Corrosion of Ferritic Stainless Steel Interconnects for Solid Oxide Cells – Challenging Operating Conditions

Solid oxide cells (SOC) have the potential to revolutionize electricity production by being able to both produce electricity with very high efficiency from a variety of fuels or to produce fuels from electricity and abundant raw materials such as water or carbon dioxide. Some material challenges remain to be solved before large-scale commercialization can be achieved. Interconnects made from ferritic stainless steels are key components in solid oxide cells, but the conditions within the cells cause them to degrade from high temperature corrosion. This thesis seeks out the potentially demanding operating conditions for solid oxide cells and focuses on investigating the effect of changing the environment on the degradation of ferritic stainless steels. Tests in which steel coupons were exposed to different atmospheres were performed to simulate the degradation of an interconnect inside an operating solid oxide cell. The effect of operating solid oxide fuel cells in electrolysis mode was specifically investigated, which means that interconnects were exposed to pure oxygen instead of ambient air and higher steam content on the fuel side. It was found that at 850\ua0\ub0C, ferritic stainless steels with 18-26% chromium content did not oxidize faster when the oxygen pressure was increased. However, the microstructure of the formed oxide scales on the steels was found to depend on oxygen concentration which caused oxide spallation for some steels at lower oxygen pressures.Experiments in hydrogen with high steam content, representing the other side of the interconnect, revealed an increase in the oxidation rate of the steel if the chromium content in the steel was too low, due to a change of the oxidation mechanism. Dilution of the same atmosphere with argon changed the oxidation mechanism to more protective behavior, which led to new insights in designing relevant simulated solid oxide cell fuel side conditions. It was also found that the oxidation rate of ferritic stainless steels in fuel side atmosphere can be significantly reduced by the physical vapor deposition (PVD) of cerium onto the surface. Even with applied cerium, however, steels with lower chromium content might still be at risk of rapid oxidation due to iron-rich oxide formation.A close-to-reality atmosphere was also simulated by exposing a ferritic steel simultaneously to air on one side and hydrogen on the other, which resulted in severely accelerated corrosion at 600 \ub0C. Areas of up to 30 \ub5m thick iron oxide were formed on the air side after 1000 h and grew to cover most of the surface after 3000 h. This dual atmosphere effect was concluded to have an inverse relation to temperature since accelerated corrosion was not observed at 700 and 800 \ub0C. In addition, it was found that the corrosion resistance could be improved if the steel was pre-oxidized in air before exposure to dual atmosphere

Temperature dependence of corrosion of ferritic stainless steel in dual atmosphere at 600–800 \ub0C

The ferritic stainless steel AISI 441 (EN 1.4509) is exposed for 1000 h to air - 3% H2O on one side and to Ar - 5% H2 – 3% H2O on the other at temperatures 600, 700, and 800 \ub0C. Conditions are chosen to mimic the environment of metallic interconnects in an operating solid oxide fuel cell (SOFC). At 600 \ub0C, ∼25 μm thick Fe2O3/(Fe,Cr)3O4 forms on large parts of the air side of the samples. Reference samples exposed to air - 3% H2O on both sides form thin protective layers of (Cr,Mn)3O4/Cr2O3 at the same temperature. At higher temperatures, 700 and 800 \ub0C, all samples form protective layers of (Cr,Mn)3O4/Cr2O3 regardless of exposure to single or dual atmosphere. It is concluded that corrosion resistance in a dual atmosphere has an inverse dependence on temperature. Different hypotheses for the underlying cause for the dual atmosphere effect are discussed and compared to the experimental data

The effect of pre-oxidation parameters on the corrosion behavior of AISI 441 in dual atmosphere

Dual atmosphere conditions have been shown to be detrimental for the ferritic stainless steel interconnects used in solid oxide fuel cells (SOFC) under certain conditions. In the present work, we analyze the influence of pre-oxidation on corrosion resistance in dual atmosphere with regard to two parameters: the pre-oxidation time and the pre-oxidation location (pre-oxidation layer on the air-facing side or the hydrogen-facing side). The steel AISI 441 is investigated and pre-oxidation is achieved in air at 800 \ub0C. To examine the influence of pre-oxidation time on corrosion behavior, five different pre-oxidation times are used: 0, 11, 45, 180, and 280 min. The samples are exposed discontinuously to dual atmosphere for 1000 h at 600 \ub0C. Photographs, taken throughout the exposure, show that the pre-oxidation time correlates with the onset of breakaway corrosion. To analyze the influence of pre-oxidation location on corrosion behavior, the samples are pre-oxidized for 180 min, and then a pre-oxidation layer is removed from one side of the sample. Subsequent dual atmosphere exposure at 600 \ub0C for 500 h shows that the pre-oxidation layer on the hydrogen-facing side is more important for corrosion resistance in dual atmosphere than the pre-oxidation layer on the air-facing side

Oxidation behavior of selected FeCr alloys in environments relevant for solid oxide electrolysis applications

If society is to move towards using more renewable energy sources the power fluctuations of e.g. wind and solar energy need to be balanced. Energy storage by production of hydrogen from solid oxide electrolysis cells (SOEC) offers a solution to overcome these obstacles. However, the performance of current SOECs decreases too fast for the technology to be commercialized. This drop in performance over time is partly due to the degradation of metallic interconnects (bipolar plates) within the cell stacks. This study investigates the corrosion performance of selected ferritic steels in simulated SOEC environments. Ferritic steels have many properties that are desirable for interconnects but suffer from oxidation and chromium evaporation over time. Four different FeCr alloys have been exposed in different partial pressures of dry O2 (anode side) and in 34% H2O -3% H2-Ar (cathode side) at 850°C and gravimetrical measurements have been performed to study oxidation rates. Chromium evaporation has been measured and compared for the oxygen containing environments. Chromium evaporation was found to vary largely with oxygen partial pressure, while the oxidation rate of the steels did not vary substantially in the different oxygen partial pressures. Differences in oxidation behavior of the steels were observed between the exposures in different partial pressures of dry O2 and in 34% H2O -3% H2 -Ar. Both reduced and increased oxidation rates were observed in the steam with hydrogen environment compared to the oxygen environments for different materials. Keywords: Solid oxide electrolysis cell, SOEC, SOFC, degradation, high temperature corrosion, chromium evaporation, denuder techniqu

Degradation of Ferritic steel interconnects in SOEC environments

This study investigates the corrosion performance of selected ferritic steels in simulated solid oxide electrolysis cell (SOEC) environments for exposure times up 500 h. Ferritic steels have many properties that are desirable for interconnects but suffer from oxidation and chromium evaporation over time. Four different FeCr alloys have been exposed in different concentrations of dry O2 (anode side) and in 34 % H2O -3 % H2-Ar (cathode side) at 850 °C and gravimetrical measurements have been performed to study oxidation rates. Chromium evaporation has been measured and compared for the oxygen containing environments. Chromium evaporation was found to vary largely with oxygen partial pressure, while the oxidation rate of the steels did not vary substantially in the different oxygen partial pressures. Differences in oxidation behavior of the steels were observed between the exposures in dry O2 and in 34 % H2O -3 % H2 -Ar. Both reduced and increased oxidation rates were observed in the cathode side atmosphere compared to the oxygen side atmosphere for different materials

Degradation of ferritic stainless steels under conditions used for solid oxide fuel cells and electrolyzers at varying oxygen pressures

Four commercial ferritic stainless steels were tested at 850 °C in oxygen pressures ranging from 10-4 to 1 atm, in order to investigate the isolated effect of oxygen pressure on corrosion, in the context of solid oxide electrolysis cells. The oxidation rates of all steels were essentially independent of oxygen partial pressure, which indicates n-type behavior. FIB/SEM analysis revealed that the grain size of the oxides was found to decrease at lower oxygen pressures. Volatile Cr species evaporation in pure oxygen was significantly lower than what has been reported for simulated solid oxide fuel cell environments with humid air

Severe dual atmosphere effect at 600 \ub0C for stainless steel 441

AISI 441 foils of 0.2 mm thickness were exposed in a dual atmosphere setup in which one side was exposed to air -3% H2O and the other to Ar -5% H2 - 3% H2O. The experiment was performed at 600 \ub0C and was referenced against exposures in air +3% H2O on both sides. The exposure conditions were chosen to simulate the conditions of an interconnect in intermediate temperature solid oxide fuel cell stacks (IT-SOFC). A strong dual atmosphere effect was observed: local breakaway corrosion was discovered after only 1000 h on samples exposed to dual atmospheres. After 3000 h iron oxide had propagated to cover the entire surface area of the sample. In comparison, the samples exposed in single atmosphere formed thin protective chromia scales on both sides even after 3000 h of exposure

Copper Iron Conversion Coating for Solid Oxide Fuel Cell Interconnects

A conversion coating of iron and copper was investigated with the purpose of increasing the performance of Sanergy HT as a potential SOFC interconnect material. Samples were exposed to a simulated cathode atmosphere (air, 3 % H2O) for durations of up to 1000 h at 850 \ub0C. Their performance in terms of corrosion, chromium evaporation and electrical resistance (ASR) was monitored and compared to uncoated and cobalt-coated Sanergy HT samples. The copper iron coating had no negative effects on corrosion protection and decreased chromium evaporation by about 80%. An Area Specific Resistance (ASR) of 10 mΩcm2 was reached after 1000 h of exposure. Scanning Electron Microscopy revealed well adherent oxide layers comprised of an inner chromia layer and an outer spinel oxide layer

Influence of pre-oxidation on dual atmosphere effect on AISI 441 interconnects for solid oxide fuel cell applications

In previous studies an extreme dual atmosphere effect on the airfacing side of AISI 441 at 600\ub0C was observed. However indications showed that pre-oxidation of the material might have a beneficial effect on the corrosion resistance in dual atmosphere. To examine this further we pre-oxidized AISI 441 samples for 0 min, 11 min, 45 min, 180 min and 280 min and subsequently exposed these at 600 \ub0C for 500 h under dual atmosphere conditions. Photographs of the air-facing sides were taken throughout exposure to monitor the corrosion behavior. SEM analysis was performed on all samples after exposure