| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 99 === The purpose of this study is to investigate the nickel-plating effects under various Ni-Al and Fe-Al layers on phase transformation, corrosion kinetics, and thermal-cycling oxidation behavior of hot-dipping low-carbon steel at 750 ℃ in dry air with and without NaCl deposits. The results will compare to those of the same steel directly hot-dipped Al (without Ni-plating), and discuss the possible reasons to form AlN in the substrate after air oxidation.
The results showed that the Ni-Al layers formed from top coating to the substrate consisted of NiAl3, Ni2Al3, NiAl, Ni3Al, and pure-nickel layers. The NiAl3, layer was transferred into the Ni2Al3 layer after an initial inter-diffusion reason about 20 min, while most the Ni2Al3 phase was transferred into the NiAl phase after a 120-hr exposure. Due to the lower activation energies to grow both NiAl and Ni3Al phases, their growth rates were much faster than those of other Ni-Al phases. It was found that the steel with nickel-plating treatment exhibited an excellent oxidation and thermal-cycling resistance with respect to that of the free nickel-plating steel.
In addition, the surface of the nickel-plating steel became mostly NiAl phase after hot corrosion at 750 ℃ when mostly Al was preferential consumed under a reaction with NaCl. Besides, the AlN precipitates formed on the Ni-plating steel after the oxidation in dry air. Very likely, the observed AlN was due mostly to reducing the nitrogen solubility in the substrate when the Fe2Al5 phase transferred into FeAl, which in turn caused inward diffusion of nitrogen into the substrate and reacted with Al to form AlN.
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