Degradation and Corrosion of Metal Components in High-Temperature Fuel Cells and Electrolyzers: Review of Protective Approaches

Abstract

High-temperature fuel cells and electrolyzers, particularly molten carbonate fuel cells (MCFCs) and Molten Carbonate Electrolyzers (MCEs), are expected to play a critical role in clean power generation, hydrogen production, and integrated CO₂ separation. Unfortunately, despite their potential, these technologies have not yet reached full commercialization. The main reason for this is material degradation. In particular, the corrosion of metallic components continues to be a leading cause of performance loss and system failure. This review provides a comprehensive assessment of degradation mechanisms in MCFC and MCE systems. It examines key metallic components, such as current collectors and bipolar plates, focusing on the performance of commonly used materials, including stainless steels and advanced alloys, under prolonged exposure to corrosive environments. To address degradation issues, this review evaluates current mitigation strategies and discusses material selection, protective coatings application, and the optimization of operational parameters. Advances in alloy development, coatings, surface treatments, and process controls have been compared in terms of effectiveness, scalability, and long-term stability. The review concludes with a synthesis of current best practices and future directions, emphasizing the need for integrated, multi-functional solutions to achieve the lifetimes required for full commercialization. By linking materials science, electrochemistry, and systems engineering, this review offers directions for the development of corrosion-resistant MCFC and MCE technologies in support of a hydrogen-based, carbon-neutral energy future.

1. Introduction

Due to the rising threat of global warming, the Paris Agreement was adopted in 2015 to limit the global average temperature increase to 1.5 °C. The European Union has since set climate targets for 2030, 2040, and 2050. These targets include a 55% reduction in greenhouse gas emissions by 2030 and a 90% reduction by 2040, compared to 1990 levels [1]. These targets will be a basement for achieving net-zero emissions and complete climate neutrality by 2050. Reaching these milestones requires innovation and transformation across multiple sectors, especially the energy sector, where the focus should be on energy production, transportation, and industrial processes.

According to these long-term strategies, there are seven building blocks that will help achieve these goals, including the development and deployment of renewable energy sources, improvements in energy efficiency, and the implementation of technologies such as carbon capture, storage, and synthetic fuel production [1]. One of the most promising pathways involves the “Power-to-X” concept, in which surplus renewable electricity is converted into synthetic fuels or hydrogen either for direct use or stored and then converted back into electricity when needed. In this context, hydrogen and fuel cells play a key role not only as integral components of Power-to-X systems but also in supporting the achievement of broader climate targets. Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electricity without any intermediate stages [2]. Depending on their operating temperature, fuel cells are classified into low-, intermediate-, and high-temperature types. While low-temperature fuel cells are mostly suited for mobile and portable applications, high-temperature fuel cells (HTFCs) are more appropriate for stationary power generation and cogeneration due to their higher efficiency and fuel flexibility.

Among HTFCs, there are two types of fuel cells: molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). The operating temperatures of these HTFCs range between 600 and 1000 °C, which provides the benefit of the use of inexpensive non-precious metal catalysts, the internal reforming of hydrocarbon fuels, and the ability to utilize carbon monoxide as a fuel, which is toxic to low-temperature systems like Proton Exchange Membrane Fuel Cells (PEMFCs) or Alkaline Fuel Cells (AFCs). This review will focus specifically on MCFCs.

MCFCs use carbonate ions (CO₃²⁻) as charge carriers, enabling them to generate electricity and act as CO₂ concentrators or separators. They can utilize CO₂-rich gas streams such as flue gases from industrial plants to produce electricity or they can operate in reverse mode, as an electrolyzer, to produce hydrogen when electricity prices are low [3,4]. This dual mode functionality aligns with the vision of a hydrogen-based circular economy and supports Europe’s long-term decarbonization objectives. For the year 2025, the summarized capacity of large-scale (>100 kW) MCFC systems is exceeding 0.35 GW, while all large-scale fuel cell types reach nearly 5 GW of installed electrical power (Figure 1); achieving 2050 climate neutrality will require a dramatic increase in deployment [5,6,7,8,9,10].

Fuel cells can really play a central role in future power generation and hydrogen production systems. As European economies transition toward hydrogen-based energy infrastructures, HTFCs will provide baseload support and facilitate large-scale hydrogen production [11]. However, for full commercialization, fuel cells must demonstrate stable long-term operation, which is widely acknowledged to be between 35,000 and 40,000 h of operation. Reaching these lifespans remains challenging, primarily due to performance degradation over time [12]. In HTFCs and, in particular, MCFCs, this degradation is driven by a combination of electrolyte loss, electrode particle coarsening, material dissolution, and poisoning. However, the most critical and limiting factor is corrosion.

Corrosion affects nearly all aspects of the cell, from the microstructure of the electrodes to metallic components such as current collectors and bipolar plates. It leads to increased internal resistance, electrolyte contamination, mechanical degradation, and ultimately reduced system efficiency and lifespan.

In the available literature, there is plenty of research [13,14,15] focused on the study of metallic elements used SOFCs. However, the results obtained in SOFC-related corrosion studies cannot be directly extrapolated to MCFC, where the materials operate under fundamentally different conditions. In MCFCs, metallic components are exposed to a highly corrosive environment due to the presence of molten alkali carbonates at elevated temperatures. This environment promotes aggressive chemical reactions, including basic fluxing, dissolution, and the formation of unstable surface layers, which are not typical for the oxide-based electrolytes used in SOFCs. As a result, corrosion mechanisms, degradation rates, and protective strategies must be investigated specifically within the MCFC context. This review aims to provide a comprehensive overview of degradation mechanisms in molten carbonate fuel cells, focusing on corrosion, and to critically evaluate the strategies proposed in the literature to mitigate these effects. The review addresses both material-based and system-level solutions, identifying the most promising pathways to enhance the durability and commercial viability of MCFC and MCE technologies.

2. Molten Carbonate Fuel Cells

The basic principle of operation of MCFCs, as well as their reverse operation as Molten Carbonate Electrolyzers (MCEs), is illustrated in Figure 2. MCFCs operate at high temperatures (typically 600–700 °C) and utilize carbonate ions (CO₃²⁻) as mobile charge carriers (Equation (1)). These ions are transported from the cathode to the anode through a molten carbonate electrolyte (Equation (2)), enabling electrochemical reactions that convert chemical energy directly into electricity. The overall reaction of the MCFC is defined by Equation (3).

$$\mathrm{C}{\mathrm{O}}_2+{{\displaystyle \frac{1}{2}}\mathrm{O}}_2+2{\mathrm{e}}^{-}\to \mathrm{C}{\mathrm{O}}_3^{=}$$

(1)

$${\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_3^{=}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{e}}^{-}$$

(2)

$${\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$$

(3)

State-of-the-art materials used in MCFCs are nickel-based alloys used as the fuel electrodes (anodes). The materials include Ni–Al, Ni–Cr, and Ni–Al–Cr alloys, which offer good electrochemical activity and thermal compatibility. The oxygen electrode (cathode) typically consists of lithiated nickel oxide, which balances electronic conductivity with corrosion resistance under oxidizing conditions [2,17].

The electrolyte consists of a eutectic mixture of lithium and potassium carbonate, typically in a 62:38 wt% Li₂CO₃–K₂CO₃ ratio. However, other carbonate compositions are also in use, including Li/Na (50:50 wt%) and Li/K (50:50 wt%) mixtures, depending on the desired melting point, ionic conductivity, and corrosion characteristics [17].

The electrolyte is retained within a porous supporting matrix, most commonly composed of γ-LiAlO₂ due to its chemical stability, thermal compatibility, and low solubility in molten carbonates. Recent research, however, has explored the incorporation of yttria-stabilized zirconia (YSZ) into the matrix to provide dual ionic conductivity, facilitating both carbonate and oxygen ion transport, which could improve electrolyte utilization and electrode kinetics [18,19].

These elements allow MCFCs to operate at high efficiencies with internal fuel reforming capabilities (including liquid fuels [20]) and CO₂ capture functionality, making them highly suitable for stationary, industrial, and grid-support applications. However, the aggressive high-temperature environment and chemically reactive electrolyte introduce complex degradation phenomena, particularly corrosion, which remain the main barriers to long-term operation and commercialization.

3. Degradation of Metal Components in High-Temperature Fuel Cells and Electrolyzers

MCFCs and MCEs, as mentioned before, operate under extreme conditions, including high temperatures (600–700 °C) in dual gas atmospheres and direct exposure to molten alkali carbonate electrolytes. These factors introduce complex and interconnected degradation, primarily involving corrosion, oxidation, carburization, interdiffusion, and thermal/mechanical fatigue. These processes weaken the electrochemical and structural integrity of components such as current collectors, bipolar plates, electrodes, and seals.

The local electrochemical environment strongly influences degradation mechanisms. On the cathode side, oxidative degradation is dominant due to exposure to oxygen-rich atmospheres and molten carbonates. In contrast, carburization, sulfidation, and internal oxidation are more common on the anode side under reducing and hydrogen-rich conditions. In MCE mode (reverse operation), polarity and gas environments are inverted, often intensifying corrosion rates and triggering failure mechanisms not commonly observed during standard fuel cell operation [21].

Corrosion increases ohmic resistance, structural degradation, and electrolyte contamination with metallic ions such as Cr and Ni. This can result in electrolyte wear out, seal failure, gas leakage, and an eventual decrease in cell performance. In addition, corrosion may affect the nickel-coated copper bus bars [22], which are essential for current connection. In MCFC systems, these components operate under extreme conditions of high temperatures and currents, making direct inspection difficult.

3.1. Common Degradation Mechanisms

There are several primary mechanisms driving degradation in MCFCs and MCEs. Together with their descriptions, these are presented in

Table 1. Degradation mechanisms and their description.

Degradation Mechanism	Description/Features
High-temperature oxidation and fluxing	Formation of oxide scales like Cr2O3, Fe2O3, and (Fe,Cr,Ni)3O4 that dissolve or destabilize in molten carbonates [23].
Carburization and internal oxidation	Carbon and hydrogen ingress on the anode side leads to grain boundary embrittlement and oxide scale degradation [24].
NiO cathode dissolution and redeposition	NiO dissolves in carbonate and redeposits on the anode or matrix, causing electrical shorts and clogging [25].
Chromium migration and electrolyte contamination	Stainless steel Cr forms soluble chromates, contaminating the electrolyte and poisoning the cathode [24,26].
Redox cycling and thermal fatigue	Thermal expansion mismatches during redox cycling cause oxide cracking and delamination [27,28].
Electrolyte loss through evaporation	Molten electrolyte evaporates or migrates, increasing resistance and risking dry-out near cell edges.
Electrode coarsening and poisoning	Cathode and anode degradation from sintering, carbon deposition, and fuel contaminants like sulfur.
Seal degradation	Electrolyte migration to seals triggers corrosion or incompatibility at ceramic–metal interfaces.

.

In addition to the primary mechanisms listed in Table 1, several issues worsen cell materials’ degradation behavior. Electrolyte loss, whether through evaporation or migration, is a particularly undetectable failure mode, leading to uneven ionic conductivity and increased ionic resistance. This issue is often most noticeable near cell edges and seal zones.

Electrode degradation is also a significant problem. NiO cathodes tend to suffer from coarsening, sintering, and a loss of porosity, especially under fluctuating redox and thermal conditions. These changes reduce the active surface area available for reaction and increase polarization resistance. Ni-based anodes are similarly affected by carbon deposition (especially under hydrocarbon-rich or -low steam-to-carbon ratio conditions) and sulfur poisoning when even small amounts of contaminants are present in the fuel stream [29].

Redox cycling represents another degradation pathway, particularly affecting Ni-based electrodes. Shutdowns and startups or oxidant/fuel leakages can cause repeated oxidation and reductions in Ni or NiO electrodes [23,30]. These redox transitions involve significant volumetric changes and mechanical strain at the particle and interface level. Over time, these stresses can lead to fuel electrode cracking, structural coarsening, delamination, and the formation of electronically conductive paths through the electrolyte matrix due to nickel migration. An air electrode subjected to reducing conditions may lose porosity and experience irreversible morphological changes, contributing to rising polarization losses [21,31].

Thermal fatigue can also contribute to material degradation, particularly during startup and shutdown cycles [9]. Repeated thermal cycling imposes mechanical stresses arising from mismatched thermal expansion coefficients among cell components, especially between the molten carbonate electrolyte, ceramic matrix, and metal hardware [9]. These stresses can result in microcracking within the matrix, electrode delamination, and the degradation of electrical contact at interfaces. The solidification of the carbonate melt during cooling amplifies these effects, often leaving the cell susceptible to increased leakage, seal failure, and a loss of electrochemical performance [32].

Seal degradation is also often overlooked but can play a significant role in system longevity and safety. The capillary migration of molten carbonate into the seal zone can initiate galvanic reactions, promote material incompatibility, and result in local failure at metal–ceramic interfaces. These interactions are difficult to predict and mitigate but can be severe enough to cause cell failure.

A summary of primary degradation mechanisms occurring at different components and interfaces within the MCFC cell is presented in Figure 3.

Together, these degradation mechanisms highlight the need for an integrated approach to material selection, protective strategies, and operational control in order to extend the operational lifetime and performance of MCFC and MCE stacks.

3.2. How Corrosion Affects Cell Components

Corrosion in high-temperature electrochemical systems arises from the combined effects of elevated temperatures, reactive gas atmospheres, and the presence of aggressive ionic species such as carbonate, oxide, or hydroxide ions. In molten carbonate-based systems, whether MCFC or MCE, this process is further intensified by the molten electrolyte, which dissolves metal oxides, facilitates redox reactions, and transports corrosive ions across interfaces (Figure 4).

The electrolyte in these systems consists of a eutectic mixture of lithium and potassium or sodium carbonates. This molten salt enables efficient ionic transport but is also highly corrosive to most structural and conductive metals. Even alloys considered corrosion-resistant in dry environments degrade rapidly in the presence of molten carbonate due to the solubility of their oxide layers and high ion mobility.

In MCFC mode, the cathode is exposed to an oxygen and CO₂ mixture, resulting in a strongly oxidizing environment. This facilitates the formation of oxide scales, typically rich in chromium, nickel, or iron. Additionally, these oxides can dissolve into the electrolyte, migrate across the cell, and redeposit in other places within the cell, where they interfere with electrochemical reactions or clog electrode pores. Cr-containing alloys are especially affected, where continuous oxidation and dissolution deplete the alloy and deposit insulating Cr-based particles that poison the cathode [33,34].

Conversely, the anode operates in a reducing environment rich in hydrogen or hydrocarbons. Such an environment promotes carburization (Figure 5), where carbon diffuses into the metal, embrittling grain boundaries and changing the microstructures. Small amounts of sulfur compounds in the fuel can also lead to sulfidation, forming unstable and non-conductive sulfide films on electrodes [24]. A summary of corrosion influence depending on the cell zone in MCFCs is presented in Table 2.

In MCE mode, electrochemical conditions reverse, meaning the cathode functions as an anode and is subjected to intense oxidative stress. These reversed conditions can degrade materials by shifting corrosion equilibria and initiating oxide layer breakdown. Additionally, the gas composition also changes and oxygen evolution at the anode increases oxidation, while the presence of hydrogen and CO₂ on the cathode affects the local reactions and further promotes corrosion.

Transient operation and higher current densities, which are more typical in electrolyzer mode, further worsen corrosion. These conditions lead to thermal gradients and redox fluctuations that can accelerate the dissolution of protective oxides and increase the mobility of dissolved metal species [21].

One of the concerns regarding MCE systems is electrolyte contamination caused by corrosion products. High-purity gas requirements mean that even small concentrations of dissolved Fe, Cr, or Ni can decrease performance, facilitate side reactions, and reduce electrochemical efficiency.

The complexity of corrosion in MCFCs and MCEs lies behind its systemic and irreversible character. While mechanical failures are typically localized and immediate, corrosion propagates progressively in many places of the cell/stack simultaneously. This can start with minor changes like a slight increase in internal resistance that can later develop into performance loss, delamination, or short circuits.

Bipolar plates/interconnects and current collectors, which serve both mechanical and electrical roles, are especially affected. Corrosion causes oxide scale growth, alloy depletion, and the formation of mobile species that contaminate adjacent components. These cascading effects distinguish corrosion from more isolated degradation forms, such as electrode sintering or seal failure [23,25,36].

Moreover, the formation of thick or cracked oxide scales decreases electrical conduction and introduces thermal gradients, further accelerating system degradation. Even minor coating defects can serve as starting points for degradation and eventual mechanical failure [37,38].

Table 2. Summary of corrosion behavior by cell zone in MCFCs.

Zone	Corrosive Conditions	Key Material Issues	Reference
Cathodic	High O2 and CO2 concentrations; oxidation of Fe and Cr to LiFeO2 and LiCrO2; chromate formation over time.	Chromate formation compromises cathode function and contaminates electrolyte.	[25,26,36]
Anodic	Reducing gases (CO, H2, CH4); promotes carburization, Cr diffusion, Ni alloy degradation, and internal oxidation.	Ni-clad 310S is vulnerable to intermetallic formation and internal oxidation.	[23,35,39]
Sealing	Thermal and chemical gradients; electrolyte wicking; galvanic coupling; corrosion mitigated by Al-based LiAlO2 coatings.	Al-based coatings are chemically stable but electrically insulating; not suitable for current-carrying components.	[23,40,41]

Table 2. Summary of corrosion behavior by cell zone in MCFCs.

Zone	Corrosive Conditions	Key Material Issues	Reference
Cathodic	High O2 and CO2 concentrations; oxidation of Fe and Cr to LiFeO2 and LiCrO2; chromate formation over time.	Chromate formation compromises cathode function and contaminates electrolyte.	[25,26,36]
Anodic	Reducing gases (CO, H2, CH4); promotes carburization, Cr diffusion, Ni alloy degradation, and internal oxidation.	Ni-clad 310S is vulnerable to intermetallic formation and internal oxidation.	[23,35,39]
Sealing	Thermal and chemical gradients; electrolyte wicking; galvanic coupling; corrosion mitigated by Al-based LiAlO2 coatings.	Al-based coatings are chemically stable but electrically insulating; not suitable for current-carrying components.	[23,40,41]

The detection of corrosion is challenging due to its gradual nature. It can only be indicated by long-term voltage decreases, increases in polarization losses, and poor current distribution.

Differences in MCFC and MCE operation require tailored mitigation strategies, as no single material or coating can offer equal protection in both environments.

Corrosion is one of the most dangerous degradation mechanisms in high-temperature fuel cells and electrolyzers. It impacts nearly every component and interface within the cell stack. Effective protection therefore requires an in-depth understanding of corrosion behavior under both fuel cell and electrolysis conditions and a combination of material, surface, and operational strategies.

4. Ways of Dealing with Degradation

As stated before, protection strategies or a combination of strategies must be applied to mitigate degradation, particularly corrosion, in MCFC and MCE metallic components like current collectors and bipolar plates. These involve a selection of new, more resistant alloys, the application of protective coatings, and maintaining controlled operational parameters. Each method has specific advantages and limitations depending on the component type, location in the cell/stack, and electrochemical environment.

The complexity of degradation phenomena across different zones in the cell stack requires a specific material design for each zone. Cathodic current collectors require electrically conductive and oxidation resistant materials, anodic surfaces require carburization resistance and structural integrity under reducing conditions, and seal areas prioritize chemical inertness and thermal compatibility.

The following subsections examine the materials and technologies employed to improve durability and reduce performance losses caused by corrosion.

4.1. Material Selection for Current Collectors and Bipolar Plates

Current collectors and bipolar plates are the components most affected by corrosion due to their exposure to both reactive gases and molten carbonates. These parts must combine high electrical conductivity with mechanical strength and chemical stability [17].

Austenitic stainless steels such as AISI 310, 310S, 316, 316L, and 304 are commonly used in MCFCs. Type 310S (25% Cr, 20% Ni) offers strong oxidation resistance and mechanical stability but can suffer from chromium volatilization and chromate formation. Type 316L, containing molybdenum, resists corrosion via stable passive film formation but shows limited CO₂ tolerance and thicker oxide growth under high-temperature operation [24,26]. More affordable options like 304 stainless steel provide easier manufacturability but lower oxidation resistance.

Under continuous operation, stainless steels form oxide scales such as Fe₂O₃ and (Fe,Ni)Cr₂O₄, which can transform into lithium-rich compounds like LiFeO₂ and LiFe₅O₈ due to lithium infiltration from the electrolyte (Equations (4) and (5)). In 310S, Cr₂O₃ may react with Li₂O to produce soluble chromates like LiCrO₂ (Equation (6)), accelerating corrosion and degrading the electrolyte [26].

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{L}{\mathrm{i}}_2\mathrm{O}\to 2\mathrm{L}\mathrm{i}\mathrm{F}\mathrm{e}{\mathrm{O}}_2$$

(4)

$$\mathrm{L}\mathrm{i}\mathrm{F}\mathrm{e}{\mathrm{O}}_2+2\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\to \mathrm{L}\mathrm{i}\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_8$$

(5)

$$\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3+\mathrm{L}{\mathrm{i}}_2\mathrm{O}\to 2\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{r}{\mathrm{O}}_2$$

(6)

High-Mn austenitic steels such as DIN 1.3816 and Nitronic 50 have gained interest due to their forming of stable Mn-containing spinels (e.g., LiMn₂O₄, (Fe,Mn)Cr₃O₄), which exhibit better conductivity and corrosion resistance while reducing dependence on costly Cr and Ni [25,26].

Ferritic Fe–Cr–Al alloys show excellent stability in wet-seal and anode environments due to LiAlO_2- and Al-rich oxide layers forming. However, their low electrical conductivity limits their use in current-carrying components [23,28].

Experimental alloy compositions, such as 22Cr-11Ni-Mn-Nb-W-N, have demonstrated distinguished long-term performance. Such alloys form dual-layer oxide scales where there is a Cr-rich inner layer and Mn-doped LiFeO₂ outer layer, combining low resistance between faces with extended corrosion resistance for over 2000 h [28].

On the anodic side, Ni-clad steels are used for their chemical stability under reducing conditions. However, extended operation leads to grain boundary diffusion and the eventual degradation of the protective Ni layer [35,42].

Bipolar plates, which experience both oxidative and reductive stress, undergo complex degradation. FeCrMnNi alloys such as 1.3965 are particularly promising, forming stable (Fe,Ni,Mn)_xCr_3−xO₄ spinel layers that resist Fe diffusion and maintain structural and electrical integrity over time [25].

Material selection for current collectors and bipolar plates involves a compromise between cost, corrosion resistance, mechanical stability, and electrical performance. High-performance alloys like Inconel 625 or 254 SMO may offer excellent corrosion resistance but are expensive. Less expensive steels or alloys may require protective coatings to remain stable. Optimizing material choice depends on the application, stack configuration, and cost constraints.

A combination of experimental validation and diagnostic techniques, such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRPD), and electrochemical impedance spectroscopy (EIS), may help characterize corrosion products and assess protective layer integrity. Techno-economic assessments help to balance performance benefits against manufacturing scalability and total system cost in material selection.

4.2. Alloy Development

To improve corrosion resistance and reduce dependency on expensive noble metals, substantial research has focused on developing advanced alloys for MCFC and MCE environments. The objective is to form stable, adherent oxide scales that will act as an effective protection against metal ion diffusion and corrosive electrolyte attack, while maintaining good electrical and mechanical performance.

One promising approach is the incorporation of Mn, Ti, Nb, and W into Fe–Cr-based alloys. Manganese promotes the formation of Mn-doped LiFeO₂ spinels, which exhibit enhanced conductivity and chemical stability. Niobium and tungsten promote the formation of compact, slow-growing oxide layers, lowering scale resistivity and corrosion rates [28,43].

Optimizing Cr content can also play an important role. In alloys with less than 20 wt.%, Cr often fails to develop continuous protective oxides, while those exceeding 25 wt.% may generate thick, resistive LiAlO₂ scales that are susceptible to dissolution and electrolyte contamination [26,44]. Similarly, excessive Al content can be beneficial in forming LiAlO₂ but raises electrical resistivity and is best limited to sealing applications.

Low-Ni, high-Mn alloys are also gaining attention, as they mitigate cost and supply chain issues. These compositions use Mn and N as austenite stabilizers and have demonstrated corrosion resistance comparable to high-Ni stainless steels under cathodic conditions [26]. Studies reported by Ahn [45] show the impact of minor alloying elements, specifically Si, Mn, and Al, on the corrosion resistance of stainless steel 310S when used as a cathode current collector (CCC) material in MCFCs. In a 62 mol% Li₂CO₃–38 mol% K₂CO₃ melt at 650 °C, potentiodynamic and potentiostatic polarization tests revealed that the addition of these elements led to a decrease in corrosion resistance. While the corrosion potential of modified 310S gradually increased after 9 h due to an active-to-passive transition, steels alloyed with Si, Mn, or Al exhibited a more rapid potential shift within the first 6 h, indicating a higher reactivity. These alloying elements increased steady-state corrosion current densities and accelerated corrosion at the cathode operation potential (−40 mV vs. reference), underscoring their detrimental effect on CCC performance under typical MCFC operating conditions.

In addition to compositional changes, microstructural engineering can bring further improvements. Fine-grained, strengthened structures reduce grain boundary attacks and limit the diffusion of aggressive species. Secondary phase control and tailored carbide/nitride precipitation also contribute to corrosion stability, particularly in fluctuating redox environments.

Research into rare-earth element additions, such as yttrium, lanthanum, and cerium, has shown promising results in enhancing oxide scale adhesion and reducing cation diffusion. These elements are typically used in small quantities but can substantially extend oxide lifetime, especially under thermal cycling [46].

Relatively new alloy classes, including high-entropy alloys (HEAs) and compositionally complex alloys (CCAs), are being explored for their potential to combine multiple corrosion-resistant mechanisms in a single material system. These alloys offer new ways to balance corrosion protection, electrical performance, and mechanical compatibility [47,48].

Unfortunately, alloy development can face challenges related to scalability and processing complexity. High-performance alloys often mean high costs and/or complex fabrication. Therefore, alloy design is most effective when integrated into broader strategies, including protective coatings, which collectively can mitigate corrosion and degradation in MCFC and MCE systems.

4.3. Protective Coatings

Protective coatings play an important role in increasing corrosion resistance in MCFCs and MCEs, particularly in regions exposed to aggressive chemical environments such as cathode current collectors and seal zones. These coatings serve as physical and chemical barriers, isolating the metallic parts from molten carbonate exposure while maintaining electrical conductivity and thermal compatibility (Figure 6).

Effective coatings must exhibit chemical stability at 600–700 °C, mechanical stability and durability under thermal cycling, and compatibility with both oxidizing and reducing environments. They must also be adherent, dense, and compatible with the thermal expansion behavior of the materials to which they are applied.

Ceramic oxide coatings, especially those based on lithium cobaltite (LiCoO₂), lithium ferrite (LiFeO₂), and perovskites like LaFeO₃ and LaMeO₃, have shown promising results. These coatings, often applied via sol–gel or slurry deposition, create compact layers that hinder chromium migration and suppress electrolyte contamination [27]. Mixed compositions such as Li(Co_0.5Fe_0.5)O₂ offer a balance between conductivity and corrosion resistance.

Conversion coatings formed in situ, such as LaFeO₃ via molten salt synthesis, exhibit high stability and strong adhesion. These coatings avoid porosity-related degradation and ensure minimal underlayer corrosion during long-term use [50].

Non-oxide ceramic coatings including nitrides (CrN, TiN), carbides (TiC), and silicides have also demonstrated superior corrosion resistance. CrN, in particular, offers low contact resistance (~21.8 mΩ·cm²) and strong performance under both oxidative and reductive conditions. Physical Vapor Deposition (PVD)-deposited CrN/TiN multilayers further improve mechanical robustness and reduce pinhole defects [51,52,53].

Electrodeposited metal coatings such as Ni-Mo-P and Ni-Co-P layers provide another approach to corrosion protection. These coatings maintain low polarization resistance, prevent Cr ion release, and maintain surface integrity in molten carbonates [54].

Aluminized coatings are ideal for wet-seal regions, where LiAlO₂-forming layers created via aluminum cladding or slurry techniques demonstrate strong adhesion and long-term chemical stability. Cladded coatings offer superior uniformity and defect resistance compared to slurry applied layers (Figure 7) [40].

Multilayer coatings combining ceramic diffusion barriers and metallic conductive layers have also been proposed to enhance performance by balancing corrosion resistance, conductivity, and mechanical strength. Such strategies, while effective, often involve higher processing complexity and cost.

Application methods significantly influence coating performance. Techniques include sol–gel deposition, pack cementation, chemical vapor deposition (CVD), PVD, plasma spraying, and electrophoretic deposition. Each method offers trade-offs in terms of film uniformity, scalability, thickness control, and adherence [55].

Although protective coatings add processing complexity, they remain essential for extending the lifetime of MCFC/MCE components, especially when bulk alloy resistance is insufficient. When correctly designed and applied, coatings can significantly reduce degradation and enable the use of more cost-effective substrates without compromising system performance.

4.4. Other Mitigation Strategies

In addition to alloy development and protective coatings, a variety of supplementary strategies have been explored to manage corrosion in MCFCs and MCEs. These include electrolyte composition modification, electrode doping, surface engineering, and architectural adaptations.

One approach involves adjusting the electrolyte composition to lower its corrosivity. Using a more basic melt such as (Li_0.52Na_0.48)₂CO₃ instead of the standard (Li_0.62K_0.38)₂CO₃ has been shown to reduce oxide spallation and scale growth. However, such changes can alter conductivity and melting point, limiting their practical applicability [26,44].

Electrolyte doping with small quantities of BaCO₃, MgCO₃, or La₂O₃ can stabilize oxide scales and promote the in situ formation of protective perovskites like LiFeO₂. These additives also shift metal corrosion potentials to nobler values, improving passivation. Still, long-term operation may deplete or destabilize these dopants [56,57].

Cathode material modifications have also shown effectiveness. Dy-doped NiO cathodes demonstrate reduced solubility in carbonates and more stable surface lithiation. Dy additions increase the Ni³⁺/Ni²⁺ ratio and slow the rate of Ni dissolution without compromising conductivity (Figure 8) [58].

More advanced architectures like core–shell cathodes, which involve combining a corrosion-resistant shell (e.g., LiCoO₂ or MgFe₂O₄) around a conductive NiO base, have maintained porosity and minimized Ni migration under extended operating conditions [21,59].

Anode material modifications have also proven beneficial. Cu–50Ni–5Al alloys reinforced with 1% CeO₂ nanoparticles exhibit significantly enhanced wear and corrosion resistance in molten carbonate environments. Compared to the unreinforced alloy, the CeO₂-doped composite showed a 50% reduction in the coefficient of friction (COF) and a 25% decrease in scratch depth under progressive loading. The enhanced performance is linked to the formation of protective Al₂O₃, CuO, and NiO phases stabilized by CeO₂ addition, indicating superior electrochemical stability and mechanical integrity under MCFC-relevant conditions [60].

On the system design level, modifications such as thermally insulated seal zones, spring-loaded electrical contacts, and pressure-controlled stack assemblies help reduce thermal mismatch and mitigate corrosion in edge and seal areas [23]. Gas purification systems are also critical, removing sulfur, halides, or particulates to prevent any damage, especially under electrolysis mode.

The way in which the cell/stack is operated can also influence corrosion rates; this involves the gradual temperature increase or decrease during startup and shutdown, avoiding sudden thermal cycling, and maintaining reducing atmospheres during standby periods. All of these factors can contribute to a longer lifespan for the components. Thermal control can prevent capillary-driven electrolyte leakage and oxidation in seal areas. A summary of corrosion mitigation strategies in metallic components of MCFC/MCE systems are presented in

Table 3. Summary of corrosion mitigation strategies in MCFC/MCE systems.

Method	Material/Strategy	Side	Main Advantages	Limitations/Notes	Ref.
Material Selection	AISI 310S/316L stainless steels	Cathode/Anode	Commercially available, moderate corrosion resistance	Chromate formation, thick oxide growth	[24,26]
	High-Mn steels (e.g., DIN 1.3816)	Cathode	Spinel formation, reduced reliance on Cr/Ni	Less industrial data, long-term aging unclear	[25,26]
	FeCrAl, FeAl	Wet-seal/Anode	Forms LiAlO2, excellent passivation in CO2 rich zones	High resistivity, not suited for conductive components	[23,50]
	Ni-clad steels	Anode	Good initial resistance in reducing environments	Cr diffusion, coating breakdown over time	[25,35]
Alloy Development	22Cr-11Ni-Mn-Nb-W-N	Cathode	Compact oxides, Mn doping improves conductivity	Advanced alloying raises cost	[28,43]
Coatings	FeCrMnNi (1.3965)	Bipolar plates	Long-term stable oxide scales	Limited to specific applications	[25]
	LiCoO2/LiFeO2 (sol–gel)	Cathode	Moderate corrosion resistance, cost-effective	Porosity, oxide growth underneath	[26,27]
	LaFeO3 (conversion coating)	Cathode	Dense structure, adherent, low resistance	More complex application process	[50]
	TiN (PVD)	Cathode/Bipolar plates	Excellent electrical conductivity, low CR	Pinhole-induced failure risks	[51,52,53]
	Ni-Mo-P/Ni-Co-P (electroplated)	Cathode	Barrier to Cr leaching, stable morphology	Long-term stack validation pending	[54]
	Al cladding/LiAlO2 layer	Seal	High chemical stability, no chromate formation	Electrically insulating	[23,40]
Other Strategies	Li/Na carbonate blend	Electrolyte	Slightly lower corrosion rate, less Cr solubility	Temperature-sensitive, conductivity changes	[26]
	BaCO3, La2O3 additions	Electrolyte	Enhances surface passivation, perovskite formation	Additive consumption, potential melt destabilization	[26,56,57]
	Dy-doped NiO	Cathode	Reduced Ni solubility, improved lithiation	REE cost, material synthesis control	[58]
	Core-shell (e.g., NiO@LiCoO2)	Cathode	Less Ni dissolution, robust surface chemistry	Fabrication complexity	[21,59]
	Wet-seal design adaptations	Seal	Limits electrolyte creep, accommodates expansion	Mechanical complexity	[23,50]

.

Surface modification techniques, such as nitriding, carburizing, and laser surface melting, can offer additional barriers to enhance protection. These processes can improve grain boundary strength and surface hardness, limiting the localized penetration of corrosive species.

The most effective mitigation strategies involve combining the methods described above to achieve good protection. It is essential to address the harsh conditions in molten carbonate fuel cells and achieve long-term system reliability.

5. Discussion

The degradation of metallic components in MCFCs and MCEs presents a significant challenge to the development of this technology. This degradation is driven by a combination of electrochemical, thermal, and mechanical factors. Addressing this challenge requires an approach that involves advanced materials, protective surface treatments, and additional system-level design optimizations.

Among widely used stainless steels, AISI 310S exhibits excellent oxidation resistance but suffers from chromium volatilization and electrolyte contamination due to chromate formation. 316L, while effective in reducing environments, tends to form thick, resistive oxide scales under oxidizing conditions. Newer alternatives, such as high-Mn austenitic steels and FeCrMnNi alloys like 1.3965, have shown strong corrosion resistance due to the formation of stable spinel oxides. However, their long-term behavior under MCEC conditions, particularly during redox cycling and polarity reversals, remains underexplored.

Protective coatings remain essential to mitigating corrosion in sensitive zones, especially the cathode and wet-seal areas. Sol–gel-derived spinel and perovskite coatings offer process simplicity but often fail due to porosity and sub-coating oxidation over time. Conversion coatings like LaFeO₃ demonstrate denser, adherent layers with improved corrosion resistance, although they have scalability limitations. PVD-deposited nitrides, like TiN and CrN, show good electrical conductivity and resistance under both oxidative and reductive environments but remain sensitive to defects. A comparative analysis reveals that while coatings offer promising corrosion protection, their performance often deteriorates under thermal cycling and extended redox conditions, underscoring the need for long-term validation studies and improved deposition techniques.

Among surface treatment approaches, nanostructured Ni-Mo-P and Ni-Co-P electrodeposited films have demonstrated industrial scalability and electrochemical robustness. Their ability to suppress Cr dissolution and maintain low polarization resistance presents a commercially attractive pathway, providing that substrate compatibility and thermal stability are ensured. Aluminized coatings and FeAl-based alloys, while electrically insulating, are ideal candidates for wet-seal regions where mechanical and chemical stability take precedence over conductivity. Future developments could focus on hybrid or functionally graded coatings that integrate insulating and conductive zones for more versatile protection.

Advanced alloy development has produced compositions that optimize Cr, Mn, and Nb content to create dual-layer oxides and low-resistance spinels. However, economic viability and manufacturability still limit widespread adoption, especially for alloys requiring high purity. The need for dual-functionality in bipolar plates, stable in both anodic and cathodic zones, further narrows the range of suitable compositions.

Electrolyte modification remains a relatively underdeveloped but potentially transformative area of research. Current carbonate mixtures are inherently corrosive due to their capacity to dissolve metal oxides and facilitate ionic transport. Substituting or doping electrolytes with stabilizing additives (e.g., alkaline earth carbonates or rare earth oxides) could reduce their aggressiveness and stabilize surface oxides in situ. However, more systematic studies are required to assess the thermodynamic compatibility, ionic conductivity, and operational stability of such modified electrolytes. The development of corrosion-resistant electrolyte formulations remains a key opportunity to reduce material requirements across the entire system.

Beyond material and coating technologies, other methods like gas purification, surface treatments, and mechanical design can reduce stress concentrations and electrolyte migration. These methods are often more cost-effective and scalable in already existing systems.

A comparative evaluation reveals that no single strategy universally applies across all components. For instance, bipolar plates/interconnects exposed to both oxidizing and reducing conditions benefit from coatings more than bulk alloying, while current collectors may demand materials with intrinsic resistance to long-term oxide growth. Electrolyzer operation, in particular, introduces asymmetrical redox environments and more aggressive oxidative stress, requiring materials that can maintain stability under polarity inversion and high oxygen activity.

From a techno-economic point of view, hybrid approaches combining moderately priced alloys with applied protective layers or electrolyte additives can offer the best compromise between performance and cost. Future directions will likely emphasize multifunctional and adaptive solutions. Achieving this will require the close integration of electrochemistry, materials science, and mechanical engineering.

In conclusion, effective long-term corrosion mitigation in molten carbonate systems will rely not on a single solution, but on customized strategies that combine material properties, operating conditions, and cost-performance considerations.

6. Conclusions

The degradation and corrosion of metallic components remain among the most significant challenges to the reliable, long-term operation of high-temperature electrochemical systems, such as MCFCs and MCEs. These systems operate under harsh conditions that include high temperatures, reactive gas environments, and corrosive molten carbonate electrolytes, all of which impose severe chemical and mechanical stresses on cell materials.

This review has synthesized the current understanding of corrosion mechanisms and evaluated mitigation strategies in MCFC and MCEC systems, from advanced Fe-Cr-Mn alloys to functional coatings such as LaFeO₃, CrN, and nanostructured Ni-Mo-P. While each approach offers specific advantages, they also come with limitations related to cost, scalability, or long-term stability under operational stress. No single solution currently meets all the demands imposed by the varied and aggressive environments within these systems.

The most promising approaches lie in integrated, zone-specific strategies that combine protective coatings, alloy optimization, and system-level design modifications. Future research should prioritize scalable, cost-efficient methods that are capable of maintaining durability for over 35,000 h, with a particular emphasis on electrolyte stability, redox-resilient materials, and multifunctional protective layers.

To enable widespread deployment, further progress will require coordinated advances across materials science, electrochemical engineering, and stack design. Successfully addressing corrosion will be key to unlocking the full potential of molten carbonate technologies in a hydrogen-driven, low-carbon energy future.