Improving the Physicochemical Properties of Fe–25C

Authors: Tomasz Brylewski Aleksander Gil Anna Rakowska Sebastien Chevalier Anna Adamczyk Jaroslaw Dabek Andrzej Kruk Miroslaw Stygar Kazimierz Przybylski

Publish Date: 2012/12/30

Volume: 80, Issue: 1-2, Pages: 83-111

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Abstract

The present study investigated the role of the reactiveelement effect REE in improving the corrosion resistance chromium vaporization rate and electrical conductivity of the Fe–25Cr ferritic steel modified either by means of yttrium implantation or chemical deposition of yttrium oxide from metaloorganic compound vapors The corrosion kinetics of the Fe–25Cr steel both pure and modified were determined under isothermal conditions in air and an Ar–H2–H2O gas mixture at 1073 K A significant improvement in corrosion resistance was observed after surface modification XRD and SEM–EDS investigations showed that the protective Cr2O3 layer formed the main part of the scale Measurements of Cr vaporization rate in the air–H2O gas mixture revealed that both surface modifications of the steel significantly suppressed the formation of volatile chromium compounds to a large degree The yttriumimplanted steels oxidized both in air and the Ar–H2–H2O mixture were characterized by the lowest area specific resistance and thereby did not exceed the acceptable ASR level 01 Ω cm2 for interconnect materials in the temperature range of 973–1073 K unlike pure steel and the steel coated with Y2O3Planartype solid oxide fuel cells SOFCs composed of ZrO2 stabilized with Y2O3 YSZ are among the most promising designs for the motor industry on account of their high power output of up to 25 kW m−2 simple geometry and compact size 1 2 Fuel cells connected in parallel feature interconnects in the form of bipolar plates which serve the following functions providing mechanical support within the cell gastight separation of the cathode and anode spaces ensuring electrical conduction between the cells as wells as the distribution of current to external devices However the design is hampered by one major flaw which cylindrical SOFCs are not affected Namely there are significant problems with ensuring sufficient tightness between the interconnect’s plates and the remaining flat elements of the fuel cell In effect the durability of these devices is significantly decreased Consequently in the recent years many efforts have focused on developing technological solutions that would enable the production of planartype SOFCs suitable for operation at low temperatures in the range of 873–1073 K In such a range of low temperatures fuel cell casings and interconnect plates may be composed of heatresistant ferritic stainless steels FSS which are much cheaper than ceramic materials or expensive metallic alloys whereas glass or silver may be used to ensure sufficient tightness 3 Aside from offering advantages such as the ease of mechanical treatment good weldability and high resistance to hightemperature corrosion in air the atmosphere typical for the cathode space and H2–H2O H2–H2O–H2S and CH4–H2O gas mixtures typical for the anode space ferritic stainless steels also have a thermal expansion coefficient TEC similar to that of the anode material 4 This makes it possible to construct anode supported cells ASCDuring hightemperature oxidation of ferritic steel with a chromium content of up to 25 wt a twolayer scale is formed The thin outer layer is composed of the MnCr2O4 spinel while Cr2O3 forms the thick inner layer 5 6 7 8 9 10 The spinel layer inhibits the formation of volatile chromium oxides and oxyhydroxides mostly CrO2OH2 which increase the activation polarization of the cathode 11 The main issue faced when applying these steels in interconnect construction is the gradual increase in area specific resistance ASR connected with the growing thickness of the scale 5 6 7 8 9 10 12 An overly high resistance of the scale may significantly inhibit the flow of electric current from the cathode to the interconnect which results in the decrease in the fuel cell’s power output 2 The growth rate of the Cr2O3 is therefore an important factor affecting the resistance of the SOFC which should ideally be maintained at the lowest possible level throughout the cell’s operation life One of the ways in which this may be achieved is by triggering the socalled reactive element effect REEExtensive experimental studies have shown that the addition of small amounts of active elements such as Y La Ce Hf Nd or their oxides in the form of dispersed particles is an effective method of improving the corrosion resistance of the alloys that form the protective Al2O3 or Cr2O3 scales 13 14 15 Active elements improve the adhesion of the abovementioned scales to the metallic phase and in the case of the Cr2O3 scale additionally decrease its growth rate It is therefore to be expected that as the thickness of the compact chromia scale decreases so will the electrical resistance of the steel/scale system since its electrical properties are determined predominantly by the physicochemical properties of chromia itself as well as the surface area of its contact with the metallic core 16The aim of the present paper was to determine the combined effect of the REE and the heattreatment atmosphere on the corrosion resistance chromium vaporization rate and electrical conductivity of the Fe–25Cr steel modified by means of yttrium implantation or deposition of yttria on its surface using metaloorganic chemical vapor deposition MOCVDThe ferritic steel Fe–25Cr 171532 CSN EN 10204 31B DIN 50049 Valcovny Plechu as FrydekMistek Czech Republic with the composition of Cr2455 Mn028 Si074 Ni099 C004 P003 S0013 and Ti001 wt was used in the investigations Experiments on oxidation featured two kinds of samples of the abovementioned material 1 rectangular samples with the approximate dimensions of 20 × 10 × 05 mm cut from metal sheets were used to study the oxidation kinetics of pure steel and steel either implanted with yttrium or with a deposited Y2O3 layer 2 square samples with a total surface area of 2 cm2 and a thickness of ca 01 cm were used in the measurements of electrical resistance and the rate of chromium volatilization The chromium volatilization rate was also measured for the Fe–25Cr model alloy which was treated as a reference material for steel The surfaces of the samples were ground with 100–1200 grade SiC abrasive paper and then polished in an aqueous Al2O3 slurry with a mean gradation of 03 µm until a mirrorlike was obtained The samples were then rinsed using water with a detergent and ethyl alcohol and degreased in acetone by means of an ultrasonic washerYttrium was implanted underneath the surface of the Fe–25Cr steel by means of ion beam implantation IBI using the apparatus owned by the Polish Academy of Sciences’ Institute of Nuclear Physics in Krakow 17 Depending on the intended application the samples were implanted either on one side only for morphological observations or on both sides for kinetics and electrical resistance studies Plasma containing ionized yttrium was obtained via glow discharge in YCl3 vapors A beam of singly ionized yttrium atoms—Y—was accelerated to the energy level of 25 keV using 1 × 1016 Y ions per cm2 The distribution of the implanted yttrium ions calculated based on the LindhardScharfSchiott LSS model was Gaussianlike The highest concentration of yttrium for the abovespecified ion dose was equal to 127 at while the effective range of yttrium ions was 173 nmDuring the deposition of the Y2O3 oxide on the surface of the Fe–25Cr steel performed by means of metaloorganic chemical vapor deposition TtmRd3Y was used as the metaloorganic precursor 18 This precursor and N2 carrier gas at 483 K were introduced into the reactor’s chamber at a flow rate of 125 cm3 min−1 along with oxygen supplied at a rate of 42 cm3 min−1 In the reactor yttrium was deposited directly on the steel substrate which was heated up to 873 K for 25 min A thin layer with 180 nm average thickness of yttria formed as a result of the reaction between yttrium and oxygen In order to distribute yttrium evenly across the steel surface the substrate was rotated around its axis Surface modification by means of the MOCVD method was carried out at the Institute Carnot de Bourgogne ICB in Dijon France

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