The reaction of fluoride ions with alumina was found to strongly depend on the concentration of fluoride ions in the aqueous solution. At low concentrations ([fluoride ions] < 0.1 mol/l in the case of potassium fluoride), the aqueous concentration of aluminum ions is relatively high as measured by using inductively coupled plasma optical emission spectroscopy (ICP-OES), and the aluminum oxide dissolves as a fluoride complex. At high concentrations of fluoride ([fluoride ions] > 0.5 mol/l in the case of potassium fluoride), a new structure is formed on the alumina surface involving fluoride, aluminum, potassium, and oxygen (in the case of potassium fluoride). The structure was characterized by using X-ray powder diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS). The resulting structure improved the passivation of alumina, the solubility of aluminum ions decreasing compared to the untreated alumina. Aluminum surfaces that were fluoride-treated showed a better resistance to dissolution in both acidic and basic media.

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