Corrosion resistance of austenitic stainless steel separator for molten carbonate fuel cell

STS310S and SC-STS310S (simultaneously co-deposited chromium and aluminum onto 310S austenitic stainless steel substrate by pack-cementation process) were used as separator materials on the cathode side of a molten carbonate fuel cell. With the STS310S, corrosion proceeded via three steps; a formation step of unstable corrosion product, a protection step against corrosion until breakaway, and an advance step of corrosion after breakaway. The final corrosion product was LiFeO₂ and the loss of mass was 6.5 mg/cm² after a corrosion test of 480 hr at 650°C. The SC-STS310S showed more effective corrosion resistance, however, than did common STS310S. There was especially no corrosion loss on the SC-STS310S after the 480 hr corrosion test. It is anticipated that it will be very useful as an alternative separator on the cathode side off the MCFC in the future.     

碳酸盐熔盐燃料电池的NiO阴极溶解

碳酸盐熔盐燃料电池阴极材料NiO在熔盐电解质中的溶解 ,是该技术能否实用的关键之一。实验表明 ,NiO在Li2 CO3 Na2 CO3熔盐中的溶解度在CO2 气氛和 650℃条件下约为 5× 1 0 - 6(摩尔分数 )。LiFeO2 或NaFeO2的添加虽能降低NiO浓度 ,但在CO2 气氛下会分解生成α Fe2 O3。     

SVET study of the corrosion protection of electrodeposited Zn and Zn‐Co‐Fe alloy coated steels

The scanning vibrating electrode technique (SVET) was applied to study the corrosion resistance of partially coated (Zn and various Zn‐Co‐Fe alloys) and partially exposed steel samples in 10 mM NaCl solution. The sacrificial properties and the protection range decreases with increase in Co content in the alloy. For high Co content in the alloy, the coating becomes more noble to steel and loses its sacrificial protection. The barrier resistance of the coatings increases with the increase in Co content in the alloy coating. Zn‐Co‐Fe alloys with high Co content (i.e., 32 wt% Co and 1 wt% Fe) showed excellent barrier properties due to passivation after dezincification protecting the underlying steel. An intermediate region of compositions can be distinguished in which the coatings provide a good combination of sacrificial and barrier resistance properties and also a reasonable protection range.     

Corrosion behavior of Zn and Ni coated carbon steels in 3% NaCl

Corrosion behavior of electrolytic coated carbon steel in 3% NaCl solution was investigated using potentiodynamic polarization, lineer polarization resistance, chronoamperometry, open circuit potential as a function of time and impedance (EIS) measurements. Zn and Ni coated carbon steel surfaces immersed into 3% NaCl solution during 1, 5 and 10 days were characterized using scanning electron microscope (SEM). According to the experimental results, nickel coated carbon steel corrosion resistance was higher than that of Zn coating. Accordingly, the corrosion rate of nickel coated carbon steel decreased by 89.6% compared to the zinc coating.     

High temperature corrosion of low and high alloy steels under molten carbonate fuel cell conditions

The corrosion behavior of eight low and high alloy steels was investigated under simulating the conditions at the cathode of a molten carbonate fuel cell at 650°C. Different Li-containing iron oxides (LiFeO₂ and LiFe₅O₈) were formed in contact with the eutectic (Li, K)-carbonate melt depending on the Cr-content of the steel. These oxides show low solubility in the melt and protect the metallic material against further corrosive attack last growing scales of Fe₃O₄ and LiFe₅O₈ were observed on the low alloy ferritic steel 10 CrMo 9 10. Higher alloy steels form LiFeO₂ in contact with the melt and mixed Fe-Cr-spinels underneath. Steels with Cr-contents over 20 wt.% Cr form a mixed LiCr_1-xFe_xO₂ and LiCrO₂ layer in contact with the metal. Marker experiments on the commercial steel 1.4404 (X2 CrNiMo 17 13 2) show that the outer LiFeO₂ layer grows mainly by outward diffusion of iron ions (Fe³⁺), whereas the inner (Fe,Ni)Cr₂O₄ spinel layer grows inward. After 500 hours, LiFe₅O₈ was formed between the spinel and the LiFeO₂ layer, but it had disappeared after several thousand hours of exposure as it was fully transformed to LiFeO₂. Co-containing LiFeO₂ was found after 500 hours on the high Co-containing steel 1.4971 (X12 CrCoNi 21 20), but is not stable after several thousand hours exposure. Co diffuses outward to form a protective LiCoO₂ layer of a few microns in thickness. Protective Cr₂O₃ layers were not observed on steels with high Cr-content (≥ 25 wt.% Cr) due to peroxide ions in the melt, which cause oxidation Cr₂O₃ and flux to chromate, which is highly soluble in the melt. Further quantitative investigations on total corrosion considering the chromate formation have shown that high alloy steels with high amounts of Cr form mainly K₂CrO₄.     

Molten salt synthesis of perovskite conversion coatings: A novel approach for corrosion protection of stainless steels in molten carbonate fuel cells

A novel conversion coating process has been developed to meet the stability requirements of stainless steel hardware in the demanding MCFC fuel cell environments. The process applies a perovskite-based coating by exploiting spontaneous oxidizing reactions of the metallic surface with La₂O₃ in eutectic alkali carbonate mixtures. By using well controlled synthesis procedures, conversion coating layers covering the entire metallic surface with a uniform and compact structure could be obtained. The as-formed coatings with a surface morphology of agglomerated crystallite particles consisted of a thin (<5 μm) LaFeO₃ perovskite layer grown over a thicker (>5 μm) LiFeO₂-rich layer. Test coupons of 316L stainless steel with the perovskite conversion coating were analyzed for corrosion protection and interfacial resistivity properties. It was found that the conversion coating is highly conductive while showing excellent long-term corrosion stability in simulated MCFC environments. These results suggested that perovskite coatings formed by molten salt conversion reactions could be particularly attractive to confer optimal protection and electrical continuity to MCFC current collectors.     

固体氧化物燃料电池阴极材料第一性原理研究进展

固体氧化物燃料电池(SOFC)具有能量转换效率高和燃料适应性广等突出优势,被认为是未来最有前景的清洁能源技术之一。目前SOFC研究热点是降低工作温度到500～800℃中低温区,以降低运行成本、增加可靠性,进而加速SOFC的商业化进程。阴极作为SOFC的重要组元,合理的设计和优化中低温下对氧还原反应具有较高催化活性的阴极材料至关重要。具有钙钛矿结构或由钙钛矿结构衍生出的层状结构的电子-离子混合导电型（MIECs）氧化物是目前研究最多的SOFC阴极材料。第一性原理可以弥补实验方面信息的缺失,能够提供电子结构、几何参数、吸附能及过渡态等相关信息,可以为合理设计和开发高性能的新型SOFC阴极材料提供科学依据和理论指导。本文通过对钙钛矿阴极氧空位的形成及迁移,氧分子在阴极（包括贵金属引入）表面上的吸附、解离、扩散过程及其规律进行了综述并总结了我们前期的研究成果,最后针对当前研究存在的问题及今后钙钛矿阴极的计算模拟研究方向进行了总结与展望。     

Uniform corrosion and intergranular corrosion behavior of nickel‐free and manganese alloyed high nitrogen stainless steels

The uniform and intergranular corrosion behavior of two kinds of nickel‐free and manganese alloyed high nitrogen stainless steels (HNSSs) were investigated. A type of 316L stainless steel (316L SS) was also included for comparison purpose. Both solution annealed (SA) and sensitization treated (ST) steels were examined. It was found that the SA HNSSs had much weaker resistance to uniform corrosion compared to the SA 316L SS. The addition of molybdenum, to some extent, improved the uniform corrosion resistance of the HNSSs. The sensitization treatment had little influence on the uniform corrosion resistance of all the steels. The HNSSs showed an obvious susceptibility to intergranular corrosion, in particular the ST HNSSs. The intergranular corrosion rates of the sensitized HNSSs were much higher than that of the sensitized 316L SS. The degree of interganular attack for the ST HNSSs was much more serious than that for the 316L SS. The addition of molybdenum obviously improved the resistance of the ST HNSSs to intergranular corrosion. The double loop electrochemical potentiokinetic reactivation tests also proved that the HNSSs were rather susceptible to the sensitization treatment compared to the 316L SS. The relatively weak resistance of the HNSSs to uniform and intergranular corrosion may be due to high manganese promoted anodic dissolution. The improvement of uniform and intergranular corrosion resistance caused by the addition of molybdenum could be attributed to the synergistic effects of molybdenum and nitrogen in the HNSSs on the formation and stability of passive film.     

Corrosion resistance of hot‐dipped zinc and zinc alloy coated sheet steels under offshore atmospheric environment

The application of hot‐dipped zinc and zinc‐aluminum alloy coatings were introduced. Exposure tests of the steels with these coatings were conducted in the offshore atmosphere in Qingdao and Xiamen for 12 years separately. Effects of the coating thickness, alloy composition and atmospheric environment on the corrosion performance were studied. Results of the onsite exposure tests were compared with the results of a previous indoor salt spray accelerated corrosion tests. The study supports that zinc‐aluminum alloy coatings are useful in providing better corrosion resistance and can be further developed for future applications.     

Corrosion behaviour of highly alloyed stainless steels in mixed chloride and fluoride aqueous solutions

Laboratory weight loss and cyclic potentiodynamic polarization corrosion tests were performed on two types of corrosion resistant alloys, a duplex alloy (ferritic-austenitic stainless steel) and two austenitic stainless steels, in mixtures of chloride (3000, 9000 and 15000 ppm) and fluoride (4800 and 15000 ppm) ions at pH 3. Two temperatures were tested, 60 and 70°C. The electrochemical results indicate that the duplex stainless steel presents high corrosion resistance. Weight loss results show low corrosion rates of the two types of stainless steels after 60 days exposure. Sonic pits-crevices were found under the corrosion crust deposits on the duplex stainless steel.     

Determination of the weaker phase in the pitting corrosion of non‐standard low‐Ni high‐Mn‐N duplex stainless steels

In this paper, the use of Energy Dispersive Spectrometry (EDS) is proposed to determine the partition coefficients of the elements of a new family of duplex stainless steels that are characterized by having low contents of nickel, together with high levels of manganese and nitrogen. From the values of the partition coefficients, the chemical compositions of the constituting phases have been determined, in order subsequently to calculate the value of the Pitting Resistance Equivalent Number (PREN) of each phase. The proposition put forward in this study is that the phase having the lower PREN determines the pitting corrosion behaviour of these types of steels. Results obtained by means of optical and scanning electron microscopy have provided confirmation that the pitting corrosion behaviour of these new materials gets determined by the resistance of the weaker phase and consequently by the phase having the lower PREN value. Lastly it has been proved possible to determine the existence of an exponential relationship between the alloys pitting potential (Ep) and the weaker phase PREN; this can be utilized for the low‐nickel duplex stainless steels design in which the pitting corrosion resistance is controlled.     

The influence of the ferritic and austenitic phases and of silicon on the Corrosion resistance of alloyed cast steels

The paper deals with the influence of the α/gamma; phase ratio and silicon on the corrosion resistance of low carbon alloyed cast steels in strongly oxidizing media. Electrochemical investigations in 65% HNO₃ + 5g Cr⁶⁺ ions/I showed that none of the four tested cast steels, with γ_R = 100%, γ_R = 75%, γ_R = 50% and γ_R = 0%, achieved a passive state, and that the corrosion process was under cathodic control. Under such conditions the calhodic activation overpotential is considerably greater than the anodic activation overpotential, and, as well as this, the Tafel coefficient for a cathodic reaction has higher values than those which are characteristic for an anodic reaction. The austenitic cast steel is the most resistant to corrosion, since for the corrosion process to take place the greatest activation energy is needed. The results of investigations of corrosion resistance in 12M HNO₃, at 115°C, for 240 hours, confirmed the results of the electrochemical measurements and the predominant role of the austenitic cast steel with γ_R = 100%. By means of AES analysis it was found that the good resistance to corrosion of such a cast steel is based on the building up of a thin mechanically-protecting barrier, which mainly consists of silicon.     

The passivation behaviour of copper-containing high-performance ferritic stainless steels in the cathode environment of polymer electrolyte membrane fuel cells

The passivation behaviour of copper-containing high-performance ferritic stainless steels in the cathode environment of polymer electrolyte membrane fuel cells was investigated using electrochemical tests and XPS surface analyses. The addition of copper to the alloy deteriorated the corrosion resistance at a passive region of 0.6 V_SCE. With an increase of the copper content, the passivity of the alloy degraded due to an increase of the formation of Cr-containing inclusions, and a decrease of the Cr oxide and hydroxide in the passive film.     

Corrosion resistance of environment‐friendly sealing layer for Zn‐coated sintered NdFeB magnet

In this paper, a protective sealed Zn coating (SZC) was prepared on sintered NdFeB magnet by the combination of electrodeposition and sol–gel method. The unsealed Zn coating (UZC) was also studied for a comparison. The surface morphology of UZC and the cross‐section morphology of SZC were investigated using scanning election microscope (SEM). The microstructure of Zn coating and structure of sealing layer were studied by X‐ray diffraction (XRD) and Fourier transform infrared (FT‐IR) spectrum, respectively. The corrosion characteristics of SZC and UZC in neutral 3.5 wt% NaCl solution were evaluated using electrochemical measurements including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test, indicating that the anticorrosive properties of SZC coated specimens increased 20 times compared with that of UZC coated specimens. In order to further investigate the anticorrosive properties of SZC, a long‐term immersion test was carried out in neutral 3.5 wt% NaCl solution using EIS. The results of long‐term corrosion test showed that the SZC could provide long‐term protection in neutral 3.5 wt% NaCl solution for NdFeB magnet.     

Long‐time behavior of non‐galvanized and galvanized steels for geotechnical stabilization applications

The contribution is dedicated to the durability of geotechnical stabilization applications. The long‐term corrosion behavior of unprotected and galvanized thread‐bars for micro‐piles and nails in different soils is described. The bars in the ground are embedded in an up to 20 mm thick cement mortar, which may be cracked and locally destructed. First, the corrosion‐affecting parameters of the steel corrosion in the ground are described and soil classes are defined. Furthermore, an estimation of the long‐term (up to 50 years) corrosion behavior was assessed on the base of information from the literature. Here the corrosion of the thread‐bar in direct contact with the soil was in the center of interest. The corrosion protection of the cement mortar cover was not taken into consideration, in order to be on the safe side. On the other hand, a macrocell‐corrosion of the thread‐bars cannot be completely excluded, if the cement mortar casing exhibits defects and constructional and environmental corrosive conditions in the ground are fulfilled. Two cases are discussed: ? element formation between sections along the thread‐bar, ? element formation between the thread‐bar and other reinforced concrete parts.     

Corrosion behavior of nickel alloyed and austempered ductile irons in 3.5% sodium chloride

In this study, the nickel alloying and austempering effects on corrosion behavior of ductile irons were investigated. The microstructure of austempered ductile iron (ADI) was analyzed by XRD, and the polarization corrosion tests were conducted using 3.5 wt.% NaCl solution. The results showed Ni-alloyed as-cast has less nodule counts than the unalloyed one; therefore, the former is more corrosion resistant than the latter. For the ADI, the nickel addition increases the retained austenite content, resulting in having better corrosion inhibition than the unalloyed ADI. Comparatively, the order of corrosion resistance in 3.5 wt.% NaCl solution is as follows: 4%Ni-ADI > ADI > 4%Ni-DI > DI.     

Corrosion behavior of various steels in the simulated fluidized bed boiler environment

The corrosion behavior of various austenitic stainless steels and high-alloy steels has been studied in simulated fluidized bed boiler environment to develop a new corrosion resistant austenitic stainless steel for the superheater tube. The superheater is usually not installed within the bed position, which is different from the evaporator installed within the bed position. Therefore, the superheater tubes are exposed to an oxidizing environment; but it is also necessary to estimate the corrosion resistance of the steels in a reducing environment. It is already known that the high temperature corrosion behavior in conditions where CaSO₄ is coated on the steels is more important than the erosion of the superheater tubes. The main results in this present study are as follows: The Nb bearing steels and low C steels showed good resistance to high-temperature corrosion in CaSO₄/CaO, e.g. 347, 304L and HR3C. The corrosion rate of all steels used increased with increase in temperature, particularly at temperatures higher than 650°C. Internal penetration was not detected at temperatures lower than 550°C, but it was detected at temperatures higher than 600°C, in particular, higher than 650°C. The corrosion thickness loss was almost the same as the internal penetration depth at 700 and 750°C in the 300 series steels placed in CaSO₄/CaO, including the fine grained 347 steel, while the internal penetration depth was larger than the corrosion thickness loss in high-alloyed materials such as Alloy 800 and 310 steels. At temperatures higher than 800°C, the same result was also obtained for the fine grained 347 steel. The corrosion during exposure to oxidizing or reducing gases without CaSO₄/CaO or CaS was slight, but when the test specimens were placed in CaSO₄/CaO or CaS, the corrosion rate sharply increased, regardless of the atmospheric gas composition. Cr, Si, Mn (less than 5 %), Mo and Nb are beneficial elements while C, Cu and Al are harmful elements. From the above results, the following steel was developed for high temperature corrosion resistance in CaSO₄/CaO: low C-22/25Cr-17/25Ni-3/5Mn-(2Mo)-Nb-0.08/0.2N-Al-(B).