The preparations and corrosion mechanisms of enamel and TiAlN coatings were studied in the present thesis. The corrosion behavior of a traditional enamel coating was investigated. In order to further improve its performances, two kinds of novel enamel based composite coatings were prepared on superalloys by high temperature firing method, and corresponding corrosion and protection mechanisms were also studied. Moreover, a TiAlN coating was deposited on high temperature titanium alloys by means of arc ion plating (AIP) technique, then the corrosion behavior of this coating was studied. It was found that the corrosion resistance of TiAlN coating could be remarkably improved by pre-oxidation treatment.
At 700°C, enamel coating effectively improved the high temperature oxidation and hot corrosion resistance of titanium alloys due to its high thermal stability and better compatibility with the substrate. However, the corrosion resistance of enamel coating decreased with increasing temperature. The rates of chemical reactions between molten salts and enamel coating increased at 800oC. During the oxidation, some oxides formed at the interface of enamel coating/substrate.
The enamel frit and Al2O3 particles could form a dense enamel-Al2O3 composite coating after firing at 1000°C. The addition of Al2O3 particles effectively improved the high temperature performances of this composite coating, which exhibited excellent high temperature oxidation and hot corrosion resistance in molten sulfate salts. The protection mechanism of enamel-Al2O3 composite coating was different from those of traditional metallic coatings. It was no longer oxidized during tests and acted as a barrier to effectively hinder the corrosive species from migrating into substrate.
The enamel-metal composite coating exhibited very good thermal shock resistance at 1000°C, much better than solo enamel coating. The dispersion of ductile NiCoCrAlTaY particles into the enamel matrix effectively increased the thermal expansion coefficient (TEC) of this composite coating. Therefore, the enamel-metal composite coating had relatively good match of TEC with the underlying substrate, and the stress evolving within the composite coating during thermal shock tests could be remarkably reduced. The enamel-metal composite coating had very dense structure and high thermal stability, which made it possess good oxidation and hot corrosion resistance, better than the traditional metallic NiCoCrAlY coating.
TiAlN coating showed excellent compatibility with the titanium alloy. TiAlN coating could effectively hinder the accelerated corrosion induced by the solid NaCl deposit and water vapor at 450°C due to its high thermal stability. It could act as an effective protective coating to improve the corrosion resistance of Ti6Al4V alloy in such marine environment. However, the atmospheric oxidation of TiAlN coating occurred to form a thick and mixed oxide scale after 300 h corrosion.
It was found that substrate alloy had the effect on the corrosion behavior of TiAlN coating coated with solid NaCl in water vapor. At 600°C, the TiAlN coating deposited on 1Cr11Ni2W2MoV exhibited excellent resistance against corrosion incuced by solid NaCl and water vapor, and provided effective protection for substrate. However, when TiAlN coating was prepared on Ti6Al4V，it suffered serious corrosion and large spallation in the same conditions. The difference of thermodynamic characters between substrates caused such different corrosion behavior.
The corrosion resistance of TiAlN coating could be improved by pre-oxidation treatment. After pre-oxidation at 700°C for 1 h, the surface of TiAlN coating was covered by a mixed oxide scale with the thickness about 150nm. These oxides hindered the formation of medium products and retarded the diffusion of corrosive medium. Therefore, such pre-oxidation treatment remarkably improved the resistance against corrosion induced by solid NaCl and water vapor at 600°C.
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