| Some ceramic materials not only have good corrosion-resistance, heat resistance and wear-resistance, but also have good properties of electric, magnetic and biological activity, therefore, metal matrix ceramic coatings obtained by coating the ceramic evenly on the surface of metal through certain technologies has both metal’s good physical mechanical property and ceramic’s advantages. NiFe2O4spinel not only has good properties of electrical and magnetic, but also has good thermal stability, chemical stability and high temperature corrosion resistance in molten salts, etc. Thus, it has also been considered to have potential as inert anode materials for aluminium electrolysis. Interconnect component of inert anode for aluminium electrolysis. However, there are some serous problems for alloy interconnect materials at the high operating temperature of960℃including poor high-temperature oxidation resistance and low adhesion of the oxide film during the process of aluminium electrolysis. This thesis is funded by the National High Technology Research and Development Project (863project) and the State Key Project of Fundamental Research (973project). In order to obtain a kind of inert coating used as interconnect component which has stable structure and better corrosion resistance, the preparation NiO/NiFe2O4coating by electrophoresis, composite electrodeposition and alloy electrodeposition respectively, followed by oxidation treatment, were investigated respectively. The preparation parameters of electrophoresis, electrodeposition and high temperature oxidation process were analyzed. The hot corrosion properties of these coatings in the cryolite molten salt atmosphere were evaluated. Finally, several coatings having effective protection of the cryolite molten salt atmosphere corrosion were obtained. Above all main conclusions can be drawn as followed:1. Electrophoretic deposition (EPD) technique followed by oxidation treatment was employed for the first time for fabricating NiO/NiFe2O4coating, including:firstly, a Ni coating was electrodeposited on the Ni-based superalloy; secondly, the ferric oxide (Fe2O3) thick films were deposited on the Ni coating by electrophoretic deposition (EPD) technique; and then pre-oxidation processing method was used to obtain the NiO/NiFe2O4composite coating. Electrophoretic deposition (EPD) technique was employed for fabricating the uniform ferric oxide (Fe2O3) films without holes and cracks on the Ni substrate. Electrophoretic deposition conditions (deposition voltage, deposition time) have a significant impact on the deposition layer thickness, deposition rate and film quality; During the oxidation process, a double-layer structure of NiO/NiFe2O4composite coating with the NiFe2O4spinel thickness of about10～15μm can be obtained under the low oxidation temperature or short oxidation time. However, a single-layer structure of NiO mainly in the coating can be obtained under the high temperature or long oxidation time. Dispersed NiFe2O4precipitates distributed in the NiO grains can also be observed.2.(Ni,Co)O/(Ni,Co)Fe204-spinel composite oxide coating can be prepared by pre-oxidation of the Ni-Co-Fe2O3coated specimens. The effects of plating parameters on the chemical composition, structural and morphological characteristics of electrodeposited Ni-Co-Fe2O3composite coatings were investigated. A well-adherent (Ni,Co)O/(Ni,Co)Fe2O4-spinel composite oxide coating was prepared by pre-oxidation treatment in air at1000℃for6h. The coating consists of three layers:the outer oxide layer was mainly composed of (Ni,Co)O phases, the middle layer was mainly composed of (Ni,Co)Fe2O4phase; and the inner layer mainly consisted of NiO phase.3. NiO/NiFe2O4-spinel composite oxide coatings were prepared by pre-oxidation of the electroplated Ni-Fe alloys. An adherent composite oxide coating with a very dense and flat structure can be obtained in this way. In addition, NiO/NiFe2O4-spinel coatings with different spinel morphologies can also be obtained by regulating the composition of Ni-Fe alloy, oxidation temperature and oxidation time. Then, these factors on the formation mechanism of NiFe2O4morphology and position are highlighted. During oxidation at low temperature, as for the Ni-Fe alloys with lower Fe content, spinel phases formed at the grain boundaries of NiO columnar grains on the scale surface, whereas as for the Ni-Fe alloys with higher Fe content, a continuous dense spinel can formed on its most surface of the oxide film, and the spinel thickness was increased with Fe content increase. However, during oxidation at high temperature, regardless of the Ni-Fe alloys with high and low Fe content, spinel phases in the form of a discontinuous layer, in which many small NiO phase existed. A large number of’dot’or’cross-shaped’ NiFe2O4spinel precipitation was also formed in the grains of NiO phase.4. Corrosion tests at960℃in an oxidising Na3AlF6-AlF3-CaF molten salt atomsphere showed that these three kinds of composite coatings have better corrosion resistance than the bare Ni substrates. Compared to the first two kinds of coatings, the pre-oxidised Ni-7Fe coating by alloy electrodeposition (pre-oxidision at1200℃in air for6h) exhibits better hot corrosion resistance during50h exposure to molten salts at960℃. The reason lies mainly in that both a compact and adherent NiO/NiFe2O4scale and intergranular NiFe2O4spinel precipitates were formed during the oxidation treatment, which prevent the inward penetration of molten salts. The newly formed NiAl2O4spinel layer in the outermost oxide scale during the corrosion tests could also act as a barrier to corrosion.In order to preparation NiO/NiFe2O4composite coating, three kinds of novel coating technology (electrophoretic deposition-oxidation, composite deposition-oxidation and alloy deposition-oxidation), had been tried. The study confirms the feasibility of preparation of NiO/NiFe2O4composite coating on metal surfaces by these methods. Additionally, this article focuses on study of the hot corrosion behaviors of these coatings in a special molten salt atmosphere. Performance tests of high temperature coatings in this molten salt atmosphere are rarely reported. Therefore, this study by the preparation and application of NI-Fe spinel composite coatings made a useful attempt and exploration, and providing analogies and references for further research. Therefore, this research work makes a useful attempt and exploration for the preparation and application of NiO/NiFe2O4composite coating, and provides analogies and data references for further research. |
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