No abstract

A material is needed for bipolar plate materials in molten carbonate fuel cells (MCFCs) that combines the low oxide resistivity of 316L stainless steel (SS) with the low corrosion rate of the type 310 SS. We tested a group of materials that included Nitronic 50 SS and a newly developed high-temperature nickel-rich alloy, having chromium contents ranging from 16 to 31 wt %. Our results indicate that chromium content is the primary determinant of oxide scale composition and resistivity. In the MCFC cathode compartment, all tested alloys formed a duplex structure with an inner Cr-rich layer and an outer Fe-rich one. The composition of the inner Cr-rich layer was determined by the chromium content of the base alloy and has a controlling effect on scale resistivity. Oxide scale resistivity was measured for three electrolyte compositions: Li/K, Li/Na, and newly developed (Li, Na, Ca, Ba) carbonates. Changes in the physical/mechanical properties (spallation/cracking) in the oxide scale of 316L SS provided an understanding of its resistivity fluctuations over time.

The corrosion behavior and the electrical resistivity of the oxide scale that forms on alternative materials for bipolar plates in molten carbonate fuel cells (MCFCs) were investigated. Commercial stainless steels (SS) containing cobalt (Haynes 556) and manganese (Nitronic 30, Nitronic 50, and Nitronic 60) were tested under cathodic MCFC conditions Additionally, 316L SS coated with cobalt by thermal spraying was studied. Oxide‐scale resistivity measurements were coupled with observations of microstructural/compositional changes over time. All tested materials formed multilayered oxide scales. The composition of these phases was the key factor in determining the interfacial electrical resistivity. The high cobalt content of Haynes 556 (18 wt%) did not decrease its electrical resistivity or improve its corrosion resistance. Thus, Co‐containing stainless steels, such as Haynes 556, do not appear to be candidate bipolar plate materials for MCFCs. In contrast, the cobalt coating on 316L SS did lead to improved corrosion resistance. The Nitronic alloys formed Mn‐containing oxide scales, which appear to have a beneficial effect on lowering the resistivity of the oxide scale. The corrosion resistance of these Mn‐containing stainless steels was greater than that of 316L SS, the present bipolar plate material.

The sintering behaviour of molybdenum pre-alloyed sintered steels was investigated. A scanning electron microscope with a hot stage attachment was used, enabling quasi-in-situ observations of the sintering process. The material MSP 1.5Mo (Fe-1.5% Mo) sinters in the austenitic phase, while MSP 3.5Mo (Fe-3.5% Mo) sinters completely in the ferritic phase, due to the increased molybdenum mass content. The significantly higher self-diffusion coefficient of iron in the ferritic phase leads to the accelerated sintering of MSP 3.5Mo, compared to MSP 1.5Mo. The effect on the sintering behaviour by adding the alloying elements chromium and phosphorus was also studied. While phosphorus accelerates sintering processes, chromium decreases the sintering rate. A pronounced shrinkage during nonisothermal heating, due to cooperative particle movement, was detected for both sintered steels. Quasi-in-situ-Untersuchungen des Sinterverhaltens molybdiinlegierter Sinterstiihle. In dieser Arbeit wurde das Sinterverhalten molybcanleqlerter Sinterstahle untersucht. Mit Hilfe eines Heiztisch-Rasterelektronenmikroskops wurden Sintervorgange quasi-in-situ beoabachtet. Oer Werkstoff MSP 1.5Mo (Fe-1.5% Mo) sintert in der Austenitphase, der Werkstoff MSP 3.5Mo (Fe-3.5% Mo) hingegen in der Ferritphase. Grund hierfQrist der erh6hte Massengehalt an Molybdan. Oer wesentlich hOhere Oiffusionskoeffizient von Eisen im Ferrit fQhrt zu einer beschleunigten Sinterung von MSP 3.5Mo im Vergleich zu MSP 1.5Mo. Oas wurde mit Hilfe des Heiztisch-Rasterelektronenmikroskops nachgewiesen. Zudem wurde der EinfluB der Legierungselemente Chrom und Phosphor untersucht. Wahrend Legieren mit Phosphor die Sinterung beschleunigt, verlangsamt Chrom die Sintervorgange. Aile untersuchten Sinterstahle zeigenunabhangig von der chemischen Zusammensetzung-schon beim nicht-isothermen Aufheizen starke Schwindungsvorgange, die sich durch kooperative Teilchenbewegung erklaren lassen.