The Performance of Ce Surface Treated Ferritic Stainless Steels for Solid Oxide Fuel Cell Interconnects

This research deals with the effect of a Ce surface treatment on the behavior of Fe-Cr-Mn ferritic stainless steels which may have application in SOFC technology. This treatment consisted of applying a slurry of CeO2 and a halide activator to the surface of coupons. After the slurry dried the coupons were heated to 900C in a controlled atmosphere furnace for 12 hours. The effectiveness of the treatment on commercial (Type 409 (12Cr), Type 430 (18Cr), Crofer 22APU (22Cr), Type 446(26Cr)) and experimental (NETL F9 (12Cr) and NETL F5 (22Cr)) alloys as a function of Cr content will be presented. The oxidation behavior of the alloys was assessed by exposing coupons (untreated and treated) to moist air at 800C. Area specific resistance (ASR) was measured at 800C. In general, the rare earth treatment effectively reduced the oxidation rate, resulting in thinner oxide scales and less internal oxidation

Surface Modifications for Oxidation Resistance

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Influence of alloy content and a cerium surface treatment on the oxidation behavior of Fe-Cr ferritic stainless steels

The cost of solid oxide fuel cells (SOFC) can be significantly reduced by using interconnects made from ferritic stainless steels. In fact, several alloys have been developed specifically for this application (Crofer 22APU and Hitachi ZMG323). However, these steels lack environmental stability in SOFC environments, and as a result, degrade the performance of the SOFC. A steel interconnect can contribute to performance degradation through: (i) Cr poisoning of electrochemically active sites within the cathode; (ii) formation of non-conductive oxides, such as SiO2 or Al2O3 from residual or minor alloying elements, at the base metal-oxide scale interface; and/or (iii) excessive oxide scale growth, which may also retard electrical conductivity. Consequently, there has been considerable attention on developing coatings to protect steel interconnects in SOFC environments and controlling trace elements during alloy production. Recently, we have reported on the development of a Cerium surface treatment that improves the oxidation behavior of a variety alloys, including Crofer 22APU [1-5]. Initial results indicated that the treatment may improve the performance of Crofer 22APU for SOFC application by: (i) retarding scale growth resulting in a thinner oxide scale; and (ii) suppressing the formation of a deleterious continuous SiO2 layer that can form at the metal-oxide scale interface in materials with high residual Si content [5]. Crofer 22 APU contains Fe-22Cr-0.5Mn-0.1Ti (weight percent). Depending on current market prices and the purity of raw materials utilized for ingot production, Cr can contribute upwards of 90 percent of the raw materials cost. The present research was undertaken to determine the influence of Cr content and minor element additions, especially Ti, on the effectiveness of the Ce surface treatment. Particular emphasis is placed on the behavior of low Cr alloys

Microchannel devices

The fabrication of stainless steel microchannel heat exchangers was examined through microlamination, the process of diffusion bonding precision machined metallic foils. The influence of diffusion bonding parameters, as well as the device geometry on the strength of the bond between the foils and embedded channel integrity, was investigated. During diffusion bonding, high temperatures and/or pressures result in well bonded foils, but these conditions cause the embedded channels to deform, which will degrade the efficiency of fluid flow through the channels. Alternatively, low temperatures and/or pressures result in undeformed channels but weakly bonded foils. This causes failure of the device due to fluid leakage. Thus, a processing envelope exists for producing a sound device with no fluid leakage and no degradation of fluid flow properties. The theoretical limit on aspect ratio within two-fluid counter-flow microchannel heat exchangers was also investigated. A counter-flow device is comprised of alternating layers of microchannels, which allow the two fluids to flow in opposite directions separated by fins. A theoretical model for interpreting the span of the fin as a function of the fin thickness was established. The model was verified experimentally by fabricating specimens to simulate the counter-flow device. The results of these investigations were used to aid in the design and processing of prototype microchannel devices

Development of low coefficient of thermal expansion (CTE) nickel alloys for potential use as interconnects in SOFC

This paper deals with the development of low coefficient of thermal expansion (CTE) nickel-base superalloys for potential use as interconnects for SOFC. Ni-Mo-Cr alloys were formulated with CTE on the order of 12.5 to 13.5 x10-6/°C. The alloys were vacuum induction melted and reduced to sheet via a combination of hot and cold working. Dilatometry was used to measure CTE of the alloys. Oxidation behavior of the alloys at 800°C in dry and moist air is reported. The results are compared to results for Haynes 230 (a commercial Ni-base superalloy) and for Crofer 22APU (a commercial ferritic stainless steel designed specifically for use as an SOFC interconnect)

Oxidation resistance of 9-12% Cr steels: effect of rare earth surface treatment

Medium Cr steels have been used in fossil fired power plants for many years because of their excellent high temperature stability and mechanical properties. The environment in a fossil fired power plant is extremely aggressive in terms of corrosion, especially oxidation. This is only accelerated as the operating temperature increases to 650C and beyond. For any new steel to be qualified for power plant use, in addition to adequate strength at the operating temperature, material wastage from all corrosion processes must be kept to a minimum acceptable level. The use of medium Cr steels provides a means to improve overall corrosion resistance. Three medium Cr are under development for use as high temperature power plant steels: 0.08C-(9-12)Cr-1.2Ni-0.7Mo-3.0Cu-3.0Co-0.5Ti. Oxidation tests were performed on the steels for times greater than 1000 hours in order to determine the oxidation kinetics and extent of material wastage. Also, rare earth oxides were incorporated into the outer surface layers of the steels to see if the oxidation resistance could be improved. These results will be compared to current power plant steels

Rural-metropolitan health differential for young persons with eating disorders referred for specialist treatment

Objective: The aim was to explore associations between residing in a rural area and clinical characteristics of children and adolescents with eating disorders presenting to a specialist eating disorders program. Method: The data source was the Helping to Outline Paediatric Eating Disorders (HOPE) Project registry (N ~ 1000), a prospective ongoing registry study comprising consecutive paediatric tertiary eating disorder referrals. The sample (N = 399) comprised children and adolescents presenting with a DSM-5 eating disorder, with ages ranging from 8 to 16 years (M = 14.49, 92% female). Results Consistent with the hypotheses, living in a rural area was associated with a lower body mass index z-score, and a higher likelihood of medical complications at intake assessment. Contrary to our hypothesis, eating pathology and living in a rural area were negatively associated. No relationship was observed between living in a rural area and duration of illness or greater percentage of bodyweight lost. Conclusions: The results suggest that living in a rural area and being a greater distance from specialist services is associated with more severe malnutrition and medical complications by the time the young person and their family obtain specialist care. These findings have implications for service planning and provision for rural communities. The modifications to service delivery in the study setting will be described