Research progress of ferrite wear-resistant plate connectors
In 1997, Hitachi Metal Company developed the metal material Fe-22Cr ferritic system alloy ZMG232 for SOFC connectors with an operating temperature of 1000℃. The ferritic material was selected as the SOFC connector because the ferritic material has a lower coefficient of thermal expansion than austenite, which is more similar to the SOFC component material. The company then adopted a series of process improvement measures, such as reducing the content of impurity oxides, and developed the ZMG232L wear-resistant plate in 2005.
ZMG232L wear-resistant plate can form a dense Cr2O3 oxide layer in the working environment, so it has an inherent good electrical conductivity at the same time, but also has outstanding oxidation resistance. Although the oxidation resistance of the connector can also be obtained by forming an Al2O3 oxide layer, Al2O3 has no electrical conductivity at the operating temperature of the ZMG232L wear-resistant plate, so the Cr2O3 oxide layer alloy with small resistance at high temperatures is suitable for SOFC connector metal materials.
With the continuous development of SOFC, higher requirements have been put forward for connector metal materials - to further improve the oxidation resistance and reduce the amount of Cr evaporation generated by metal materials to reduce the impact on the performance of power generation components. Based on ZMG232L wear-resistant plate, Hitachi Metal Company realized the design of a film and densification alloy composed of spinel layer and chromium oxide layer with electrical conductivity by adding alloying elements and optimizing components, and developed a new alloy ZMG232G10 wear-resistant plate with oxidation resistance and electrical conductivity increased by about 2 times.
The oxide layer structure and alloy design of ZMG232G10 wear-resistant plate are briefly introduced below.
When the ZMG232L wear-resistant plate is heat treated in an oxidizing atmosphere, a double-layer oxide layer is formed on the surface of the workpiece, the surface layer is the (Mn,Cr) 3O4 oxide layer, the inner layer (matrix) is the Cr2O3 oxide layer. The oxidation resistance depends on the Cr2O3 oxide layer, but the evaporation of the Cr element contained in it will lead to the deterioration of the performance of the power generation element, although the (Mn,Cr) 3O4 oxide layer will have a certain inhibitory effect on the evaporation of Cr, but the oxidation rate of (Mn,Cr) 3O4 makes the protection effect limited.
To solve the above problems, the following measures are taken in the alloy design.
1) Improve oxidation resistance: reduce Mn content and reduce the oxidation rate of (Mn,Cr) 3O4;
2) Inhibit the evaporation of Cr: Add Cu, Cu diffuses into the (Mn,Cr) 3O4 oxide layer, densifying the (Mn,Cr) 3O4 oxide layer.
Due to the above composition design measures, the ZMG232G10 wear-resisting plate has better oxidation resistance than the ZMG232L wear-resisting plate, and the evaporation amount of Cr is reduced.
ZMG232G10 wear plate and other wear plate comparison
In order to commercialize SOFC, the production cost of each component must be reduced. In recent years, the application of ferrite wear-resistant plate in SOFC connectors has been studied. The oxidation resistance of ZMG232G10 wear-resistant plate connector developed by Hitachi Metal Company was compared with other wear-resistant plates, and the test sample was 10mm thick. Because the thinner the plate thickness, the worse the oxidation resistance of the alloy, it is necessary to pay attention to the thickness design when the wear-resistant plate is used as the SOFC connector material. When ZMG232G10 is used as SOFC connector material, thinner material than ordinary wear-resistant plate can be used.
The smaller the oxidation increment change with time, the better the oxidation resistance, such as ZMG232G10 wear plate oxidation resistance is significantly better than SUS430. Under normal circumstances, the oxidation increment of the alloy forming oxide layer increases according to the parabolic rule with the increase of time, but the oxidation increment of SUS430 increases sharply with the increase of time in the initial oxidation treatment. Before the oxidation time of 2500h, the change trend of SUS444 oxidation increment is basically the same as that of ZMG232G10, but after the increase, SUS444 oxidation increment decreases rapidly, which is the result of spalling of its oxidation layer with oxidation resistance. The above comparison shows that compared with ZMG232G10 wear-resistant plates, SUS430 and SUS444 have poor oxidation resistance.
