Chromium Volatilization Research Articles

Effects of Mn concentration on durability of Fe-Cr ferritic steels dispersed with La2O3 for solid oxide fuel cell interconnects

This study investigates the effect of Mn concentration in La2O3-dispersed ferritic steel on the oxidation behavior, area-specific resistance (ASR), and chromium volatilization for solid oxide fuel cell (SOFC) interconnects. The results show that increasing Mn concentrations lead to a gradual increase in the oxidation rate of the alloy samples. The alloy with 0.3 wt% Mn primarily forms Cr2O3 during the initial oxidation stage, whereas the alloys containing more than 1.0 wt% Mn promote the growth of (Mn,Cr)3O4, leading to an accelerated oxidation rate. The ASR value exhibits a decreasing trend initially, followed by an increasing trend, as the Mn concentration in the alloy increases from 0.3 to 2.0 wt%. The alloy with 1.0 wt% Mn shows the lowest ASR value of 12.2 mΩ cm2 after 1000 h. The alloys demonstrate a tendency to reduce chromium volatilization as the Mn concentration increases, which can be attributed to the growth of (Mn,Cr)3O4 at the topmost layer of the oxide. The study suggests that controlling the Mn concentration in the Fe-Cr alloys can help in reducing the ASR values and Cr volatilization, making the ferritic steel alloy with 1.0 wt% Mn an effective choice for SOFC interconnects.

Oxidation and Chromium Volatilisation of AISI 430 Stainless Steel in O<sub>2</sub>-H<sub>2</sub>O and Ar-CO<sub>2</sub>-H<sub>2</sub>O at 800°C

To investigate the behaviour of oxidation and chromium volatilisation, AISI 430 stainless steel was oxidised in O2-H2O and Ar-CO2-H2O atmospheres for 96 h at 800°C with varying water vapour content. In the O2-H2O environment, the volatilisation rate of Cr and Mn increased as the water vapour content increased from 5% to 20%. In the case of Ar-CO2-H2O conditions, the presence of water vapour in the Ar-20%CO2 environment reduced the oxidation rates of the steel. However, increasing the H2O content in Ar-20%CO2 had an insignificant effect on the oxidation rate. Water vapour was found to accelerate the volatilisation rate of Cr and Mn. Breakaway oxidation occurred in Ar-20%CO2-40%H2O, resulting in the highest volatilisation rates of Cr and Mn.

An investigation of Cr vaporization from SUS430 metallic interconnect and deposition on (La,Sr)MnO3 cathode of intermediate temperature solid oxide fuel cells

In solid oxide fuel cells (SOFCs), chromium deposition and poisoning of metallic interconnect is one of the most important issues in the electrochemical activity and stability of the cathode for the oxygen reduction reaction (ORR). In this paper, the chromium evaporation characteristics, oxide scale growth, and deposition of chromium on the ORR on (La,Sr)MnO3 (LSM) cathode are studied at 700–800 °C for SUS430 alloy interconnect material in the intermediate-temperature solid oxide fuel cells. The chromium vaporization rate is measured using the transpiration technique and the surface morphologies, and cross-sections of oxide scales are examined by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The results indicate that chromium volatilization is correlated with the composition and morphology of the oxide scale, the Cr volatilization process presents two stages according to the structural evolution of the oxide scale. In addition, deposition and poisoning of Cr species released from metallic interconnect on the LSM cathode are studied and the formation of MnCr2O4 phase on the LSM backbone degrades the ORR electrochemical activity, causing the increase of overpotential of the cathode. The results obtained give insight into the mechanism of the Cr poisoning effect in intermediate-temperature.

Fabrication of Mn–Co Alloys Electrodeposited on AISI 430 Ferritic Stainless Steel for SOFC Interconnect Applications

Mn–Co alloys were electroplated on AISI 430 stainless steel using an electrodeposition technique with the aim to reduce oxidation and chromium volatilization. The electroplating parameters were designed to improve the coating quality. The increased current density with decreased MnSO4 content resulted in a denser coating layer. A sample coated with 0.10 M CoSO4 and 0.50 MnSO4 at 350 mA cm−2 showed the best oxidation resistance after being oxidized at 800 °C for 90 h. The X-ray diffraction (XRD) result revealed that the oxide growth on the surface of the coated samples mainly formed oxides of MnCo2O4, MnCr2O4, and Cr2O3. The chromium volatilization was evaluated by exposing the coated samples to humidified synthetic air at 800 °C for 96 h. The mass flux of Cr volatilization was on the order of 10−11 g cm−2 s−1. Furthermore, different heat treatments in O2 and CO2 atmospheres were compared. Annealing in CO2 at 800 °C for 4 h helped increase the Mn–Co coating density. The relationship between the porosity and its failure behavior was also discussed.

Corrosion study of 430 stainless steel with cobalt electrodeposited obtained from the recycling of Li-ion batteries

Abstract In this paper an interesting alternative for recycling of Li-ion battery cathode and and improving the corrosion resistance of 430 stainless steel is presented. The spent cathode composition has molecular formula approximately LiCoO2. The cobalt electrodeposition onto 430 SS was performed using the cobalt bath obtained by spent LiCoO2 lixiviation. In air atmosphere and high temperatures the metallic cobalt is transformed into a Co3O4 layer that acts as protection against chromium volatilization. This was confirmed by energy-dispersive X-ray spectroscopy and scanning eletron microscopy measurements. Electrochemical impedance spectroscopy measurements, in 0.5 M H2SO4 after thermal treatment at 600, 700 and 800 °C show that the cobalt electrodeposition is efficient in mitigating the effects of corrosion when 430 stainless steel is subjected to high temperatures.

Evidence for chromium, cobalt and molybdenum volatilisations during high temperature oxidation of Co-27Cr-6Mo Alloy

A Co-27Cr-6Mo alloy was oxidised in pure O2 between 800 and 1000 °C for durations up to 96 h. The flow rate was varied between 2 and 5 cm.s–1. In these conditions, volatilisations of chromium, cobalt and molybdenum were observed. The chromium volatilisation values were in good agreement with calculations assuming (CrO3)g volatilisation limited by diffusion in the gas boundary layer. On the contrary, the measured flux of evaporated Co was higher than the calculated Co volatilisation flux from the metallic Co. The theoretical relation between the solid Co particle size and its vapour pressure was suggested to help explaining such difference.

Volatilization of chromium from AISI 441 stainless steel: Time and temperature dependence

In this work, a photospectrometric method was used to quantify chromium evaporation from the AISI 441 ferritic stainless steel. Experiments were conducted for 250 h in the temperature range of 650–850 °C to evaluate the time and temperature dependence of chromium evaporation in the initial oxidation stage. The results showed a thermally activated process with an activation energy of 218.3 kJ·mol−1 and parabolic evolution over time at 650 and 750 °C, while a change in the evaporation kinetics was observed at 850 °C, resulting in a linear trend. A diffusion-evaporation model was applied and compared to the experimental data demonstrating the role of the chromium cation diffusion on the evaporation process.

Investigation of the Cr volatilisation using the ceramic phase collector technique

An effect of chromium contamination in the lanthanum strontium manganite (LSM) cathode which is a component of solid oxide fuel cell (SOFC) leads to degradation of the device. This is caused by volatilisation of Cr-species from the iron-chromium alloy at high temperature. In this work, the ceramic phase collector technique was used to evaluate the Cr volatilisation. To prove the effectiveness of this method, AISI 430 ferritic stainless steel samples with and without cobalt (Co) spinel coatings were used to compare the Cr condensation in the LSM ceramic. The coatings were produced by electroplating followed by a heat-treatment process. The samples were characterised using SEM with EDX and XRD to confirm the microstructure and the formation of Co spinel. It was found that chromium migration was restrained for coated samples, in contrast the Cr-species deposited in the LSM for uncoated samples. The chromium volatilisation evaluated by the contents of Cr species in the LSM collector was compared to the traditional condensation technique which analysed chemical contents by inductively coupled plasma-mass spectrometry (ICP-MS).

High-Temperature Oxidation of AISI441 Ferritic Stainless Steel for Solid Oxide Fuel Cells

Several ferritic stainless steel grades are widely studied and used in solid oxide fuel cells (SOFCs) technology as interconnect materials. Their high-temperature oxidation behavior is interesting to evaluate their applicability at SOFCs operating conditions and to design degradation tests and models predicting the lifetime of a SOFC stack. In this work the AISI441 grade was oxidized in static air at 850°C to study its oxidation kinetic by weight gain measurements. It was found a parabolic growth with a rate constant of 9.42 x 10-14 g2cm-4s-1. Data calculated using the diffusion coefficients of the species involved in the oxidation process resulted in higher weight gain. Discrepancies between the measurements and the model were corrected taking into account the chromium volatilization.

High temperature oxidation and chromium volatilisation of AISI 430 stainless steel coated by Mn-Co and Mn-Co-Cu oxides for SOFC interconnect application

An AISI 430 stainless steel was coated by Mn-Co and Mn-Co-Cu oxides, then tested in O2-5%H2O at 800 °C. The coated samples had reduced rates of oxidation and Cr loss due to Cr-species volatilisation with the higher rates for the Mn-Co-Cu oxide coated steel due to more Cr diffusion to the coating layer. The measured rates of Cr loss were in the order of 10–11 g cm–2 s–1. The graphical representation of the mass flux of Cr loss as a function of water vapour content was proposed to help visualise the volatilisation effect.

Obtaining Mn-Co Alloys in AISI 430 Steel from Lithium-Ion Battery Recycling: Application in SOFC Interconnectors

The recycling of exhausted lithium-ion batteries from mobile phones originate five solutions with different Co and Mn proportions that were used as electrolytic solutions to obtain Mn-Co spinel coatings on the surface of AISI430 stainless steel. The coatings are intended to contain chromium volatility in the working conditions of Solid Oxide Fuel Cells (SOFC) metallic interconnectors. Potentiostatic electrodeposition was the technique used to obtain Mn-Co coatings from low concentration electrolytes at pH = 3.0 and potential applied −1.3 V. Charge efficiency data were used for sample optimization. Three optimized samples were subjected to oxidation heat treatment at 800 °C for 300 h and then characterized by XRD, SEM and EDS. The results showed that the addition of manganese ions instead of cobalt ions in the electrolytic bath produces more stable and well-distributed deposits as the ratio of the two ions becomes equal in the electrolytic bath. Thin, homogeneous and stable spinel coatings (Mn, Co)3O4 2.8 μm and 3.9 μm thick were able to block chromium volatility when exposed to SOFC operating temperature.

CHAPTER 3 Characterisation of Thermal Oxide Scales on Stainless Steels

This chapter aims at reviewing the characterisation techniques that are commonly used for high temperature oxidation study, especially on stainless steels. In addition, the experimental studies about the high temperature oxidation i.e. thermogravimetric method and chromium volatilisation measurement are explained. The various kinds of characterisation techniques for physico-chemical and electronic properties of thermal oxide scales are reviewed, starting from optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), focused ion beam coupled with scanning electron microscope (FIB/SEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), and photoelectrochemical characterisation (PEC). The review focuses on the basic concepts and shows how the characterising tools can be applied to thermal oxide characterisation.

Effective Reduction of Chromium-oxy-hydroxide Evaporation from Ni-Base Alloy 690

The corrosion of the Ni-base alloy 690 (60Ni, 30Cr and 10Fe) in humidified air was studied at 500–800 °C, and the rate of CrO2(OH)2 volatilization was measured quantitatively as a function of exposure time using a denuder technique. Different gas velocities were employed in exposures with a maximum duration of 200 h. Corrosion morphology was investigated by SEM/EDX using BIB-milled cross sections. The rate of chromium volatilization increased with increasing temperature and gas velocity. The rate of volatilization decreased with exposure time. Two oxide scale morphologies were observed, depending on temperature and gas velocity. In the 500–700 °C range, the scale consisted of chromia-rich corundum-type oxide, while exposures with high gas velocities at 800 °C produced an entirely different type of scale that included a Ni-rich and Cr-poor cap layer. The latter scale morphology is suggested to result from extensive chromium depletion of the alloy substrate which triggers a new mode of oxidation involving formation of NiCr spinel oxide. Continued volatilization of CrO2(OH)2 causes the NiCr spinel to decompose into a Ni-rich oxide that forms a cap layer on the scale surface. This cap layer is very efficient in decreasing the rate of chromium volatilization, allowing the chromium levels in the substrate to recuperate. We show that volatilization of chromium (VI) from the alloy can be mitigated by an oxidation pre-treatment that allows the Ni-rich cap layer to form.

Volatilization kinetics of chromium oxide, manganese oxide, and manganese chromium spinel at high temperatures in environments containing water vapor

AbstractPerformance degradation of solid oxide fuel cells due to chromium volatilization is a well‐investigated issue in the literature. Therefore, retention coatings were developed to distinctly reduce the chromium volatilization. One approach was by alloying with manganese to ferritic steels to form manganese chromium spinel which is reported to decrease chromium volatilization by 61–75%. In the present paper, the volatilization rates of pure manganese chromium spinel ceramics were examined as well as those of the two oxides forming this spinel—pure chromium oxide and pure manganese oxide—in synthetic air containing 10% water vapor (high p(O2)) and argon/hydrogen containing 10% water vapor (low p(O2)) at 850°C, 950°C, and 1,050°C. Chromium oxide showed higher volatilization rates in high p(O2), whereas manganese oxide demonstrated higher volatilization rates in low p(O2). Contradictory to the literature, manganese chromium spinel displayed the highest volatilization rates in both atmospheres and nonlinear kinetics behavior. This deviation from linear behavior can be attributed to diffusion‐controlled volatilization in high p(O2).

Volatility and chemical stability of chromium, molybdenum, and tungsten hexacarbonyls

The sublimation vapor pressures of three metallic (group 6) hexacarbonyls, Cr(CO)6, Mo(CO)6, and W(CO)6, were measured using the Knudsen mass-loss effusion method. The standard (po = 0.1 MPa) molar enthalpies, entropies, Gibbs energies, and heat capacity difference between the gas and solid state, at T = 298.15 K, were derived from the experimental results combined with selected literature ones covering a wide range of temperature. The temperatures and molar enthalpies of fusion of those compounds were measured using differential scanning calorimetry. The thermodynamic stability of the hexacarbonyls was evaluated taking into account the standard Gibbs energies of formation in the crystalline and gaseous phases. Gas-phase absolute entropies, heat capacities, and enthalpies of formation of the three compounds studied as well as the bond distances M–C and C–O were calculated.

Modelling Microstructural and Chemical Degradation of Ferritic Stainless Steels for SOFC Interconnects

Ferritic Fe-based (FeCrX) alloys forming a protective chromium oxide film are used for providing electrical interconnection in high temperature Solid Oxide Fuel Cells (SOFC) are susceptible to degradation mechanisms in the presence of high temperatures (>600oC), and humid air. These translate by the increase in interconnect electrical contact resistance and the formation of volatile chromium species causing poisoning of the adjacent cathode. Modelling tools and techniques are increasingly becoming popular to gain better understanding of the degradation mechanisms that lead to the formation of volatile chromium species. This research work examines the oxidation behaviour of uncoated ferritic stainless steels system operated at temperature of 700oC in both wet (3% humidity) and dry conditions. A model is presented based on the physical processes involved. The model is able to predict the weight gain of the samples upon oxidation, the oxide film thickness, the volatilization of chromium and the useable lifetime of interconnects based on chromium depletion. Parameter determination from standard experiments is discussed along with the physical and design insights from the model. Perhaps most importantly the model provides recommendations regarding accelerated testing of FeCrX interconnects and control of the Chromia film thickness. Acknowledgements: This work was supported by the European FCH JU in project SCORED 2:0 under contract no. 325331 and the EPSRC.

Reduction of chromium volatilisation from stainless steel interconnector of solid oxide electrochemical devices by controlled preoxidation

• Chromium volatilisation from AISI 441 at 800 °C in O 2 + 5%H 2 O was measured and was shown to depend on gas velocity.

Chromium vaporization from mechanically deformed pre-coated interconnects in Solid Oxide Fuel Cells

Cathode poisoning, associated with Cr evaporation from interconnect material, is one of the most important degradation mechanisms in Solid Oxide Fuel Cells when Cr2O3-forming steels are used as the interconnect material. Coating these steels with a thin Co layer has proven to decrease Cr vaporization. To reduce production costs, it is suggested that thin metallic PVD coatings be applied to each steel strip before pressing the material into interconnect shape. This process would enable high volume production without the need for an extra post-coating step. However, when the pre-coated material is mechanically deformed, cracks may form and lower the quality of the coating. In the present study, Chromium volatilization is measured in an air-3% H2O environment at 850 °C for 336 h. Three materials coated with 600 nm Co are investigated and compared to an uncoated material. The effect of deformation is investigated on real interconnects. Microscopy observations reveal the presence of cracks in the order of several μm on the deformed pre-coated steel. However, upon exposure, the cracks can heal and form a continuous surface oxide rich in Co and Mn. As an effect of the rapid healing, no increase in Cr vaporization is measured for the pre-coated material.

Protective Coatings for Metallic Components in Solid Oxide Fuel Cell Systems

To make solid-oxide fuel cell (SOFC) systems more manufacturable and reduce system costs, SOFC developers, wherever possible, have substituted lower cost stainless steel into the stack design, including replacing ceramic interconnects within the stack and high-cost nickel-based superalloys in balance-of-plant components. However, for successful implementation of these steels, protective coatings are necessary to protect the air-facing metal surfaces from high temperature oxidation and to minimize chromium volatilization from the metal, because chromium volatiles poison the cathode and degrade cell performance. NexTech Materials has developed a cost-effective coating technology to improve the high temperature performance of stainless steel components. Coatings include an electrically conductive, manganese cobaltite (MCO) coating tailored for the cathode active-area of metallic interconnects and a complementary diffusion-based aluminide coating for the non-active, seal-area of interconnects and also readily amenable to balance of plant components. In this paper, the suitability of this coating technology for SOFC applications will be demonstrated. Microstructural analysis in combination with long-term electrical area-specific resistance (ASR), oxidation and chromium volatility testing will illustrate the stability and functionality of the coatings under application specific conditions. Stack-level testing will demonstrate the successful scale-up of the technology.

Multilayered Precoated AISI 441 for Solid Oxide Fuel Cell Interconnects

Sandvik Materials Technology develops nanocoatings for SOFC stainless steel interconnects. In this study a multilayered PVD-coating has been investigated. Cyclic oxidation for more than 20,000 hours at 800°C, chromium volatilization and area specific resistance studies has been conducted on three different coatings and compared with the uncoated steel, in this case AISI441 (EN 1.4509). In this work we have shown that pre-coated AISI 441 show very promising results for SOFC interconnectors at 800°C. We have studied and compared state of the art CeCo coating with a new four layered coating that shows slightly higher cumulative chromium volatilization, similar ASR values and reduced mass gain in the cyclic oxidation experiments. Figure 1