Resistance improvement of aluminum surface to corrosion through reactions with fluoride ions

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.