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Corrosion phenomena of alloys and electrode materials in Molten Carbonate Fuel Cells
Author(s) -
Biedenkopf P.,
Bischoff M. M.,
Wochner T.
Publication year - 2000
Publication title -
materials and corrosion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.487
H-Index - 55
eISSN - 1521-4176
pISSN - 0947-5117
DOI - 10.1002/(sici)1521-4176(200005)51:5<287::aid-maco287>3.0.co;2-8
Subject(s) - corrosion , materials science , spinel , metallurgy , alloy , oxide , magnetite , maghemite , cathode , carbonate , hematite , molten carbonate fuel cell , intergranular corrosion , electrolyte , electrode , chemistry
The corrosion behavior of different alloys and the electrical conductivity of the growing corrosion scales was investigated under simulated and real molten carbonate fuel cell conditions. The corrosion of the usually used NiO cathode material was also investigated. In several exposure tests in oxidizing atmospheres, the FeCrMnNi steel 1.3965 showed a higher corrosion resistance to the aggressive carbonate media than the FeCrNi alloy 1.4404 (SS316L). This superior corrosion resistance is explained by the formation of a mixed (Fe,Ni,Mn) x Cr 3‐x O 4 spinel layer, which reduces the outward diffusion of iron ions more than the mixed (Fe,Ni)Cr 2 O 4 spinel formed on austenitic FeCrNi steels. Oxide debris, which spalls off the current collectors, was investigated by XRD. The corrosion scales spalled off mainly at the curved area of the current collector and not at the cathode/current collector interface. The debris was strongly magnetic and consisted of several, in some cases lithiated iron oxides, whereby α‐Fe 2 O 3 (hematite), γ‐Fe 2 O 3 (maghemite) and Fe 3 O 4 (magnetite) formed most of the debris. The investigations of the electrical conductivity of the corrosion scales have shown that the electrical conductivity is limited by the inner, Cr‐containing oxide of the multi‐layered corrosion scale. Cr‐rich alloys which contain more that 20 wt.% Cr showed extremely high ohmic resistance of the corrosion scale, much higher than that of alloys containing less than 20 wt.% Cr due to the formation of highly conductive mixed spinel layers. Small additions of Al in the alloy increased the ohmic resistance of the corrosion scale by many orders of magnitude. Corrosion tests in the fuel environment showed, that common uncoated stainless steels are not suitable for the use as anodic current collectors. The corrosion resistance in the anodic gas atmosphere is determined by the Cr and the Ni contents of the alloy. Only the model alloy NKK which contains 45 wt.% Ni and 30 wt.% Cr showed an acceptable corrosion resistance. The NiO dissolution and the Ni precipitation was investigated by single cell tests. These tests showed, that the replacement of the metallic Ni by an Al support (which is necessary to avoid cracks inside the ceramic) decreases the amount of metallic Ni in the ceramic matrix significantly. Therefore shorting of a fuel cell having a NiO cathode and a LiAlO 2 matrix with an Al support for the mechanical support is not expected in the target lifetime of 40 000 h. The double layer LiCoO 2 ‐NiO cathode also showed a significant reduction in Ni precipitation after testing. Due to the improvements and development in materials the MCFC‐lifetime has been trebled in the last few years.