Oxide film formation and diffusion processes in near-surface layers of current collectors in solid oxide fuel cells

A method is developed for modifying the surface of current collectors in solid-oxide fuel cells (SOFC) prepared from ferrite stainless steel (Crofer22APU). Diffusion of the protective coating material into the Crofer22APU bulk and reverse diffusion of steel components into the coating are studied. The cross-sectional microstructure and composition are studied by the electron-microscopic technique. The elemental composition of the junction between the current collector and the lanthanum-strontium manganite cathode is studied depending on the time of service-life tests in the SOFC working mode (50–6000 h). The formation of the Cr₂O₃ oxide islet structure on the current collector surface at the steel/coating interface is observed. It is shown that the mutual diffusion of coating components (Ni) and Crofer22APU steel together with the redox reaction at the interface prevent the chromium diffusion to the surface and protect the steel current collector from oxidation.     

High-Temperature Oxidation Resistance of PDC Coatings in Synthetic Air and Water Vapor Atmospheres

This work is aimed at the development and investigation of the oxidation behavior of ferritic stainless-steel grade AISI 441 and polymer-derived ceramic (PDC) protective coatings. Double-layer coatings of a PDC bond coat below a PDC top coat with glass and ceramic passive fillers’ oxidative resistance were studied at temperatures up to 1000 °C in a flow-through atmosphere of synthetic air and in air saturated with water vapor. Investigation of the oxide products formed at the surface of the samples in synthetic air and water vapor atmospheres, at different temperatures (900, 950, 1000 °C) and exposure times (24, 96 h) was carried out on both uncoated steel and steel coated with selected coatings by scanning electron microscopy (SEM) and X-Ray diffraction (XRD). The Fe, Cr₂O₃, TiO₂, and spinel (Mn,Cr)₃O₄ phases were identified by XRD on oxidized steel substrates in both atmospheres. In the cases of the coated samples, m- ZrO₂, c- ZrO₂, YAG, and crystalline phases (Ba(AlSiO₄)₂–hexacelsian, celsian) were identified. Scratch tests performed on both coating compositions revealed strong adhesion after pyrolysis as well as after oxidation tests in both atmospheres. After testing in the water vapor atmosphere, Cr ions diffused through the bond coat, but no delamination of the coatings was observed.     

Sol-Gel Deposition of ZrO2 Films in Air and in Oxygen-Free Atmospheres for Chemical Protection of 304 Stainless Steel: A Comparative Corrosion Study

ZrO₂ coatings for corrosion protection were deposited on 304 stainless steel by sol-gel method using zirconium propoxide as precursor and densified in air and in oxygen-free (argon or nitrogen) atmospheres. XRD and IR data of the films were practically independent of the atmosphere used in the densification step showing that the ceramic oxide is properly formed from the precursor. The corrosion behavior of the stainless steel substrate was studied by potentiodynamic polarization curves in the absence and the presence of ZrO₂ coatings prepared in air, argon or nitrogen. The coatings extended the lifetime of the material by a factor of almost eight in a very aggressive environment, independently of the preparation procedure. The possibility of depositing pure or mixed oxide films by sol-gel methods in the absence of additional oxygen will allow the preparation of specific coatings onto oxygen-reactive substrates.     

Interfacial reactions of duplex stainless steels with molten aluminum

The morphology and constitution of the intermetallic layers formed on the surface of duplex stainless steels (DSSs) immersed in molten aluminum at 750 °C for 30 min have been studied in detail by scanning electron microscopy and electron probe micro‐analyzer. Compared with H13 steel, the DSSs exhibited a better corrosion resistance. The weight loss rates, as expressed in terms of weight loss per square centimeter of the specimen per minute, of DSSs are smaller than that of H13. The thickness of the intermetallic layers of DSSs is comparatively thinner. And the interface between the intermetallic layers and DSSs substrate is much flatter. The intermetallic layers of duplex stainless steels are consisted of inner continuous (Fe,Cr)₂Al₅ phase and outer porous (Fe,Cr)Al₃ phases. Microstructure observations suggest that the retarded interfacial reaction between DSSs and molten aluminum is associated with a continuous Al‐Fe‐Cr intermetallic phases layer formed on the solid/liquid interface, which acts as an effective diffusion barrier. The precipitation phase particles distributed along the austenite/ferrite and ferrite/ferrite interfaces also had a good effect on the corrosion resistance properties of DSSs to molten aluminum. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrochemical behaviour of SiO2 sol-gel coatings on stainless steel

SiO₂ coatings onto stainless steel substrates have been prepared by sol-gel in order to study the performance and mechanism of attack in different corrosive solutions. The electrochemical behaviour of the samples has been evaluated by Electrochemical Impedance Spectroscopy using NaCl and HCl as electrolytes. Comparative tests have been performed on samples with one and two silica layers as well as on uncoated ones. SiO₂ coatings produce no important protection of stainless steels subjected to electrochemical corrosion. This behaviour may be explained by micropores and microcracks produced during the coating sintering.     

Influence of protective layers on SOFC operation

Corrosion kinetics of ferritic alloys/steels (Crofer22APU, ITMLC, ZMG232L) were studied at high temperature. An extent of corrosion was evaluated by measuring the oxide scale thickness and the weight gain as a function of heating time. It is shown that even porous layer applied to interconnect can significantly reduce the rate of the steel oxidation. Contribution of the “oxide component” into the total degradation of the SOFC stack performance is estimated. Different protection materials and combinations were tested to analyze their influence on the processes of high temperature oxidation and long-term degradation of Fe-Cr steels. It has been shown that “more soft” materials on the basis of spinels (Mn(Co_{1 − x} Fe _x )₂O₄, Cu_{1 − x} Ni _x Mn₂O₄) are most suitable materials for the use as protective layers in comparison to perovskites. The efficiency of different protective materials was also tested in the real SOFC stacks designed in cooperation with company Staxera GmbH. It has been shown that applied spinel materials can effectively increase the long-term stability of the SOFC stacks.     

Synthesis and characterization of ultrafine spinel ferrite obtained by precursor combustion technique

Nanoparticles of the spinel ferrite, Co_0.6Ni_0.4Fe₂O₄ have been synthesized by the precursor combustion technique. This synthetic route makes use of a novel precursor viz. metal fumarato hydrazinate which decomposes autocatalytically after ignition to yield nanosized spinel ferrite. The X-ray powder diffraction of the ??as prepared?? oxide confirms the formation of monophasic nanocrystalline cobalt nickel ferrite. The thermal decomposition of the precursor has been studied by isothermal, thermogravimetric and differential thermal analysis. The precursor has also been characterized by FTIR, and chemical analysis and its chemical composition has been fixed as Co_0.6Ni_0.4Fe₂(C₄H₂O₄)₃·6N₂H₄. The Curie temperature of the ??as prepared?? oxide was determined by ac susceptibility measurements.     

Sol-gel ZrO2 coatings for chemical protection of stainless steel

ZrO₂ coatings deposited on 316 L stainless steel sheets were synthesized by sol-gel method using Zr(OC₃H₇)₄ as precursor and isopropanol, glacial acetic acid, and water as solvents for application with ultrasounds. Different solutions for dip-coating were prepared with compositions varying between 0.025 and 0.9 mol/dm³ of ZrO₂. X-ray diffraction shows that the films densified at 800°C are crystalline with a tetragonal structure. The thickness of the coatings varied from 0.35–0.75 m. The influence of the ZrO₂ coatings on the corrosion behavior of stainless steel substrates in aqueous NaCl was studied through potentiodynamic polarization curves at 1 mV/s. The values of the electrochemical parameters allow for an explanation of the role of the films in the increased resistance of steel against corrosion in moderately aggressive environments.     

Protective coatings for stainless steel for SOFC applications

In this paper, a coating procedure based on spin coating of metal oxide polymer precursors on stainless steel, which decreases the oxide scale growth rate, is evaluated. The yttrium and cobalt solutions were used as polymer precursors, while a ferritic stainless steel Crofer 22 APU was used for the deposition of protective coatings. The thickness of deposited protective film was about ~500 nm. The effectiveness of protective layer was evaluated by cyclic thermogravimetry, oxide scale electrical conductivity, and X-ray diffractometry. The results show that steel coated with yttrium polymer precursor has better properties than uncoated or cobalt-coated sample.     

Oxidation kinetics study of the iron-based steel for solid oxide fuel cell application

Kinetics of oxidation of Fe-Cr steel containing 25 wt.-percent Cr was studied as a function of temperature (1023–1173 K) for up to 480 h in flowing air, which corresponds to SOFC cathode environment operating conditions. The oxidation process was found to be a parabolic, suggesting that the diffusion of ionic defects in the scale is the slowest, rate determining step and it occurs predominantly by short-circuit diffusion paths. Comparison of the determined activation energy of oxidation of the studied steel with literature data indicates that at 1098–1173 K the chromia scale grows by the outward solid-state diffusion of chromium interstitials, whereas at 1023–1098 K — through a significant contribution of counter-current oxygen/chromium diffusion along Cr₂O₃ grain boundaries. The oxide scales were composed mainly of Cr₂O₃ with a continuous thin Mn_1.5Cr_1.5O₄ spinel layer on top of the chromia scale. The oxidation test results on Fe-25Cr steel demonstrate the applicability of the commercial type DIN 50049 stainless steel as interconnect for SOFC. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrochemical and XPS studies on corrosion behaviours of AISI 304 and AISI 316 stainless steels under plastic deformation in sulphuric acid solution

The corrosion behaviours of austenitic stainless steels were investigated by electrochemical methods under plastic deformation with constant strain in the naturally aerated 0.5 M H₂SO₄ + 0.2 M KCl solution at room temperature. The work addresses the influence of plastic deformation and molybdenum element on the corrosion resistance of austenitic stainless steels in the test solution. Electrochemical impedance spectroscopy presents the decreasing charge transfer resistance (R_t) and polarization resistance (R_p) values with the immersion time for AISI 304 stainless steel under constant strain deformation, and the increasing R_t and R_p values with the immersion time for AISI 316 stainless steel. The analysis of the chemical composition of the corrosion products was carried out by XPS. Molybdenum addition in AISI 316 stainless steel affects significantly the corrosion resistance because of its high ability to form Mo (VI) and MoCl₅ insoluble compounds in acid medium. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and characterization of nanosize nickel-doped cobalt ferrite obtained by precursor combustion method

Nanoparticles of the spinel ferrite, Co_1?x Ni _x Fe₂O₄ (x?=?0, 0.2, 0.3) have been synthesized by the precursor combustion technique. Novel precursors of metal fumarato-hydrazinate have been employed to yield the nanosized spinel ferrite. A characteristic feature of these precursors is that they decompose autocatalytically after ignition to give the monophasic nanocrystalline ferrite. This fact is corroborated by X-ray powder diffraction analysis. The thermal decomposition pattern of the precursors has been studied by isothermal thermogravimetric and differential thermal analysis. In order to fix the chemical composition, the precursors have been characterized by FTIR and chemical analysis and their chemical composition has been fixed accordingly. The Curie temperature of the ??as-prepared?? oxide was determined by alternating current susceptibility measurements.     

Characterization of interfaces of alumina – high alloyed steels by SST and AES depth profiling

Thin alumina films, deposited at 280°?C on several high alloyed steels by low pressure metal-organic chemical vapour deposition (LP-MOCVD), were annealed at 0.17 kPa in a nitrogen atmosphere for 2, 4 and 17 h at 600 and 800°?C. The effect of the annealing process on the adhesion of the thin alumina films was studied using a scanning scratch tester (SST) and Auger electron spectroscopy (AES). The best adhesion properties were obtained with commercial oxide dispersion strengthened (ODS) high temperature alloys, especially type PM 3030. From the “normally” high alloyed stainless steels, type AISI-321 showed the best properties. The other stainless steel – alumina combinations showed after a thermal treatment a decrease of the critical load, L_c. Using ODS alloys as the substrate resulted in an increased L_c. AES-studies revealed that the increased L_c can be explained by 1) the presence of sulphur trapping elements, avoiding segregation of sulphur near the interface which could have a detrimental effect on scale adherence, and 2) titanium and carbon enrichment at the interface resulting in a beneficial effect on the adherence between the oxide and the substrate.     

Mn–Co spinel protective–conductive coating on AL453 ferritic stainless steel for IT-SOFC interconnect applications

This paper presents research on the synthesis and properties of the Mn_1.5Co_1.5O₄ (MC) spinel powder, as well as its application for the preparation of a MC thick film on the AL453 steel to be used for metallic interconnect material in IT-SOFCs. In order to prepare the MC micropowder with excellent homogeneity of the chemical and phase compositions, EDTA gel processes were utilized. In order to improve the contact electrical resistance between an AL453 steel interconnect and the La_0.8Sr_0.2FeO₃ (LSF) cathode and protect the cathode from Cr poisoning, the surface of the AL453 steel was coated with a protective manganese cobaltite spinel matrix using screen printing in combination with an appropriate heat treatment. The oxidation of the AL453/MC composite layer carried out in the air–H₂O gas mixture at 1,073 K for 55 h showed that the spinel coating may serve as an effective barrier against outward Cr diffusion from the AL453 steel and, therefore, significantly inhibit the formation of volatile Cr vapors from the chromia scale. The contact ASR study of the interconnect–cathode interface in the AL453/MC/LSCM/LSF/LSCM/MC/AL453 system carried out in the range of 723?1,073 K in air showed a very large drop in ASR compared to the resistance of the AL453/LSCM/LSF/LSCM/AL453 system without the spinel coating.     

Structure of Ir and Ir-Al<Subscript>2</Subscript>o<Subscript>3</Subscript> coatings obtained by chemical vapor deposition in the presence of oxygen

By chemical vapor deposition Ir and Ir-Al₂O₃ coatings are obtained with a thickness of up to 40 fum on steel substrates precoated with a layer of Al₂O₃. Tris-acetylacetonates of iridium(III) and aluminium(III) are used as precursors. The deposition processes are carried out at atmospheric pressure in the presence of oxygen. The obtained coatings are studied by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The dependences of coating structures and compositions on the preparation conditions are found. An increase in the deposition temperature results in the formation of Ir coatings with loose discontinuous structure, an increase in the size of metal crystallites, and the growth of the oxygen concentration in their composition. An increase in the concentration of precursor vapors in the deposition zone at a constant deposition temperature results in the formation of Ir coatings that consist of differently structured layers (compact, columnar, and granular). Mixed Ir-Al₂O₃ coatings which composed of metal Ir and amorphous Al₂O₃ crystallites, which exhibit a pronounced iridium texture in the [111] direction, have the most perfect compact structure. The introduction of the oxide phase in the coating composition halves the Ir crystallite size.     

Effect of Sintering Temperature on the Corrosion and Wear Behavior of Protective SiO2-Based Sol-Gel Coatings

Sol-gel hybrid organic-inorganic and inorganic SiO₂-based protective coatings with and without added 3 m glass particles were developed and tested for their corrosion and wear behavior of an stainless steel substrate (AISI316L). The corrosion resistance greatly increases by incorporating glass particles in the sols. The incorporation of particles in the coatings allows the synthesis of thicker crack-free coatings. On the other hand, the corrosion resistance increases for coatings with a higher organic content obtained at lower sintering temperature. These coatings are also highly stable in saline aqueous solutions. However, the wear resistance is badly affected by the hybrid character of the SiO₂ matrix. The optimum coating process in terms of corrosion and wear resistance, appears to be a hybrid system with a dense SiO₂ network achieved at intermediate sintering temperatures.     

Characterization of the Corrosion of Nanostructured 17-4 PH Stainless Steel by Surface Mechanical Attrition Treatment (SMAT)

Abstract

The corrosion properties of nanostructured 17-4PH stainless steel facilitated by a surface mechanical attrition treatment (SMAT) process were studied using electrochemical measurements in 0.6?M NaCl aqueous solution. The microstructure of the surface layer was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results demonstrated the formation of a nanostructured surface layer on the surface of the material. By the combination of SMAT and low-temperature annealing processes, the potentiodynamic polarization measurements and X-ray photoelectron spectroscopy (XPS) spectra demonstrated an improvement in the corrosion resistance of 17-4PH stainless steel with a reduced corrosion current density of 0.241?mA/cm² and a higher chromium content. The improved corrosion resistance may be attributed to the formation of nucleation sites through which chromium may freely move from the matrix to the upper surface and thereby form a protective oxide layer on the surface of the material.     

Study of iron-zinc catalysts by Mössbauer spectroscopy

The Mössbauer parameters were determined on a series of catalyst mixtures of iron and zinc oxides with variable quantities of zinc. From these results, a change in the crystal structure of the iron oxide when introducing zinc into the samples was observed. The corundum structure of the -Fe₂O₃ phase was transformed into the spinel type of zinc ferrite when zinc oxide was present in any quantity. By means of these parameters a strong electronic interaction between the zinc ferrite and the zinc oxide present in excess was evident. The catalysts were analyzed using X-ray fluorescence /XRF/ and X-ray diffraction /XRD/.     

Corrosion behavior of the steel used as a huge storage tank in seawater

Open-circuit potential (OCP), polarization curve, and electrochemical impedance spectroscopy (EIS) measurement were used to investigate the corrosion behaviors of high-strength low-alloy (HSLA) steel and mild steel in seawater. Both steels were used in the construction of a huge oil storage tank. The OCP results show that the HSLA steel quickly reached more negative E _OCP values than the mild steel. Polarization curve results reveal that the HSLA steel exhibits higher corrosion currents and more negative corrosion potentials than the mild steel. EIS measurements reveal that both steels exhibit similar corrosion behaviors up to 144 h, one increased capacitance loop can be shown in EIS diagrams. The mild steel presents higher corrosion resistances than the HSLA steel at former stage, which is associated with the effect of the grain size. After 240 h of immersion, both steels present different corrosion behaviors. The EIS diagrams exhibit two capacitance arcs for the HSLA steel and one capacitance arc for the mild steel, which is due to the formation of intact corrosion scales on the electrode surface of the HSLA steel as to introduce a new reaction interface. The HSLA steel exhibits higher corrosion resistances than the mild steel at latter stage of experiment, which is ascribed to the synthetic actions of residual Fe₃C and the protective property of corrosion products.     

Structural Characterization by SIMS of Alumina Coatings on Stainless Steel

Al₂O₃ coatings were obtained by the alkoxide route and deposited on stainless steel using the dip coating technique. The starting precursor was aluminum sec-butoxide modified by acrylic acid in order to prevent its precipitation in the presence of water.Useful information for the structural organization of alumina coatings on stainless steel is deduced from SIMS analysis. SIMS data reveal that the coating structure brings into play two different layers: an outer alumina layer that is more or less doped, mainly by iron, and an internal layer corresponding to the alumina/steel interphase. Beneath the interphase, the presence of an oxidized steel layer on the substrate surface is detected.Whatever the coating, the alumina/steel interphase exhibits a nearly constant thickness. On the other hand, a thickness variation of the oxidized steel layer is observed between samples under study: this thickness increases with the curing time of the coating.