Corrosion behavior of SS316L in simulated and accelerated PEMFC environments

The electrochemical behavior and change in the passive film formation of SS316L are investigated under polymer electrolyte membrane fuel cell (PEMFC) simulated (pH from 3 to 6 containing F⁻, SO₄²⁻ and Cl⁻ anions) and accelerated conditions (0.5 M and 1 M H₂SO₄ + 2 ppm HF). Potentiodynamic, potentiostatic, and EIS measurements are performed to investigate the electrochemical behavior of the SS316L specimens in both the anode and cathode PEMFC environments. The chemical composition of the passive film, surface topography of the specimens, and degree of metal ion release is characterized by XPS, SEM, and ICP-OES, respectively. The results reveal that the nature of the passive film depends on the pH value, external medium/environment, as well as applied potential during polarization. The corrosion behavior of SS316L is closely related to the chemical composition and structure of the passive film.     

Factors affecting corrosion behavior of SS316L as bipolar plate material in PEMFC cathode environments

An empirical corrosion model for SS316L in simulated proton exchange membrane fuel cell (PEMFC) environments is developed based on systematic experimental data on the effects of various factors, such as acidity, fluoride ion concentration, temperature and polarization potential. Correlation parameters under different conditions are provided in tabulated forms and comparisons of the empirical model with experimental results are shown in graphical forms. The results show that the empirical model agrees very well with the experimental data except at the short initial polarization time and the model is applicable up to a polarization potential of 0.7 V. The results also show that polarization potential is the most sensitive parameter among all the parameters studied.     

Investigation of acidity on corrosion behavior and surface properties of SS304 in simulated PEMFC cathode environments

This study presents the influence of acidity on the corrosion performance and surface properties of AISI 304 stainless steel (SS304) in the simulated cathode condition of proton exchange membrane fuel cells (PEMFC) with various concentrations of H₂SO₄. The electrochemical tests indicate that the corrosion resistance of SS304 samples decreases gradually with the solution acidity ascending, but the stable current densities (0.043–0.547 μA cm^?2) in the simulated solutions after polarization (0.6 V, 5 h) are all lower than that of the relevant DOE 2025 target (i_corr < 1 μA cm^?2). Obvious pitting corrosion occur in the solutions with H₂SO₄ concentration higher than 10^?3 M. The surface wettability and interfacial contact resistance (ICR) of the potentiostatically polarized SS304 show an upward trend with the solution acidity increasing, and whether the SS304 samples are polarized or not, their ICR (0.274–1.232 Ω cm²) is far higher than the latest DOE 2025 technical target (<0.01 Ω cm²). The results reveal that surface modification is indispensable for SS304 as bipolar plates, and more attention should be paid to possessing high and stable pitting resistance, hydrophobicity, and interfacial conductivity in an acid environment.     

Effect of fluoride ions on corrosion behavior of SS316L in simulated proton exchange membrane fuel cell (PEMFC) cathode environments

In order to determine the suitability of SS316L as a bipolar plate material in proton exchange membrane fuel cells (PEMFCs), its corrosion behavior is studied under different simulated PEMFC cathode corrosion conditions. Solutions of 1 × 10⁻⁵ M H₂SO₄ with a wide range of different F⁻ concentrations at 70 °C bubbled with air are used to simulate the PEMFC cathode environment. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to study the corrosion behavior. Scanning electron microscopy (SEM) is used to examine the surface morphology of the specimen after it is potentiostatic polarized under simulated PEMFC cathode environments. Auger electron spectroscopy (AES) analysis is used to identify the composition and the depth profile of the passive film formed on the SS316L surface after it is polarized in simulated PEMFC cathode environments. Photo-electrochemical (PEC) method and capacitance measurements are used to characterize the semiconductor passive films. The results of both the potentiodynamic and potentiostatic analyses show that corrosion currents increase with F⁻ concentrations. SEM examination results indicate that pitting occurs under all the conditions studied and pitting is more severe with higher F⁻ concentrations. From the results of AES analysis, PEC analysis and the capacitance measurements, it is determined that the passive film formed on SS316L is a bi-layer semiconductor, similar to a p-n heterojunction consisting of an external n-type iron oxide rich semiconductor layer (electrolyte side) and an internal p-type iron-chromium oxide semiconductor layer (metal side). Further analyses of the experimental results reveal the electronic structure of the passive film and shed light on the corrosion mechanisms of SS316L in the PEMFC cathode environment.     

Influence of fluoride ions on corrosion performance of 316L stainless steel as bipolar plate material in simulated PEMFC anode environments

Corrosion performance of 316L stainless steel as a bipolar plate material in proton exchange membrane fuel cell (PEMFC) is studied under different simulated PEMFC anode conditions. Solutions of 1 × 10⁻⁵ M H₂SO₄ with a wide range of different F⁻ concentrations at 70 °C bubbled with hydrogen gas are used to simulate the PEMFC anode environments. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to study the corrosion behavior. Scanning electron microscope (SEM) and atomic force microscope (AFM) are used to examine the surface morphology of the specimen after it is potentiostatic polarized in simulated PEMFC anode environments. X-ray photoelectron spectroscopy (XPS) analysis is used to identify the compositions and the depth profile of the passive film formed on the 316L stainless steel surface after it is polarized in simulated PEMFC anode environments. Mott–Schottky measurements are used to characterize the semiconductor passive films. The results of potentiostatic analyses show that corrosion currents increase with F⁻ concentrations. SEM examinations show that no localized corrosion occurs on the surface of 316L stainless steel and AFM measurement results indicate that the surface topography of 316L stainless steel becomes slightly rougher after polarized in solutions with higher concentration of F⁻. From the results of XPS analysis and Mott–Schottky measurements, it is determined that the passive film formed on 316L stainless steel is a single layer n-type semiconductor.     

Electrochemical nitridation of a stainless steel for PEMFC bipolar plates

AISI446 steel has been electrochemically nitrided in 0.1 M HNO₃ + 0.5 M KNO₃ solution at room temperature. XPS analysis revealed surface NH₃ and a deeper nitride layer. The surface layer of the stainless steel modified by electrochemical nitridation was thus composed of a nitrogen-incorporated oxide film. The nitrided steel showed very low interfacial contact resistance (ca. 18 mΩ cm² at 140 N/cm²) and excellent corrosion resistance in simulated PEMFC environments. Electrochemical nitridation provides an economic way to modify the stainless steel’s surface, and is very promising for application to fuel cell bipolar plates.     

Austenitic stainless steels in high temperature phosphoric acid

Austenite 316 L, 317 L, and 904 L stainless steels were investigated in 98% H₃PO₄ at 170 °C and they experienced passivation regardless of the purged gas. When polarized at 0.1 V (hydrogen) and 0.7 V (air) (phosphoric acid fuel cell (PAFC) environments), currents at the level of mA cm⁻² were observed. Compared to carbon composite under identical conditions, 904 L showed lower currents while 316 L and 317 L showed much higher currents.

X-ray photoelectron spectroscopy (XPS) depth profiles indicated that the surface film of the fresh steels consists of a Fe-oxide-rich outer layer and a Cr-oxide-rich inner layer. After being polarized in the PAFC environments, the Fe-oxide layer was selectively dissolved and Cr-oxide dominated the passive film. Phosphorus was incorporated into the film during the process, thus the chemical composition of the passive film differed from those formed in the polymer electrolyte membrane fuel cell (PEMFC) environments. The thicknesses of the stainless steels in the passive films in PAFC environments were estimated.     

Corrosion characteristics of SS316L as bipolar plate material in PEMFC cathode environments with different acidities

The corrosion characteristics of SS316L in simulated proton exchange membrane fuel cell (PEMFC) environments with a wide range of H₂SO₄ concentrations have been systematically studied. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to determine the corrosion parameters and the results show that corrosion resistance decreases with increasing H₂SO₄ concentrations. Scanning electron microscope (SEM) is used to examine the surface morphology of the specimens after potentiostatic polarized in simulated PEMFC cathode environments and the results indicate that local corrosion occurs under all the conditions studied and local corrosion is more severe with higher H₂SO₄ concentrations. Auger electron spectroscopy (AES) analysis is used to identify the composition and the depth profile of the passive film formed on the SS316L surface and the results show that the thickness of passive film decreases with increasing H₂SO₄ concentrations. Interfacial contact resistances (ICR) between SS316L polarized and carbon paper are measured and the results show that ICR decreases with increasing H₂SO₄ concentrations. The corrosion mechanisms of SS316L in PEMFC cathode environments are analysed and discussions on choosing simulated PEMFC cathode corrosion environments for accelerated tests are also provided.     

Applicability of extra low interstitials ferritic stainless steels for bipolar plates of proton exchange membrane fuel cells

Ferritic stainless steels can be attractive bipolar plate materials of proton exchange membrane fuel cells (PEMFC), provided that the stainless steels show sufficient corrosion resistance, for instance, by eliminating interstitial elements such as carbon and nitrogen. In the present study, thus, ferritic stainless steels (19Cr2Mo and 22Cr2Mo) with extra low interstitials (ELI) are evaluated to determine the required level of chromium content to apply them for PEMFC bipolar plates. In a simulated PEMFC environment (0.05 M SO₄²⁻ (pH 3.3) + 2 ppm F⁻ solution at 353 K), the 22Cr2Mo stainless steel showed lower current density during the polarization in comparison with the 19Cr2Mo one. The polarization behavior of the 22Cr2Mo stainless steel resembles that of the type 316 one (17Cr12Ni2Mo). Similar values of interfacial contact resistance (ICR) are observed for both ferritic stainless steels. The 22Cr2Mo stainless steel bipolar plate is found to be stable throughout the cell operation, while the 19Cr2Mo stainless steel corroded within 1000 h. After the cell operation, the 22Cr2Mo stainless steel retains the chromium enriched passive film, while the chromium enriched surface film is not found for the 19Cr2Mo one, showing iron oxide/hydroxide based film. X-ray fluorescence (XRF) analysis of the membrane electrode assemblies (MEAs) after the cell operation indicates that the 22Cr2Mo stainless steel was less contaminated with iron species. The above results suggest that the 22Cr2Mo stainless steel can be applicable to bipolar plates for PEMFC, especially 22 mass% of chromium content in ferritic stainless steel with ELI system is, at least, demanded to ensure stable cell performance.     

Effects of thermal oxi-nitridation on the corrosion resistance and electrical conductivity of 446M stainless steel for PEMFC bipolar plates

Stainless steel has attracted interest as a bipolar plate material for polymer electrolyte membrane fuel cells due to its excellent mechanical properties, good corrosion resistance, and low cost. However, the application of thermal nitridation for the improvement of electrical conductivity deteriorates the corrosion resistance under PEMFC operating conditions due to the discontinuous formation of external Cr-nitride. In this study, nitridation with pre-oxidation of 446M stainless steel was performed in order to improve both the corrosion resistance and the electrical conductivity. 446M stainless steels with oxide and nitride on the surface were evaluated to assess their feasibility as a bipolar plate material for PEMFCs. The results were compared with those obtained using as-received and only nitrided 446M stainless steels. The oxide formed by the pre-oxidation protects the surface of 446M stainless steel from corrosion in corrosive environments, especially under cathode conditions, and the Cr-nitride formed by the subsequent nitridation serves as an electro-conductive channel. As a result, the pre-oxidized, nitrided 446M stainless steel exhibits improved corrosion properties and electrical conductivity under PEMFC operating environments.     

SnO2:F coated austenite stainless steels for PEM fuel cell bipolar plates

Austenite stainless steels (316L, 317L, and 349™) have been coated with 0.6 μm thick SnO₂:F by low-pressure chemical vapor deposition and investigated in simulated polymer electrolyte membrane fuel cell (PEMFC) environments. The results showed that substrate steel has a significant influence on the behavior of the coating. Coated 316L showed a steadily increasing anodic current in PEMFC environments, indicating that it is not suitable for this alloy/coating combination. Coated 349™ showed a cathodic current in the PEMFC anode environment, demonstrating its stability in the PEMFC cathode environment. Coated 317L exhibited a stable anodic current after a current peak (at ca. 14 min) in the PEMFC anode environment, and showed an extremely stable low current in PEMFC cathode environment, suggesting the possibility of using SnO₂:F coated 317L for PEMFC bipolar plate applications.     

High nitrogen stainless steel as bipolar plates for proton exchange membrane fuel cells

To investigate the applicability of high nitrogen (HN) austenitic stainless steel as bipolar plates for proton exchange membrane fuel cells (PEFCs), the polarization tests were carried out in synthetic solutions (0.05 M SO₄²⁻ (pHs 2.3, 4.3 and 5.5) +2 ppm F⁻) at 353 K. Interfacial contact resistance between the stainless steel and gas diffusion layer was measured before and after polarization. A single cell employing the HN stainless steel as bipolar plates was operated for 1000 h at 0.5 A cm⁻² (12.5 A). The single cell exhibited voltage drop of 17 mV during the operation. Corrosion products were scarcely detected for the HN stainless steel bipolar plate, as confirmed by scanning electron microscopy. After the polarization tests and single cell operation, XPS analyses were carried out to examine the resulting surface states. In the synthetic solutions to pH 4.3, the passive films mainly consisted of oxides enriched with Cr. When the solution pH was 5.5, on the other hand, the films were mainly composed of Fe-oxides. After the single cell operation for 1000 h, it was found that the passive films of the rib surface for the gas inlet part was mainly composed by Fe-oxides. On the other hand, the passive films for the ribs from center to gas outlet part were mainly made up of Cr-oxides. By combining the simulated and real operation environments, it is believed that the corrosion resistive Cr-oxides passive layer of the HN stainless steel obtained by the presence of nitrogen incorporated into the stainless steel could contribute to the maintenance of the higher cell voltage during the extensive cell operation.     

SnO2:F coated ferritic stainless steels for PEM fuel cell bipolar plates

Ferrite stainless steels (AISI441, AISI444, and AISI446) were successfully coated with 0.6 μm thick SnO₂:F by low-pressure chemical vapor deposition and investigated in simulated PEMFC environments. The results showed that a SnO₂:F coating enhanced the corrosion resistance of the alloys in PEMFC environments, though the substrate steel has a significant influence on the behavior of the coating. ICP results from the testing solutions indicated that fresh AISI441 had the highest dissolution rates in both environments, and coating with SnO₂:F significantly reduced the dissolution. Coating AISI444 also improved the corrosion resistance. Coating AISI446 steel further improved the already excellent corrosion resistance of this alloy. For coated steels, both potentiostatic polarizations and ICP results showed that the PEMFC cathode environment is much more corrosive than the anode one. More dissolved metallic ions were detected in solutions for PEMFC cathode environment than those in PEMFC anode environment. Sn²⁺ was detected for the coated AISI441 and AISI444 steels but not for coated AISI446, indicating that the corrosion resistance of the substrate has a significant influence on the dissolution of the coating. After coating, the ICR values of the coated steels increased compared to those of the fresh steels. The SnO₂:F coating seems add an additional resistance to the native air-formed film on these stainless steels.     

Corrosion resistance of chromized 316L stainless steel for PEMFC bipolar plates

Corrosion resistance of the chromized 316L stainless steel was studied in a proton exchange membrane fuel cell (PEMFC) operating condition. Cr-rich surface layer was formed by pack cementation technique and electrochemical properties of the chromized surface were examined by potentiodynamic and potentiostatic tests. Results showed that the Cr-rich layers underneath the free surface passivated the surface and protect the surface from corrosion in 0.5 M H₂SO₄ solution at 80 °C. However, the Cr-rich layers showed columnar grains with voids when the stainless steel was pack cemented for an extended period of time, resulting in drastic degradation of corrosion resistance. The optimum condition for the best corrosion resistance in the PEMFC operating condition was obtained without sacrificing the interfacial contact resistance.     

Coating of stainless steel and titanium bipolar plates for anticorrosion in PEMFC: A review

Anticorrosion coating for stainless steel (SS) and titanium bipolar plates were evaluated to improve the corrosion resistance and electrical conductivity in PEMFC. The PEMFC offers clean and environmentally friendly usage in electrical power systems. The bipolar plates contribute 60%–80% of the total components of PEMFC stack with electrical conductivity >100 S cm^?1. Therefore, high conductivity and corrosion resistance are observed for long-term operations in PEMFC. Recent works has developed the cost-effective and feasible alternative materials to replace graphite bipolar plates. Metallic materials, such as SS and titanium, possess good electrical conductivity but poor corrosion resistance. Coating of SS and titanium bipolar plates can improve the corrosion resistance of metallic bipolar plates. Excellent performance of bipolar plates was recorded by using NbC coating for stainless steel materials. The ICR value using plasma surface alloying method was 8.47 mΩ cm² with a low current density (I_corr) between 0.051 and 0.058 μA cm^?2. The criteria for both current densities (<1 μA cm^?2) and electrical conductivity (<10 mΩ cm²) met the DOE's 2020 technical targets. In addition, conventional air brush method can be used for fabricating multilayer coatings onto substrates because it is self-cleaning, low cost and offers high volume and large area production. Vapor deposition method, a highly advanced coating technology using PVD, suitable for coating bipolar plates because it is environmentally friendly and can be used in high temperatures, producing materials with good impact strength and excellent abrasion resistance. PEMFC cost is still too high for large scale commercialization, which is the cost of raw material and processing to allow fabrication of thinner plates contributes substantially to the total PEMFC cost. Some future works on fuel cell anticorrosion research with reasonable coating method is suggested to reduce the cost in order to facilitate the move toward commercialization especially for SS and titanium bipolar plates.     

Characterization and performance of UNS S63019 (21-4N) as bipolar plate material in a simulated polymer electrolyte membrane fuel cell environment

The austenitic stainless steel UNS S63019 was evaluated regarding its potential as bipolar plate material in a polymer electrolyte membrane fuel cell (PEMFC) environment. Segregated grains of niobium carbide (NbC_x) were identified in polished cross-sections of the alloy, offering a possible pathway for enhanced electrical conductivity through the passive surface oxide. Additionally, the alloy was tested for corrosion resistance in a simulated PEMFC environment. It was considered that perhaps the elevated nitrogen concentration in the alloy would provide some benefit for corrosion resistance.Results for interfacial contact resistance (ICR) testing of the air-formed surface film on UNS S63019 showed decreased electrical conductivity as compared to UNS S30400. Niobium carbide particles did not improve film conductivity due to a non-conductive niobium oxide layer that formed on the surface. Corrosion resistance of the alloy was also poor as compared with UNS S30400, demonstrating that elevated nitrogen concentration in the alloy was not adequate in itself to enhance corrosion resistance. Poor corrosion resistance was attributed primarily to high carbon content in the alloy which combined with a significant amount of chromium to form carbides.     

Experimental investigations on the corrosion resistance characteristics of coated metallic bipolar plates for PEMFC

Bipolar plates (BPPs) made of stainless steels preferred in PEM Fuel Cell (PEMFC) applications due to their high electrical conductivity, low material and production costs, low weight and mechanical strength. However, their corrosion resistances are not at desired levels for real PEMFC working conditions. To overcome this issue, different coating types are suggested. In this study, corrosion resistance behavior of 51 μm-thick SS316L metallic bipolar plates that were coated with the three different PVD coatings (TiN, CrN, and ZrN) at three thicknesses (0.1 μm, 0.5 μm, and 1 μm), and then were formed with two different manufacturing processes (stamping and hydroforming) investigated. Potentiodynamic and potentiostatic corrosion experiments were performed on the coated-formed SS316L plates, and coated-unformed blanks. Corrosion test results indicate that 1 μm ZrN coating demonstrated the highest corrosion resistance among the tested cases regardless of the manufacturing process employed. Moreover, hydroformed bipolar plates exhibited higher corrosion resistance than the stamped BPPs, but lower than the blank samples. Hardness measurements were also performed on the coated samples and resulted in higher corrosion resistance for harder surfaces.     

The effect of temperature on corrosion behavior of SS316L in the cathode environment of proton exchange membrane fuel cells

The effect of temperature on the corrosion behavior of SS316L in simulated proton exchange membrane fuel cell (PEMFC) environments has been systematically studied. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to characterize the corrosion behavior. Atomic force microscope (AFM) is used to examine the surface morphology and X-ray photoelectron spectroscopy (XPS) analysis is used to identify the composition and the depth profile of the passive film. Photo-electrochemical (PEC) measurements are also performed to determinate the band gap energy of the passive film semiconductor. Interfacial contact resistances (ICR) between polarized SS316L and carbon paper are also measured. The experimental results show that corrosion resistance decreases with temperatures even though the thickness of passive film increases with temperature, at a given cell potential, the corrosion behavior of SS316L can be significantly different at different temperatures in PEMFC cathode environments, and the band gap of passive films decrease with temperature. The results also show that within the temperature range studied (25-90 °C), after different passivation time, the corrosion current densities of SS316L are all lower than the US DOE 2015 target value of 1 μA cm⁻², but the ICR between the carbon paper and polarized SS316L does not satisfy the US DOE 2015 target.     

Improved corrosion resistance and interfacial contact resistance of 316L stainless-steel for proton exchange membrane fuel cell bipolar plates by chromizing surface treatment

The electrochemical performance and electrical contact resistance of chromized 316 stainless-steel (SS) are investigated under simulated operating condition in a proton-exchange membrane fuel cell (PEMFC). The corrosion resistance of the chromized stainless steel is assessed by potentiodynamic and potentiostatic tests and the interfacial contact resistance (ICR) is examined by measuring the electrical contact resistance as a function of the compaction force. The results show that the chromizing surface treatment improves the corrosion resistance of the stainless steel due to the high-chromium concentration in the diffuse coating layer. On the other hand, the excess Chromium content on the surface increases the contact resistance of the steel plate to a level that is excessively high for commercial applications. This study examines the root cause of the high-contact resistance after chromizing and reports the optimum process to improve the corrosion resistance without sacrificing the ICR by obtaining a chrome carbide on the outer layer.     

Manufacturing and performance assessment of stamped,laser welded,and nitrided FeCrV stainless steel bipolar plates for proton exchange membrane fuel cells

A manufacturing and single-cell fuel cell performance study of stamped, laser welded, and gas nitrided ferritic stainless steel foils in an advanced automotive bipolar plate assembly design was performed. Two developmental foil compositions were studied: Fe–20Cr–4V and Fe–23Cr–4V wt.%. Foils 0.1 mm thick were stamped and then laser welded together to create single bipolar plate assemblies with cooling channels. The plates were then surface treated by pre-oxidation and nitridation in N₂–4H₂ based gas mixtures using either a conventional furnace or a short-cycle quartz lamp infrared heating system. Single-cell fuel cell testing was performed at 80 °C for 500 h at 0.3 A/cm² using 100% humidification and a 100%/40% humidification cycle that stresses the membrane and enhances release of the fluoride ion and promotes a more corrosive environment for the bipolar plates. Periodic high frequency resistance potential-current scans during the 500 h fuel cell test and post-test analysis of the membrane indicated no resistance increase of the plates and only trace levels of metal ion contamination.