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 ([F-]<0.1M in the case of KF), the aqueous concentration of aluminum ions is relatively high (ICP measurements), and the aluminum oxide dissolves as a fluoride complex.
At high concentrations of fluoride ([F-]>0.5M in the case of KF), a new structure is formed on the alumina surface involving fluoride, aluminum, potassium ions (in the case of KF) and oxygen. The structure is characterized by XRD, SEM and EDS.
The resulting structure improved the passivation of alumina, the solubility of aluminum ions decreasing compared to the “untreated” alumina.
Aluminum surfaces “fluoride treated” showed a better resistance to dissolution in both acidic and basic media