Potentiostatic Polarization Tests Research Articles

A corrosion mechanism of NbC/α-C:H films for metallic bipolar plates in proton exchange membrane fuel cell cathode based on percolation model

A series of NbC/α-C:H films were prepared through arc ion plating for the protection of metallic bipolar plates in proton exchange membrane fuel cells (PEMFCs). Phase characterizations show that NbC grains were randomly embedded into the α-C:H matrix and their size and content increased with the decreasing of C2H2 flow rate. Results from the kelvin probe force microscope (KPFM) and conductive atomic force microscope (CAFM) presented a uniform surface potential distribution but significant local conductivity differences. DFT calculations and potentiostatic polarization tests indicated that dissolution of NbC grains were preferred in PEMFC cathode environment due to preferential adsorption of corrosive media. On the basis of theoretical calculations and experimental results, a corrosion mechanism was presented based on percolation model to better understand the corrosion process of NbC/α-C:H films in PEMFC cathode.

Nanometric CrN/CrAlN and CrN/ZrN multilayer physical vapor deposited coatings on 316L stainless steel as bipolar plate for proton exchange membrane fuel cells

Nanometric CrN/ZrN and CrN/CrAlN multilayer coatings were deposited on 316L stainless steel as metallic bipolar plates (BPs) for proton exchange membrane fuel cells (PEMFCs) by the cathodic arc evaporation-physical vapor deposition technique. The coatings were evaluated using X-ray diffraction, field emission scanning electron microscopy, surface roughness determination, and water droplet contact angle measurement. In order to study the electrochemical behavior of the coatings, potentiodynamic polarization and electrochemical impedance spectroscopy examinations were conducted at the simulated cathodic PEMFC media (0.5 M H2SO4 purging O2, 70°C). Also, the interfacial contact resistance (ICR) of the coated and uncoated 316L BPs was evaluated in different compaction forces before and after the potentiostatic polarization test. The current density at 0.644 V vs. Ag/AgCl of 0.16 and 0.76 μA cm−2 was recorded for CrN/ZrN and CrN/CrAlN, respectively. Moreover, it was found that after the potentiostatic polarization tests, the ICR values of the 316L BPs were decreased from 276 to 8.3 and 9.4 mΩ cm2 by the deposition of the CrN/ZrN and CrN/CrAlN coatings, respectively. The enhancement in the ICR and corrosion behavior of both CrN/ZrN and CrN/CrAlN multilayer coatings indicated that these coatings can be potentially suitable for PEMFCs application, according to the United States Department of Energy's targets.

Effect of Sealing Treatments on Erosion–Corrosion of a Fe-Based Amorphous Metallic Coating in 3.5 wt.% NaCl Solution with 2 wt.% Sand

In this study, a Fe-based amorphous metallic coating (AMC) was sealed with three sealants, i.e., stearic acid, aluminum phosphate and cerium salt, respectively. Two types of electrochemical tests, namely the ex situ electrochemical impedance spectroscopy test and the in situ potentiostatic polarization test, were conducted to evaluate the erosion–corrosion resistance of as-sprayed and as-sealed AMCs. The results show that the aluminum phosphate–sealed AMC exhibits the best erosion–corrosion resistance with the higher critical flow velocity compared with the as-sprayed AMC, which is attributed to the deep penetration of aluminum phosphate and high hardness of the sealed layer. In contrast, the sealants of stearic acid and cerium salt are easily removed by sand particle impacting, deteriorating their erosion–corrosion resistance.

Nanocrystalline TaCN coated titanium bipolar plate dedicated to proton exchange membrane fuel cell

To promote the corrosion resistance and surface conductivity of metal bipolar plates (BP) in proton exchange membrane fuel cells (PEMFC), a TaCN nanoceramic coating with TaN and TaC dual phase structures was engineered onto a commercially pure titanium substrate employing the double cathode glow discharge plasma (DCGDP) technique. The as-prepared TaCN coating was well-adhered to the substrate with a thickness of ∼16.5 μm. The coating exhibited a microstructure comprising equiaxed grains with an average size of ∼10.3 nm. The electrochemical behavior of the TaCN coating was evaluated in a simulated PEMFC environment containing various HF concentrations. Open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), potentiostatic polarization, and Mott-Schottky tests showed that the addition of HF enhanced the corrosion rate of both the Ti substrate and the TaCN coating. Nevertheless, the TaCN coating exhibited improved corrosion potential, lower corrosion current density and larger resistance than that of Ti for all HF concentrations. The Hilbert-Huang and corrosion morphology analysis indicated that the TaCN coating maintained a passive state with increasing HF concentration. First-principle calculations investigated the adsorption capacity of the F− ions on the passive film on the TaCN coating. Furthermore, the TaCN coating possesses desirable interfacial contact resistance (ICR) (about 10 mΩ cm2 under a compressive force of 140 N cm−2) and outstanding hydrophobicity (92.9° ± 2°). Overall, the TaCN coating, which can deliver appreciable enhancement relative to the performance of titanium, represents an extremely competitive candidate as a protective coating for bipolar plates.

The performance research of TaC modified 316L bipolar plate for proton exchange membrane fuel cell

The corrosion resistance and conductive property are two crucial factors affecting the performance of bipolar plates in proton exchange membrane fuel cell (PEMFC). In this research, a TaC coating is deposited on a 316L bipolar plate through plasma surface modification technology. The electrochemical property of bare 316L and TaC modified bipolar plate has been studied in a simulated PEMFC operation environment. The electrochemical analysis indicates that the performance of bipolar plate has been significantly improved due to the effective effect of TaC modified coating. An obviously decreased corrosion current density is represented by TaC modified bipolar plate and the corrosion potential is more positive in a simulated electrolyte environment. A satisfactory water contact angle of 80.5° is obtained to be beneficial to the water management of PEMFC after surface modification. Furthermore, the TaC-316L bipolar plate presents a minor interfacial contact resistance (ICR) and maintains a favourable result after the potentiostatic polarization test.

Corrosion Behavior of As-Cast Ti–10Mo–6Zr–4Sn–3Nb and Ti–6Al–4V in Hank’s Solution: A Comparison Investigation

Newly developed Ti–10Mo–6Zr–4Sn–3Nb has fascinating mechanical properties to be used as a biomedical material. However, there is still a lack of investigation focusing on the corrosion behavior of Ti–10Mo–6Zr–4Sn–3Nb. In this work, the microstructure and corrosion behavior of as-cast Ti–10Mo–6Zr–4Sn–3Nb was investigated by optical microscopy, X-ray diffraction, and electrochemical measurements. Hank’s solution was used as the electrolyte. A classical as-cast Ti–6Al–4V was used as reference. The results showed that Ti–10Mo–6Zr–4Sn–3Nb has a higher corrosion potential and a lower corrosion current density compared with Ti–6Al–4V, indicating better corrosion resistance. However, after applying anodic potentials, Ti–10Mo–6Zr–4Sn–3Nb shows larger passivation current density in both potentiodynamic polarization and potentiostatic polarization tests. This is because more alloying elements contained in Ti–10Mo–6Zr–4Sn–3Nb trigger the production of a larger number of oxygen vacancies, resulting in a higher flux of oxygen vacancy. This finding illustrates that the passive film on Ti–10Mo–6Zr–4Sn–3Nb is less protective compared with that on Ti–6Al–4V when applying an anodic potential in their passivation range.

Effect of cerium on the initiation of pitting corrosion of 444-type heat-resistant ferritic stainless steel

AbstractThe 444-type heat-resistant ferritic stainless steel is widely utilized in automotive exhaust pipes and solid oxide fuel cells, due to its excellent properties at elevated temperature. To meet the demands of significantly harsh service environments, rare earths were added in 444-type ferritic stainless steel. For the purpose of evaluating the effect of rare earths on pitting corrosion initiation, the metastable pitting corrosion behavior in 444-type ferritic stainless steel was studied through potentiodynamic polarization and potentiostatic polarization tests. The results demonstrated that pitting corrosion was initiated at the inclusion/alloy interface. The cerium alloying in 444-type ferritic stainless steel decreased the amount of preferential dissolution sites. The beneficial effect of Ce on pitting corrosion resulted from the formation of stable cerium oxides, as well as from the reduction in the amount and size of inclusions in 444-type ferritic stainless steel. In addition, electrochemical impedance spectroscopy test results revealed that cerium alloying enhanced the polarization resistance of passive films through insignificant thickness alteration.

ZrC Coating Modified Ti Bipolar Plate for Proton Exchange Membrane Fuel Cell

AbstractThe electrical conductivity, corrosion resistance and hydrophobic property of bipolar plate are important factors to affect the overall performance of proton exchange membrane fuel cell (PEMFC). In order to improve the performance of bipolar plate, ZrC coating has been prepared on bare Ti plate by plasma surface modification technology. From the microscopic morphology characterization, it can be found that the surface of ZrC modified plate is smooth without obvious defect. The potentiodynamic polarization and potentiostatic polarization tests are measured in cathodic and anodic environment of PEMFC. The result shows that the corrosion current density of ZrC coated Ti plate decreases by 1–2 orders of magnitude compared with bare Ti plate. Moreover, the interfacial contact resistance (ICR) decreases from 110 mΩ cm2 to 7.70 mΩ cm2 under the compaction force of 140 N cm−2. In addition, the ICR of ZrC coated Ti plate slightly increases after potentiostatic polarization, indicating that a good electrical conductivity and durability is obtained by ZrC coated Ti plate. Furthermore, the ZrC coated Ti plate has a lower water contact angle (105°) than bare Ti plate (76°), indicating a better hydrophobic performance for water management.

The influence of chromium content on wear and corrosion behavior of surface alloyed steel with Fe(16−x)Crx(B,C)4 electrode

In this study, the electrodes containing Fe(16−X)CrX(B,C)4 (X = 3,4,5) were used for surface alloying on an AISI 1010 steel substrate. The alloy structure of the coating exhibits in-situ composite structure with a large amount of reinforcing carbides and borides phases as to be hypoeutectic and hypereutectic structure in the surface alloyed layer. The obtained composition of the alloy shows that it contains much more interstitial hard phases in the alloyed layer than the conventional hardfacing alloys used in the industrial applications for the hardfacing of the steels. The morphological and microstructural properties showed that layer microstructure changed from hypoeutectic to hypereutectic structure with an increase in chromium content. The results of these studies revealed that the microstructure of hardfacing layers consists of interstitial phases of α(Fe-Cr) and (Fe,Cr)23(C,B)6 as major phases, (Fe,Cr)2(C,B) and (Fe,Cr)7(C,B)3 as minor phases in the in-situ composite structure. The composite hardness’ in the alloyed surface layers of the Fe(16−X)CrX(B,C)4 alloys ranged from 711 HV to 1164.3 HV. Wear test against alumina ball shows that the friction coefficients of the surface alloyed layers decreased with increase in applied loads and decrease in chromium content in the Fe(16−X)CrX(B,C)4 alloy composition. Coefficient of friction ranged from 0.55 to 0.79 in the present study. Wear rates of the surface alloyed steels caused to increase with decrease in chromium content and increase in applied loads. The wear rates of the alloyed layer changed between 3.07 × 10 and 5 mm3/m and 6.95 × 10−5 mm3/m. The corrosion resistance of the alloyed layers was measured by a potentiostatic polarization test. Icorr and Ecorr of the coated layers changed from 1.813 to 9.965 µA/cm2 and −704.786 to −745.792 mV, depending on alloyed layer compositions. With increasing chromium content, the corrosion resistance of the coating layer has changed in a nobler side.

Corrosion inhibition of methanol towards stainless steel bipolar plate for direct formic acid fuel cell

The corrosion properties of AISI316L stainless steel (316 L SS) as bipolar plates are investigated under aqueous acid methanol solutions (0.05 M H2SO4 + 2 ppm HF + 10 M HCOOH + x M CH3OH (x = 0, 3, 6 and 9) solutions at 70 °C) to simulate the varied anodic operating conditions of direct formic acid fuel cells (DFAFCs). When the methanol content is higher, the potentiodynamic, potentiostatic polarisation and EIS tests of the 316 L SS bipolar plates all show excellent corrosion resistance. The surface morphology and the glow discharge mass spectrometer (GDMS) illustrate that the surface corrosion on 316 L SS bipolar plates is slowed down when the methanol concentration is increased. These results indicate the methanol plays the role in retarding the corrosion rate of the 316 L SS in simulated DFAFCs anodic operating conditions by restricting the proton conductivity in the test solutions. The sample tested in higher content methanol solution has smoother corroded surface and thinner passivation film, which contributes to a lower interfacial contact resistances (ICR) value.

Balancing the corrosion resistance and through-plane electrical conductivity of Cr coating via oxygen plasma treatment

Developing an electrically conductive and corrosion-resistant coating is essential for metal bipolar plates of polymer electrolyte membrane fuel cells (PEMFCs). Although enhanced corrosion resistance was seen for Cr coated stainless steel (Cr/SS) bipolar plates, they experience a quick decrease of through-plane electrical conductivity due to the formation of a porous and low-conductive corrosion product layer at the plate surface, thus leading to an increase in interfacial contact resistance (ICR). To tackle this issue, the multilayer Cr coatings were deposited using the magnetron sputtering with a remote inductively coupled oxygen plasma (O-ICP) in the present study. After the O-ICP treatment, a Cr oxide layer (CrO*) is formed on the specimen surface. The CrO*/Cr/SS has a remarkably lower stable corrosion rate (iss) than that of the native Cr oxides (CrOn/Cr/SS). Compared with CrOn/Cr/SS, the excellent performance of CrO*/Cr/SS is attributed to a denser and thicker surface layer of CrO* with Cr being oxidized to its highest valence state, Cr (VI). More importantly, the through-plane electrical conductivity of the specimens treated by the optimized O-ICP decreases much slowly than CrOn/Cr/SS and thus, the increament of ICR of CrO*/Cr/SS after the potentiostatic polarization test is considerably smaller than that of CrOn/Cr/SS, which is benefited from the reduced iss that mitigates the deposition of corrosion products and hinders further oxidation of Cr coating. Therefore, CrO*/Cr/SS proves to be a well balanced trade-off between corrosion resistance and through-plane electrical conductivity. The results of this study demonstrate that O-ICP treatment on a conductive metal coating is an effective strategy to improve the corrosion resistance and suppress the increase of ICR over the long-term polarization. The technique reported herein exhibits its promising potential application in preparing corrosion resistant and electrically conductive coatings on metal bipolar plates to be used in PEMFCs.

Interface-induced degradation of amorphous carbon films/stainless steel bipolar plates in proton exchange membrane fuel cells

In the field of proton exchange membrane fuel cells (PEMFCs), amorphous carbon (a-C) films have attracted considerable attention as a surface functional coating for metallic bipolar plates (BPPs), given that they can endow metallic BPPs with both high corrosion resistance and electrical conductivity under harsh PEMFC environments. In this study, to determine the role of a-C/metallic substrate interfaces on the performance of PEMFCs, and clarify the associated degradation mechanism, a series of a-C films are deposited on 316L stainless steel (316Lss) samples at different sputtering powers. The composition, microstructure, interfacial contact resistance (ICR), and corrosion resistance of the a-C films are then systemically investigated, before and after electrochemical corrosion tests. The results obtained reveal that all the a-C films can greatly improve the performance of 316Lss under simulated PEMFC operational conditions, and the a-C film deposited at 0.9 kW exhibits the lowest corrosion current density (~7.52 × 10−3 μA cm−2) and minimum ICR values (2.91 and 4.00 mΩ cm2, before and after the long-time potentiostatic polarization tests, respectively). Furthermore, residual chromium oxides, which possibly result in an increased in ICR values, are observed at the a-C/316Lss interface after corrosion test; thus, an interface-induced a-C film degradation mechanism is proposed.

Effect of Al content on the corrosion resistance and conductivity of metal nitride coating in the cathode environment of PEMFCs

In this paper, TiN coatings doped with various Al contents are settled on stainless steel 316L (SS316L) as bipolar plates of proton exchange membrane fuel cells (PEMFCs) through closed field unbalanced magnetron sputter ion planting. The influence of Al doping on the phase structure, surface morphology, corrosion resistance, passive film, interfacial contact resistance (ICR) and contact angle are studied. According to results of X-ray diffraction (XRD), the TiAlN coated samples have grain refinement. From the results of scanning electron microscope (SEM), TiAlN coatings are dense and uniform. Based on the potentiodynamic polarization test results, TiAlN-2A coating exhibits the highest corrosion potential of 0.17 VSCE and the lowest corrosion current density of 1.42×10−7 A cm−2 in the simulated PEMFCs cathode environment, which completely meets the DOE 2020 technical targets (<1×10−6 A cm−2), as well as the best stability and the lowest current density in the cathode potentiostatic polarization test. In addition, the electrochemical impedance spectroscopy (EIS) test reveals that TiAlN-2A coating has the highest corrosion resistance than bare SS316L and the rest coatings. Capacitance analysis shows that TiAlN-2A coated sample has lower electron donor concentration, which indicates better corrosion resistance. According to ICR results, at compaction of 1.4 MPa, the ICR value of TiAlN-2A coating is 12.6 mΩ cm2, which is close to satisfy the DOE 2020 technical targets (≤10 mΩ cm2). In addition, the corrosion and conductivity mechanism is also investigated.

Performance of Tantalum Modified 316L Stainless Steel Bipolar Plate for Proton Exchange Membrane Fuel Cell

AbstractThe drawback of corrosion resistance and surface conductivity are two obvious defects to limit the large‐scale application of 316L stainless steel (316L SS) as bipolar plate (BP) in proton exchange membrane fuel cell (PEMFC). A tantalum (Ta) modified coating was prepared by plasma surface modification technology on the surface of 316L SS bipolar plate to improve the functional performance. The electrochemical behavior of untreated 316L SS and Ta modified bipolar plate was investigated in simulated PEMFC operating environment. The results of potentiodynamic polarization, potentiostatic polarization and electrochemical impedance spectroscopy (EIS) reveal that the Ta coating considerably improves the corrosion resistance of 316L SS bipolar plate. The potentiodynamic polarization result shows that a more positive self‐corrosion potential is obtained by Ta modified bipolar plate, and the corrosion current density is obviously reduced to 1.95 μA cm−2 and 0.14 μA cm−2 in simulated cathodic and anodic environment, respectively. The Ta modified bipolar plate exhibits a satisfactory interfacial contact resistance (ICR), and still remains lower ICR than bare substrate after potentiostatic polarization test for 4 h. In addition, the high water contact angle (94.5°) for Ta coated bipolar plate implies more superior hydrophobic performance, which is convenient for water management within the PEMFC.

Synthesis and characterization of conductive ceramic MAX-phase coatings for metal bipolar plates in simulated PEMFC environments

Conductive ceramic MAX-phase (Ti3AlC2) coated stainless-steel 304 exhibits good corrosion resistance in pH 3 H2SO4 solution containing 2 ppm F− at 70 °C in simulated PEMFC environments. The corrosion current density of the Ti-Al-C films is 0.44 μA·cm-2 were calculated using the potentiodynamic polarization, and the current density during the 24 h potentiostatic polarization test is 0.8 μA·cm-2. The interfacial contact resistance value of the Ti3AlC2-coated samples is 3.725 mΩ·cm2 at 140 N·cm-2 pressure, which is very low as compared to the bare SS304.

Influence of the individual microstructural features on pitting corrosion in type 304 austenitic stainless steel

The individual role of microstructural features viz. grain boundary character distribution (GBCD), grain size and residual strain on pitting corrosion is assessed in type 304 stainless steel employing potentiodynamic and potentiostatic polarization tests. Greater fraction of Σ3n (n ≤ 3) boundaries and J3-type triple junctions have resulted in better pitting resistance in the optimized GBCD specimen. The presence of higher dislocation density in the deformed specimen has resulted in greater pit generation rate. The fine-grained microstructure has exhibited inferior pitting resistance due to its higher random high angle boundary surface area which serve as preferential sites for nucleation and growth of pits.

Interaction between pitting corrosion and critical flow velocity for erosion-corrosion of 304 stainless steel under jet slurry impingement

The interaction between pitting corrosion and critical flow velocity (CFV) for erosion-corrosion of 304 stainless steel under slurry impingement was investigated through electrochemical tests, scanning electron microscopy and X-ray photoelectron spectroscopy. The results show that pitting corrosion affects the precise determination of CFV using the potentiostatic polarization tests, while CFV just corresponds to the case of the highest pitting corrosion resistance in erosion-corrosion process. This interaction is explained in terms of the properties of surface film. Furthermore, the guidance for quick determination of CFV is proposed and the significance of CFV in evaluating the erosion-corrosion resistance is highlighted.

Preparation and performance of nanometric Ti/TiN multi-layer physical vapor deposited coating on 316L stainless steel as bipolar plate for proton exchange membrane fuel cells

The current research mainly focuses on surface properties, corrosion resistance and interfacial contact resistance (ICR) of nanometric titanium/titanium nitride (Ti/TiN) multi-layer coating deposited on 316L stainless steel via physical vapor deposition (PVD) technique for using in proton exchange membrane fuel cell (PEMFC) bipolar plate (BPP) application. The phase and microstructure of the coated BPPs is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and contact angle examinations. The performance of the BPPs is investigated by electrochemical corrosion tests including electrochemical impedance spectroscopy (EIS), potentiodynamic and potentiostatic polarization tests at two conditions including at ambient temperature and in the simulated PEMFC cathode environment. Electrochemical test results reveal that the Ti/TiN coating increases the polarization resistance of 316L stainless steel by 26 orders of magnitude at ambient temperature and by 1000 orders of magnitude in the simulated PEMFC cathode environment. The corresponding ICR of the BPPs is measured ex-situ in a PEMFC set-up. It is observed that the coated BPP has ICR value of 11 and 18 mΩ cm2 before and after potentiostatic polarization, respectively. Hence, the Ti/TiN coating studied in this work has promising properties as BPP material and is potentially suitable alternative to replace the graphite BPP in PEMFC.

Electrochemical properties of niobium and niobium compounds modified AISI430 stainless steel as bipolar plates for DFAFC

ABSTRACTThe highly conductive and corrosion resistant diffusion layers of niobium, niobium nitrides and niobium carbides have been successfully fabricated on AISI430 stainless steel (430 SS) via a plasma surface diffusion alloying (PSDA) method as bipolar plates for direct formic acid fuel cells (DFAFCs). The surface of niobium carbides modified 430 SS (NbC 430 SS) is relative densification and uniformity without other surface flaw and micropore. Potentiodynamic and potentiostatic polarisation measurements indicate that NbC 430 SS enhances the corrosion resistance more effectively than other modified samples and its corrosion current density is maintained approximately 2.5 μA cm–2 in the simulated anodic environment of DFAFC. Moreover, NbC 430 SS presents a superior hydrophobicity with a contact angle of 105.5° and a lower interfacial contact resistance (ICR) before and after potentiostatic polarisation tests than other specimens. Therefore, NbC 430 SS developed in this work has a great potential to be bipolar plates for DFAFCs.

Corrosion Resistance and Electrochemical Behaviour of Amorphous Ni84.9Cr7.4Si4.2Fe3.5 Alloy in Alkaline and Acidic Solutions

In this work, the corrosion behaviours of the amorphous Ni84.9Cr7.4Si4.2Fe3.5 alloy and its crystalline counterpart are studied in acidic, neutral, and alkaline solutions by scanning electron microscopy, electrochemical impedance spectroscopy, and potentiodynamic and potentiostatic polarization tests. X-ray photoelectron spectroscopy and scanning Kelvin probe are employed to characterize the alloy surface. The results show that the amorphous Ni84.9Cr7.4Si4.2Fe3.5 alloy presents a better corrosion resistance compared to its crystalline counterpart, which is attributed to the uniform energy distribution of the atoms on the amorphous alloy surface, and this presents as a uniform electric potential map to effectively suppress the occurrence of the corrosion cell reaction.