reported previously [28, 30]. Other studies confirm that intergranular attack in Fe-Ni-Cr alloys is

more severe than metal loss in molten chlorides [23]. Depletion of Cr from grain boundaries in molten

salts has been commonly identified as a key corrosion mechanism in various molten salts for high Cr

alloys [11, 28, 31, 32]. However, recent research showed acceptable resistance of stainless steel 310 to

molten carbonate salts at 750 °C [3]. Generally, steel alloys with around 20 wt% Cr and/or high nickel

content show a greater resistance to high-temperature corrosion [25, 33].

This study will examine the compatibility of stainless steel 316, as TES vessel, with some eutectic

mixtures of molten salts, as PCM, for the purpose of developing economic and functionally efficient

CSP systems.

2. Experimental procedure

Austenitic stainless steel 316 (SS316), supplied by M D Lewis & Company Pty. Ltd, with the nominal

chemical composition of (in wt%) 67.45% Fe, 12% Ni, 17.5% Cr, 3% Mo and 0.05% C was examined

for compatibility in three eutectic mixtures of molten salts.

Powders of sodium chloride (CAS No.7647-14-5), sodium sulfate (CAS No. 7757-82-6), sodium

carbonate (CAS No.497-19-8), potassium carbonate (CAS No.584-08-7) and lithium carbonate (CAS

No.554-13-2) were placed for 24 h in a 180 °C furnace to dry and then were measured and mixed

according to Table 1. The eutectic mixtures melting points and test temperatures have been also

provided in the table. Test temperature for each salt was selected close to its melting point assuming

the salt will be used as a PCM.

Table 1 Chemical composition, melting point and test temperature of the salt mixtures.

Eutectic salt mixture Composition

(wt%)

Melting point

(°c) [34, 35]

Test temperature

(°c)

Chloride sulfate 26.5 NaCl + 73.5 Na2SO4 626 700

Chloride carbonate 40 NaCl + 60 Na2CO3 632 700

Ternary carbonate 33.4 Na2CO3 + 32.1 Li2CO3 + 34.5 K2CO3 397 450

3

2.1. Electrochemical corrosion investigation

Electrochemical experiments were conducted using a three-electrode cell containing the molten salts in

alumina crucibles open to air at 700 °C and 450 °C in a preheated cylindrical furnace. Test coupons of

25 mm long, 5 mm wide and 1.4 mm thick were mechanically wet ground and polished down to 0.04

m by colloidal silica, washed with ethanol and dried in air. Measurements were implemented by means

of a VMP3-based BioLogic instrument controlled by EC-Lab® software. The three-electrode cell was

implemented with the polished sample as the working electrode and two same sized platinum sheets

(25×5×1 mm) as pseudo reference and counter electrodes [36-39]. Samples were subjected to open

circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic

polarization (PDP) measurements in this order to avoid sample deterioration. Equilibrations of

potentials (OCP) were carried out for 1 h immediately after immersion. EIS measurements were then

obtained using a frequency range of 100 kHz- 100 mHz with the amplitude of ± 10 mV. Finally, PDP

was conducted at the potential scan rate of 10 mV/min and potential range of -400 to +500 mV with

respect to the open circuit potential. Tafel fit tool of EC-Lab software was used to fit PDP curves and

calculate Tafel slopes, corrosion current density and corrosion potential.

2.2. Static corrosion

Fresh metal coupons were cut to around 25×7×1.4 mm for static corrosion tests while the front sides

were mechanically wet polished down to 1 µm in colloidal silica using standard grinding and polishing

procedures. Cylindrical alumina crucibles were used as salt vessels and the furnace temperature was set

to 700 ± 10 °C for chloride carbonate and chloride sulfate salts and 450 ± 10 °C for the ternary

carbonate.

The salt containing vessels were placed into the furnace at room temperature and then gradually heated

up to the test temperature. Once the salt melted and the chamber conditions stabilized, the metal coupons

were immersed and then removed after 120 h of exposure. All coupons were then mounted exposing

the right (or left) side into a conductive resin, ground and polished down to 0.04 µm from the side of

the sample in colloidal silica using standard procedures, washed with ethanol and finally dried in air. It

is worthy to note that although 120 h does not seem long enough to study the corrosion rate and related

phenomena, the aggressive nature of molten salts makes studying short-term impacts like attack

morphology and corrosion mechanisms possible.

2.3. Microstructural investigations

Optic microscope model Leica DMi8A (magnification 1.25 x-50 x) equipped with Leica Application

Suite software was used to take macro and micro-images for microstructural and corrosion observations.

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were also

employed for further microstructural investigations using a field emission SEM; model: JEOL 7001F,

with automated feature detection equipped with secondary electron and EDS analysis system.

3. Results and discussion

3.1. Electrochemistry

OCP graphs of the alloy specimens in the studied molten salts are represented in Fig. 1a. In chloride

sulfate, SS316 shows an initial rapid drop followed by more noble OCP values after 1000 s and ending

to stable values over the 1 h of exposure. In chloride carbonate at 700 °C and ternary carbonate at 450

°C it shows more noble potential values over time. This could be attributed to the gradual development

of films on the surface.

4

Fig. 1 (a) Open circuit potential, and (b) potentiodynamic polarization curves of SS316 samples in chloride

sulfate and chloride carbonate at 700 °C and ternary carbonate at 450 °C.

Ecorr values in PDP plots in Fig. 1b reflect OCP values in Fig. 1a. Data extracted from PDP curves are

summarized in Table 2 depicting the corrosion current density values in Fig. 2b.

SS316 represents the least Icorr value in ternary carbonate at 450 °C. Between the two high temperature

salt mixtures at 700 °C, chloride carbonate seems the most aggressive one with a corrosion current

density value of around 3.3 times higher that of chloride sulfate. This could be attributed to the high

solubility of Cr and other alloying elements in chloride carbonate and the subsequent dissolution of

oxide films into the molten salt as reported previously [35]. The significant changes in Icorr and Ecorr

values could also indicate changes in mechanism between the molten salts because of dissimilar

compositions and test temperatures.

Table 2 Extracted data from polarization plots in Fig. 1b and EIS plots in Fig. 3b.

Salt mixture Ecorr

(mV)

Icorr

(µA.cm-²)

Βa

(mV/decade)

|Βc|

(mV/decade)

Rp

(Ω.cm2)

Chloride sulfate -466 74 288 262 113

Chloride carbonate -24 247 221 313 20

Ternary carbonate -539 6 56 143 225

Fig. 2 Column chart of corrosion current density values of SS316 in chloride carbonate and chloride sulfate

at 700 °C and ternary carbonate at 450 °C.

5

The corrosion mechanisms of steel alloys in the studied molten salts have been previously discussed in

detail [28, 40]. It has been shown that the corrosion mechanism in molten carbonate salts varies widely

with temperature and the salt composition while the salt becomes less corrosive at lower temperatures

[23]. Because of their ionic nature, molten salts interact electrochemically with metals leading to Redox

reactions [41-44] with corrosion highly dependent on the solubility of metal oxides in the liquid salt

[45]. Solubility of oxygen and the subsequent oxidation of metals (and alloying elements) has been

previously shown in molten carbonates [28, 32, 35, 46]. Formation of a variety of iron oxides, such as

FeO, Fe2O3 and Fe3O4, is a consequence of the interaction between molten carbonates and steel alloys

[45, 46]. However, molten chlorides have been reported to destabilize the oxide film due to the

formation of HCl and Cl2 gas that penetrate the film on the material surface and react with the substrate

metal [17, 43, 44, 47-49].

Analyses of Nyquist and Bode-Phase plots (Fig. 3) resulted from EIS measurements in the molten salts

provide more information. In agreement with Zeng et al. [50] and as confirmed in our previous research

[28, 40], the Nyquist plots confirm the formation of semi-protective films on the metal surface where

the transfer of ions in the scale is rate limiting [50, 51]. This is also in agreement with OCP plots in Fig.

1a. Bode plots in Fig. 3b show the highest impedance values (polarization resistance, Rp, in Table 2) at

low frequencies for the alloy in ternary carbonate at 450 °C versus chloride carbonate with the least

impedance and moderate values for chloride sulfate (at 700 °C); corresponding to corrosion current

density values reported in Table 2. Considering the results in Table 2, Fig. 1b and Fig. 2b, although the

deposit does not seem to be totally protective, it could have led to a slow corrosion rate for the alloy in

ternary carbonate at 450 °C compared to the other two salts at 700 °C. However, this does not disaffirm

the impact of temperature which is quite lower for the ternary carbonate than the two high temperature

salt mixtures.

Fig. 3 (a) Nyquist, and (b) Bode-Phase plots resulted from impedance measurements of SS316 samples in chloride

sulfate and chloride carbonate at 700 °C and ternary carbonate at 450 °C. Larger Nyquist plot in chloride carbonate

is also included in (a).

3.2. Static corrosion, microscopy and microanalysis

3.2.1. SS316 vs. chloride sulfate at 700 °C

SEM-EDS images of SS316 submerged for 120 h in the molten chloride sulfate at 700 ºC is presented

in Fig. 4. Grain boundary (GB) de-alloying of Cr, Fe and Mn followed by the formation of a Mo-Ni-S

oxide along GBs seems to be the corrosion mechanism. In agreement with EIS results, a very thin oxide

film is also detectable on the alloy surface. However, sulfur attack and the subsequent formation of

molybdenum sulfides have been previously reported to destabilize the oxide layer [40-42].

6

Consequently, dissolution of the surface film allows direct access of the molten salt to the underlying

metal leading to further oxidation and corrosion.

Fig. 4 SEM and the corresponding EDS map analysis of SS316 after 120 h exposure to chloride sulfate salt at 700

°C.

3.2.2. SS316 vs. chloride carbonate at 700 °C

A SEM image and its corresponding EDS map analysis of a coupon of SS316 subjected to chloride

carbonate at 700 °C for 120 h is shown in Fig. 5. Again, depletion of Fe and Cr from GBs seems to have

led to the formation of a semi-protective GB oxide as previously predicted by electrochemistry.

However, the high diffusivity of ions through the surface layer reduce the film adherence in the molten

salt [28].

Fig. 5 SEM and its corresponding EDS map analysis of SS316 after 120 h exposure to chloride carbonate salt

at 700 °C [28].

3.2.3. SS316 vs. ternary carbonate at 450 °C

Fig. 6 shows the formation of a continuous and adherent Fe-Ni-Cr-K oxide on the material surface in

ternary carbonate at 450 °C. No localized de-alloying and oxidation/corrosion attack is detectable on

the sample. Thus, it could be concluded that the oxide layer formed on the material surface acts as a

solid barrier against further corrosion in contact with the molten salt at 450 °C.

7

Fig. 6 SEM and the corresponding EDS map analysis of SS316 after 120 h exposure to ternary carbonate salt at 450 °C

[28].

Conclusion

Corrosion behavior of stainless steel 316 in the eutectic mixtures of NaCl + Na2CO3 and NaCl + Na2SO4

at 700 °C and Li2CO3 + K2CO3 + Na2CO3 at 450 °C in air was studied for compatibility in thermal

energy storage. A combination of optical microscopy, electrochemical measurements, SEM and EDS

techniques were employed to characterize the degradation mechanisms. Results are summarized as

below.

Electrochemical measurements showed the most cathodic potential value for the alloy in NaCl +

Na2CO3 at 700 °C which was found as the most aggressive salt through providing the highest corrosion

current density value. Impedance spectroscopy suggested the formation of films on the surface of the

alloy in all molten salts, which were shown not to be fully protective. Consequently, oxidation was

found as the attack mechanism to the alloy in all molten salt environments.

The corroded metal morphology was grain boundary oxidation because of de-alloying and subsequent

intergranular attack at 700 °C. However, uniform oxidative attack was observed for the alloy in contact

with Li2CO3 + K2CO3 + Na2CO3 at 450 °C with no localized de-alloying.

Acknowledgement

This work was funded by the Australian Solar Thermal Research Initiative (ASTRI), which is supported

by the Australian Government via the Australian Renewable Energy Agency (ARENA). The authors

would also like to thank AINSE Ltd for providing financial assistance (Award-PGRA-2016) to enable

work on the reported topic. The data reported in the paper were obtained at the Central Analytical

Research Facility (CARF) operated by the Institute for Future Environments at Queensland University

of Technology (QUT). Access to CARF was supported by generous funding from the Science and

Engineering Faculty, QUT.

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Table of figures

Fig. 1 (a) Open circuit potential, and (b) potentiodynamic polarization curves of SS316 samples in

chloride sulfate and chloride carbonate at 700 °C and ternary carbonate at 450 °C. Fig. 2 Column chart of corrosion current density values of SS316 in chloride carbonate and chloride

sulfate at 700 °C and ternary carbonate at 450 °C. Fig. 3 (a) Nyquist, and (b) Bode-Phase plots resulted from impedance measurements of SS316

samples in chloride sulfate and chloride carbonate at 700 °C and ternary carbonate at 450 °C. Larger

Nyquist plot in chloride carbonate is also included in (a). Fig. 4 SEM and the corresponding EDS map analysis of SS316 after 120 h exposure to chloride

sulfate salt at 700 °C. Fig. 5 SEM and its corresponding EDS map analysis of SS316 after 120 h exposure to chloride

carbonate salt at 700 °C [28]. Fig. 6 SEM and the corresponding EDS map analysis of SS316 after 120 h exposure to ternary

carbonate salt at 450 °C [28].