Oxidation Behavior of Molten Tin Doped with Phosphorus

The oxidation process of molten tin in air at 280°C was studied. We found that a trace addition of phosphorus to the tin reduced the surface oxidation greatly by forming a protective film. The total thickness of the oxide film formed on the molten Sn-0.007wt.%P alloy was about 36 nm, which was composed of a layer of 6 nm SnO₂, 10–15 nm (Sn, P)O, and a transition layer. This oxide film was approximately a quarter of the thickness that formed on pure tin. The oxidized surfaces of different tin alloys were studied by scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Much higher segregation of phosphorus was observed on the subsurface of the oxide film, and the concentration of phosphorus in the oxide film was about 500 times greater than that of the bulk concentration. Based on this result, the segregation of phosphorus on the molten surface could result in the formation of a new protective (Sn, P)O film on the subsurface of the molten tin. It is also suggested that the crystal structure of the oxide film should be studied in the future to confirm the mechanism.