Cobalt diffusion coating on ferritic-martensitic steel
|Period:||2013-10-01 to 2016-12-31|
|Funder:||Federal Ministry for Economics and Energy (BMWi, Germany) via AiF|
|Project Manager:||Dr. Diana Fähsing|
|Research Group:||High Temperature Materials|
Modern Solid Oxide Fuel Cells (SOFC) work at temperatures between 600-800°C, which allows for the replacement of expensive lanthanum chromite ceramics (LaCrO3) by ferritic-martensitic steels as a material for interconnects. The metallic interconnects exhibit many advantages over ceramic materials. They have electrically conducting oxide scales, good mechanical properties, appropriate thermal expansion behavior, very low costs, excellent machinability and they can be joined with a number of standard welding and brazing techniques.
However, there are two major issues associated with the use of stainless steels. One is the evaporation and migration of volatile chromium species from the chromia scale due to the presence of water vapor, another is the electrical resistance, which increases over time due to continuous oxidation at elevated temperatures.
This work is focused on the improvement of the oxidation behavior in water vapor environments. Due to the limited oxidation resistance in water vapor containing atmospheres, ferritic-martensitic steels (9% Cr) cannot be used at temperatures above 600°C, due to the formation of the volatile chromium oxy hydroxide species CrO2(OH)2. This species evaporates from the surface of the material and leads to accelerated breakdown of the protective chromia layer. In the literature this process has been described as breakaway oxidation. Even more critical for SOFCs, the evaporated chromium reacts with the cathode material and leads to severe degradation of the cell performance.
The aim of the present work is the improvement of the oxidation behavior of such ferritic-martensitic steels in water vapor containing environments. Therefore the metal subsurface region of ferritic-martensitic steels is enriched with manganese and/or cobalt in the form of a diffusion coating by the pack cementation process. Pack cementation is a rather simple and industrially interesting technique, however its application to produce coatings with manganese or cobalt is rarely found in the literature. The influence of several coating parameters especially a variation of the activator, selected on the basis of thermodynamic calculations was investigated to achieve a highly enriched surface diffusion zone with cobalt.back
Das IGF-Vorhaben Nr. 17872 N der Forschungsvereinigung DECHEMA e.V., Theodor-Heuss-Allee 25, 60486 Frankfurt am Main wurde über die AiF im Rahmen des Programms zur Förderung der industriellen Gemeinschaftsforschung (IGF) vom Bundesministerium für Wirtschaft und Energie aufgrund eines Beschlusses des Deutschen Bundestages gefördert.