Effect of Pre-oxidation on High-Temperature Chlorine-induced Corrosion Properties of Air Plasma-Sprayed Ni-5%Al Coatings

Plasma-sprayed Ni-based alloy coatings can be used to mitigate the high-temperature chlorine-induced corrosion of the superheater tubes of boilers. In this study, Ni-5%Al coatings were deposited by air plasma spray on the surface of 304 stainless steel substrates; the coating was pre-oxidized at 700 °C for 6 h in air. The effect of pre-oxidation on the high-temperature chlorine-induced corrosion performance of the Ni-Al coated samples was studied via comparative tests on the coating phase structure and morphology. The results showed that the microstructure of the as-sprayed coatings presented some defects. After pre-oxidation, the bonding strength of the coatings was nearly four times that of the as-sprayed coatings; the coatings formed multilayer of nickel oxide (NiO) on the surface and aluminum was oxidized within the coatings. Also, the porosity of the coatings was reduced from about 5 to 4%. After corrosion, the weight gain of the pre-oxidized coatings was lower than that of the as-sprayed Ni-5% Al coatings. The highest weight gain of the pre-oxidized coatings was reduced by 36.3% compared with that of the as-sprayed coating. This study showed that the pre-oxidation treatment improved the corrosion resistance and the bonding strength of the coatings.