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Fe-based amorphous coatings are increasingly recognised as promising candidates for the protection of coal-fired boilers against corrosion. The present study prepared Fe-based amorphous coatings on a T91 substrate by plasma spraying technology. The corrosion behaviour of the coating in hot Na 2 SO 4 + K 2 SO 4 salts at 700°C was investigated, and measurements of the mean mass gain were performed after each cycle to establish the hot corrosion kinetics of the coatings using the thermogravimetric technique. The coated specimens, especially specimens with 380-μm-thick coatings, exhibited lower mean gain rates at all operating cycles as compared to the uncoated T91 samples. The highest hot corrosion resistance was a result of the amorphous composite microstructure and high Cr and Ni elemental contents, which contributed to the formation of the protective oxides of chromium and nickel such as Cr 2 O 3 , NiO and NiCr 2 O 4 .

Yttria-stabilized zirconia (YSZ) deposits were prepared by an atmospheric plasma spray (APS) on a stainless-steel substrate preheated to different temperatures from room temperature to 11001C. The microstructure of the deposits was characterized from polished and fractured cross sections by scanning electron microscopy (SEM). The ionic conductivity of the deposits was measured using both DC and AC methods, and the relationship between ionic conductivity and microstructure of deposits was examined. SEM observation revealed that the YSZ deposits exhibit different microstructures with different deposition temperatures. With the increase of the deposition temperature, the columnar grain growth across lamellar interfaces was enhanced and subsequently the intersplat bonding ratio in the deposit was improved. The ionic conductivity of YSZ deposits at the direction perpendicular to the deposit surface was significantly increased through the microstructure development on increasing the deposition temperature. However, it was found that the intergrain resistance changed little despite a remarkable change of the deposition temperature. A microstructure model is proposed to correlate the relative conductivity to the mean lamellar interface bonding ratio. The increase in ionic conductivity of the YSZ deposit with the deposition temperature can be attributed to the increase in the lamellar interface bonding ratio.