Microstructure formation and corrosion behaviour in HVOF-sprayed Inconel 625 coatings
D. Zhang, S.J. Harris, D.G. McCartney
Materials Science and Engineering
Abstract
The nickel-based alloy Inconel 625 was thermally sprayed by two different variants of the high velocity oxy-fuel process. In this study, coatings deposited by a liquid-fuelled gun were compared with those produced by a gas-fuelled system; in general, the former generates higher particle velocities but lower particle temperatures. In vestigations into the microstructural e volution of the coatings, using scanning electron microscopy and X-ray diffraction, are presented along with r...Moreesults on their aqueous corrosion beha viour,obtained from salt spray and potentiodynamic tests. It is inferred from coating microstructures that, during spraying, powder particles generally comprised three separate zones as follows: fully melted regions; partially melted zones; and an unmelted core.However, the relati ve proportions formed in an indi vidual powder particle depended on its size, trajectory through the gun, the gas dynamics (velocity/temperature) of the thermal spray gun and the type of gun employed. Cr2O3was the principal oxide phase formed during spraying and the quantity appeared to be directly related to the degree to which particles were melted. The salt spray
test provides a sensitive means of determining the presence of interconnected porosity in coatings and those produced with the liquid-fuelled gun exhibited reduced interconnected porosity and increased corrosion resistance compared with deposits obtained from the gas-fuelled system. In addition, potentiodynamic tests re vealed that passi ve current densities are 10 /20 times lower in liquid-fuel coatings than in those sprayed with the gas-fuelled gun.
keywords:Corrosion; Coatings; Ni-based alloy; Microstructure formation; Thermal spraying
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