This paper examines the in-flight oxidation of molten aluminum sprayed in air using the twin-wire electric arc (TWEA) thermal spray process. The oxidation reaction of aluminum in air is highly exothermic and is represented by a heat generation term in the energy balance. Aerodynamic shear at the droplet surface: (1) enhances the amount of in-flight oxidation by promoting entrainment and mixing of the surface oxides within the droplet, and (2) causes a continuous heat generation effect due to the exothermic oxidation reaction that sustains droplet temperature as compared to a droplet without internal circulation. This continual source of heat input keeps the droplets in a liquid state during flight. A linear rate law based on the Mott-Cabrera theory was used to estimate the growth of the surface oxide layer formed during droplet flight. An explanation is provided for the elevated, nearly constant surface temperature (~ 2000 °C) of the droplets during flight to the substrate and it is shown that the majority of oxide content in the coating is produced during flight, rather than after deposition. The calculated oxide volume fraction of an average droplet at impact agrees well with the experimentally determined oxide content for a typical TWEA-sprayed aluminum coating, which ranges from 3.3 to 12.7%.

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