Influence of Pre-Oxidation on Dual Atmosphere Effect on AISI 441 Interconnects for Solid Oxide Fuel Cell Applications

Abstract

Interconnects are an integral part of solid oxide fuels cells (SOFC) and, even when ferritic stainless steel is used, their contribution to the overall stack cost is still substantial. Stainless steels that are produced in large volumes such as AISI 441 are particularly attractive from a cost perspective. However, in our previous work, we have shown that this material suffers from severe corrosion in dual atmosphere exposure at 600 °C typical for Intermediate Temperature-SOFC (IT-SOFC) applications. We have seen a beneficiary effect of pre-oxidation and will, in the present work, closely examine this effect. By varying pre-oxidation parameters such as length and location (anode vs. cathode side) of the oxide layer, we were able to clarify the importance of pre-oxidation. Furthermore, we see a correlation between the length of pre-oxidation, and therefore the thickness of the oxide layer, and the onset of breakaway oxidation. This correlation is expected to allow for accelerated life time testing with respect to dual atmosphere corrosion. For our studies we chose five different pre-oxidation times: 0 min, 11 min, 45 min, 180 min and 280 min. The dual atmosphere exposure was conducted at 600 °C and the exposure was interrupted after 24 h, 168 h and 500 h and photographs were taken of the air facing side of each steel sample. By means of the photographs, we can clearly correlate length of pre-oxidation with onset of breakaway. Our findings are further supported by Scanning Electron Microscopy/Energy Dispersive X-Ray (SEM/EDX) analysis. An additional study was performed to get a clear picture of the beneficial effects of the pre-oxidation layer in relation to its location. To examine this question the samples were pre-oxidized and subsequently the resulting oxide layer was removed from one side of the sample via mechanical polishing. The samples were then exposed with the pre-oxidized surface either facing the fuel side or facing the air side. The exposure was interrupted after the same intervals as in the aforementioned experiment to visualize the progress of breakaway oxidation. Also these samples were analyzed using photographs and SEM/EDX analysis.