•The VO2+/VO2+ redox reaction of the electrode could be facilitated to some extent with the increasing anodic corrosion.•A real reaction kinetic equation for the oxidation of VO2+ on the electrochemically oxidized electrode has been firstly obtained.•The establishment of the kinetic equation is conducive to predict polarization behaviors of the electrodes in engineering application.The morphology, surface composition, wettability and the kinetic parameters of the electrochemically oxidized graphite electrodes obtained under different anodic polarization conditions have been examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurements, steady-state polarization and cyclic voltammetry (CV) tests, with an attempt to investigate the inherent correlation between the physicochemical properties and the kinetic characteristics for carbon electrodes used in an all-vanadium redox flow battery (VRFB). When the anodic polarization potential raises up to 1.8 V vs. SCE, the anodic corrosion of the graphite might happen and a large number of oxygen-containing functional groups generate. The VO2+/VO2+ redox reaction can be facilitated and the reaction reversibility tends to become better with the increasing anodic potential, possibly owing to the increased surface oxides and the resulting improved wettability of the electrode. Based on this, a real reaction kinetic equation for the oxidation of VO2+ has been obtained on the electrode polarized at 1.8 V vs. SCE and it can be also well used to predict the polarization behavior of the oxidized electrode in vanadium (IV) acidic solutions.

•Co–Mn alloys are deposited in chloride solutions to protect metallic interconnects.•The Co–Mn alloy is converted into adhesive spinel coatings by heat treatment in air.•Spinel coatings consist of an external MnCo2O4 layer and an inner Cr-rich layer.•spinel coatings inhibit the oxidation of the steel at 800 °C in air and air-10%H2O.•Spinel coatings decrease the area specific electrical resistance of the steel.Ferritic stainless steels have become the candidate materials for interconnects of intermediate temperature solid oxide fuel cell (SOFC). The present issues to be solved urgently for the application of ferritic stainless steel interconnects are their rapid increase in contact resistance and Cr poisoning. In the present study, a chloride electrolyte suspension has been developed to electro-deposit a Co–Mn alloy on a type 430 stainless steel, followed by heat treatment at 750 °C in argon and at 800 °C in air to obtain Co–Mn spinel coatings. The experimental results indicate that an adhesive and compact Co–Mn alloy layer can be deposited in the chloride solution. After heat treatment, a complex coating composed of an external MnCo2O4 layer and an inner Cr-rich oxide layer has been formed on 430SS. The coating improves the oxidation resistance of the steel at 800 °C in air, especially in wet air, and inhibits the outward diffusion of Cr from the Cr-rich scale. Moreover, a low contact resistance has been achieved with the application of the spinel coatings.

•Galvanic corrosion of the couples Ni/Cr, Fe/Cr and Ni/Fe occurs in molten (Li,Na,K)F.•Galvanic corrosion effects are obviously greater than 1, with the largest value of 80.97 observed for the couple Ni–Cr.•Anodes Cr coupled with Ni or Fe, and Fe with Ni are dissolved to form metal ions which then are reduced on cathodes.The corrosion of structural materials is a great challenge for the development of a molten salt reactor using molten fluorides as fuel or coolant. The corrosion of materials in molten fluorides occurs mainly through the dissolution of alloying elements into the melt. In the present investigation, the galvanic corrosion behavior of pure Ni, Fe, and Cr as the common alloying constituents in an eutectic LiF–NaF–KF melt at 700 °C has been investigated. The experimental results indicate that the corrosion potential of the three metals decreases significantly by the order of Ni, Fe and Cr, with the largest potential difference observed between Ni and Cr. Their great potential differences lead to the occurrence of significant galvanic corrosion. The galvanic corrosion effects of the couples Ni/Cr, Ni/Fe and Fe/Cr are significantly larger than 1, with the most remarkable effectiveness obtained for the couple Ni/Cr, then Fe/Cr and Ni/Fe.Galvanic corrosion of the couples Ni/Cr, Ni/Fe and Fe/Cr in an eutectic (Li,Na,K)F melt at 700 °C. Anode: M→Mn++neM→Mn++ne, Cathode: Mn++ne→MMn++ne→M, Cr(galvanic effect = 80.97)Ni, Cr(galvanic effect = 70.65)Fe, Fe(galvanic effect = 55.94)Ni. Significant galvanic corrosion.

The major concerns with the ferritic stainless steel interconnects of solid oxide fuel cells operated at intermediate temperatures are the growth of non-protective oxide scales and the Cr evaporation into the cathode, which can increase significantly the cell resistance and polarization resistance. An additional coating is needed to solve these problems. Perovskite DyCrO3-based coatings for a type 430 stainless steel (430SS) interconnect are prepared by a high-energy micro-arc alloying (HEMAA) process using a rod of DyCrO3-20 wt.%Ni as the deposition electrode. Area specific resistance and oxidation behavior of the uncovered and covered steel at 850 °C in both air and wet air are examined. The as-deposited DyCrO3-based coatings are mainly composed of DyCrO3 with some Ni and (Ni,Fe)Cr2O4, with a metallurgical bonding to the substrate. The coated steel in both air and wet air has a relatively fast mass gain in the initial stage, followed by a very low mass gain, forming a three-layered scale with a NiFe2O4 outer layer, a thick DyCrO3 sublayer and a thin Cr2O3-rich inner layer. The DyCrO3-based coatings can inhibit significantly the oxidation of 430SS, especially the breakaway oxidation in wet air, and have a low area specific electrical resistance.Highlights► DyCrO3-based coatings for a type 430 stainless steel interconnect were studied. ► A high-energy micro-arc alloying process was employed to deposit DyCrO3 coatings. ► The coating inhibits the oxidation of the steel at 850 °C in air and air-10%H2O. ► The coating consists of external NiFe2O4, middle DyCrO3 and inner Cr2O3 layer. ► The coating can decrease the area specific electrical resistance of the steel.

The corrosion of bipolar plates is a great obstacle to the industrial application of molten carbonate fuel cell (MCFC). No electrical conductivity is needed for the wet-seal materials, thus the application of aluminum coatings is a very effective method to inhibit the corrosion of wet-seal areas of MCFC by the formation of a protective Al2O3 scale. In this work, a high-energy micro-arc alloying (HEMAA) process is attempted to prepare FeAl coatings on the type 316 stainless steel (316SS) as wet-seal material of MCFC using an FeAl intermetallic compound rod as the deposition electrode. The microstructure of the FeAl coatings is analyzed, and its corrosion behavior in molten (0.62Li, 0.38K)2CO3 at 650 °C in air is examined by electrochemical impedance spectroscopy. The experimental results indicate that the FeAl coating on 316SS is microcrystalline, with a metallurgic bonding with the substrate. The Nyquist plots for the corrosion of both the bare and the coated steel are all composed of double capacitive loops, but with significantly larger impedance values observed for the coated steel. The FeAl coatings increase greatly the corrosion resistance of 316SS by forming a compact and adhesive Al2O3 scale.Highlights► FeAl coatings for wet-seal materials of molten carbonate fuel cell were studied. ► High-energy micro-arc alloying process was employed to prepare FeAl coatings. ► FeAl coatings are microcrystalline, with a metallurgic bonding to the substrate. ► FeAl coatings have significantly larger impedance values than 316 stainless steel. ► FeAl coatings increase greatly the corrosion resistance of 316 stainless steel.

A two-electrode probe was used for electrochemical impedance studies of hot corrosion kinetics of molybdenum-containing Ni3Al-base alloy IC6 covered with a solid Na2SO4 film at 750 and 800 °C in air. The alloy was subject to catastrophic corrosion at both temperatures, forming a thick porous oxide layer, as a result of the formation of Na2MoO4–MoO3–Na2SO4 melt. The experimental temperature affected the formation of the melt, the ionic conduction of the corrosion layer, and thus the impedance characteristics. For the corrosion at 800 °C the Nyquist plots were composed of two capacitive loops at high-mid frequency and a line at low frequency indicating a diffusion-controlled reaction. At 750 °C, however, the plots consisted of a single capacitive loop in the initial stage, followed by the same impedance features as at 800 °C. Two equivalent circuits were proposed to fit the impedance spectra at the two temperatures. Based on the precise measurements of diffusion impedance, the diffusion flux of oxygen through the salt layer was calculated, and the main reduction reaction was also discussed.

Currently used ferritic stainless steel interconnects are unsuitable for practical applications in solid oxide fuel cells operated at intermediate temperatures due to chromium volatility, poisoning of the cathode material, rapidly decreasing electrical conductivity and a low oxidation resistance. To overcome these problems, a novel, simple and cost-effective high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO3-based coatings for the type 430 stainless steel interconnects. However, it is much difficult to deposit an oxide coating by HEMAA than a metallic coating due to the high brittleness of oxide electrodes for deposition. Therefore, a Cr-alloying layer is firstly obtained on the alloy surface by HEMAA using a Cr electrode rod, followed by a LaCrO3-based coating using an electrode rod of LaCrO3–20 wt.%Ni, with a metallurgical bonding between the coating and the substrate. The preliminary oxidation tests at 850 °C in air indicate that the LaCrO3-based coatings showed a three-layered microstructure with a NiFe2O4 outer layer, a thick LaCrO3 sub-layer and a thin Cr2O3-rich inner layer, which thereby possesses an excellent protectiveness to the substrate alloy and a low electrical contact resistance.

Polyaniline coating doped with dodecylbenzesulfonate anions is electrodeposited galvanostatically on type 304 stainless steel used as bipolar plates of proton-exchange membrane fuel cell from a basic solution of 0.3 M aniline monomer solution containing sodium dodecylbenzesulfonate as a supporting electrolyte. Electrochemical measurements in 1 M H2SO4 and in 0.3 M HCl show that the polyaniline coating increases the free corrosion potential of the steel by more than 300 mV and 450 mV, respectively, with a corrosion rate more than two orders of magnitude lower than that of the uncoated steel. Long-term exposure studies show that the coating is highly stable and inhibits the corrosion of the steel effectively.

Chromium volatility, poisoning of the cathode material and rapidly decreasing electrical conductivity are the major problems associated with the application of ferritic stainless steel interconnects of solid oxide fuel cells operated at intermediate temperatures. Recently, a novel and simple high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO3-based coatings for the type 430 stainless steel interconnects using a LaCrO3–Ni rod as deposition electrode. In this work, a Cr–La alloying layer is firstly obtained on the alloy surface by HEMAA using Cr and La as deposition electrode, respectively, followed by oxidation treatment at 850 °C in air to form a thermally grown LaCrO3 coating. With the formation of a protective scale composed of a thick LaCrO3 outer layer incorporated with small amounts of Cr-rich oxides and a thin Cr2O3-rich sub-layer, the oxidation rate of the coated steel is reduced remarkably. A low and stable electrical contact resistance is achieved with the application of LaCrO3-based coatings, with a value less than 40 mΩ cm2 during exposure at 850 °C in air for up to 500 h.

The corrosion resistance of 12CrMoV and SS304 steels in contact with a molten mixture of (55–45) mol.% ZnCl2–KCl, similar to that found in waste-incineration plants, has been examined in air at 400 °C by the electrochemical-impedance-spectroscopy (EIS) technique. The initial Nyquist plots of 12CrMoV showed a semicircle at high frequency and a line at low frequency indicating a diffusion-controlled reaction. At a later stage, the Nyquist plots are composed of a small semi-circle at high frequency, a line at medium frequency and a large semi-circle at low frequency, similar to that shown by SS304 during the whole experimental test. The larger impedance of SS304 as compared to 12CrMoV may be attributed to the presence of Ni and to the higher Cr content of SS304. Equivalent circuits representing the features of the corrosion of 12CrMoV and SS304 are proposed to fit the corresponding impedance spectra, and the electrochemical parameters in the equivalent circuits are calculated.

A bilayer conducting polymer coating composed of an inner layer of polypyrrole (Ppy) with large dodecylsulfate ionic groups obtained by galvanostatic deposition, and an external polyaniline (Pani) layer with small SO42− groups obtained by cyclic voltammetric deposition was prepared to protect type 304 stainless steel used for bipolar plates of a proton-exchange membrane fuel cell. The corrosion performance of the bare and coated steel in 0.3 M HCl was examined by electrochemical impedance spectroscopy, polarization and open-circuit potential measurements. The experimental results indicated that both the composite Ppy/Pani coatings and the single Ppy coatings increased the corrosion potential of the bare steel by more than 400 mV (saturated calomel electrode), and increased the pitting corrosion potential by more than 500 mV (saturated calomel electrode). The bilayer coatings could reduce the corrosion of the alloy much more effectively than the single Ppy coatings, serving as a physical barrier and providing passivity protection, with acceptable contact resistance.

The dissolution of NiO cathodes and the balance between the corrosion and contact ohmic resistance of current collector materials in molten carbonates are great obstacles to the commercialization of molten carbonate fuel cells (MCFC). Rare-earth element dysprosium was proposed to modify the NiO cathodes and to alloy nickel to explore its possible applications in MCFC. The measured solubility of NiO impregnated with 0.5–3 wt.% Dy in (0.62Li,0.38K)2CO3 at 650 °C in 60% CO2–40% O2 indicated that Dy addition increased noticeably the stability of NiO, while 1 wt.% Dy content produced better effectiveness. An investigation of the passive behavior of nickel and of Ni–Dy alloys containing 1–10 wt.% Dy in molten (Li,K)2CO3 with dynamic polarization measurements as well as X-ray diffraction and X-ray photoelectron spectroscopy indicated that the addition of Dy to nickel decreased its passive anodic current, and thus improved its corrosion resistance. The lithiation of NiO in the melt was a very significant reaction that could be promoted by the Dy additives to a certain extent, and increased the electrical conductivity of NiO.

Metallic bipolar plates look promising for the replacement of graphite due to higher mechanical strength, better durability to shocks and vibration, no gas permeability, acceptable material cost and superior applicability to mass production. However, the corrosion and passivation of metals in environments of proton exchange membrane fuel cell (PEMFC) cause considerable power degradation. Great attempts were conducted to improve the corrosion resistance of metals while keeping low contact resistance. In this paper, a simple, novel and cost-effective high-energy micro-arc alloying process was employed to prepare compact titanium carbide as coatings for the type 304 stainless steel bipolar plates with a metallurgical bonding between the coating and substrate. It was found that TiC coating increased the corrosion potential of the bare steel in 1 M H2SO4 solution at room temperature by more than 200 mV, and decreased significantly its corrosion current density from 8.3 μA cm−2 for the bare steel to 0.034 μA cm−2 for the TiC-coated steel. No obvious degradation was observed for the TiC coatings after 30-day exposure in solution.