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Effect of Alloying Element on the High Temperature Property of Ferritic Stainless Steel for

Effect of Alloying Element on the High Temperature Property of Ferritic Stainless Steel for
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Because of the sharp rise of energy demand, sustainable use of fossil fuel is set to face challenges
depletion of fossil fuel reserves, significant fuel price rise and environmental concern. Therefore, the solid oxide fuel cells (SOFCs) are considered as a promising power source in the future due to its high efficiency and low environmental pollution. The interconnect is one of the essential components of SOFC stack systems and it provides an electrical connection between the anode of one unit cell and cathode of the neighboring cell. Ferritic stainless steels are promising materials for SOFC interconnect, because they are relatively cost-effective and have similar thermal expansion coefficient to that of other SOFC components. Most of the alloy candidates of ferritic stainless steel for SOFC interconnects contain rare earth elements such as La and Y to improve oxidation resistance of steels through the reactive element effect. However, the effect of reactive elements on the multi-oxide layers formed on the ferritic stainless steel is not clearly understood, yet. Moreover, the alloy candidates with rare earth elements are considered “specialty” alloys, because the addition of rare earth element requires extra process such as high vacuum melting. These extra processes are not inexpensive or readily available for general steel making process for ferritic stainless steels Therefore, new alloying elements which can play role similar to rare earth element without extra process are necessary. In the this dissertation, the oxidation behaviors of ferritic stainless steels with rare earth elements and other alloying element, which has high oxygen affinity such as Ti and Nb, has been discussed in terms of oxide scale microstructure, oxidation kinetics, electrical property and Cr evaporation behavior of scale. The rare earth elements such as La and Y suppress the growth of Cr2O3 and inner Mn-Cr spinel layers, but enhance that of outer Mn-Cr spinel. These modifications of scale microstructures induce the property of ferritic stainless steel to reduce oxidation rate, to increase electric conductivity and to suppress Cr evaporation. The effect of rare earth elements are related with their segregation behavior in the scale. Therefore, other alloying elements, which have high oxygen affinity and tend to segregation in the scale, can play a role similar to that of rare earth elements. Ti tends to be concentrated at the scale and minor addition of Ti can enhance oxidation resistance of ferritic stainless steels similar to rare earth elements. However, the effects of addition of La and Y to ferritic stainless steel are not exactly same to that of Ti. Ti addition induced the generation of ionic defects in the oxide layer and modified the growth kinetics of Cr2O3 and MnCr2O4, but in different manner depending on Ti amount. A high amount of Ti (~ 1 wt %) in ferritic stainless steel generated excess ionic defect in the scale and reduced the oxidation resistance. On the other hands, the minor addition of Ti (0.05 ~ 0.07 wt%) is effective for reducing the oxidation rate and electrical resistance. Co-addition of a small amount of Ti and La enhances Ti segregation at the scale/alloy interface without generation of excess ionic defect and it can improve both the electric conductivity and Cr evaporation resistance. Nb addition in a range of 0.5 ~ 1.2 wt% induce the Nb segregation at outermost oxide scale as NbO2, and near the oxide scale/alloy interface as both Nb2O5 and Laves phase. These Nb segregations promote modification of oxide scale microstructure and enhance oxidation resistance, electric conductivity and Cr evaporation resistance of ferritic stainless steel. On the other hand, minor addition of Nb (<0.06 wt%) does not enhance Nb segregation at the scale, and excess Nb (>4.7 wt%) suppresses precipitation of Nb2O5 at the scale/alloy interface, because of rapid Laves phase growth. The effect of Nb on the oxidation behavior of ferritic stainless steel depending on oxygen active element such as Ti and Si has been discussed. Si suppresses Nb enrichment near the scale/alloy interface and it reduces the precipitation of both Nb2O5 and Laves phase. Because Nb also suppresses Si enrichment and the formation of continuous Si oxide at the scale/alloy interface, Nb addition has beneficial function to reduce negative effect of Si impurity in SOFC interconnect in terms of electrical conductivity. On the other hand, Nb enhances selective Ti oxidation at the scale/alloy interface without significant reduction of Nb enrichment. Therefore, co-addition of Nb and Ti has beneficial effect to enhance oxidation resistance, electrical conductivity and Cr evaporation resistance of ferritic stainless steel in SOFC environment.
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