The Rate and Mechanism of Dissolution of Purest Aluminum in Hydrofluoric Acid

Abstract

Pure aluminum (99.99%) dissolves in hydrofluoric acid with a rate comparable to that of titanium in the same acid. The rate of dissolution, (mm3/cm2 min of hydrogen), in 0.1–6N hydrofluoric acid is in agreement with the empirical equation , and for . Recrystallized aluminum showed the same rates within the error limits (±14%). Of the noble metal salts added to the acid, only nickel and gold increased the dissolution rate; deposits obtained from other salt solutions did not adhere to the surface of the dissolving aluminum. Ammonium fluoride and ammonium chloride additions to the acid decreased the dissolution rate (passivation) because a salt film was formed on the aluminum surface. The potential of aluminum in 2N hydrofluoric acid was −1.22 v, indicating that the extent or thickness of protective surface films was negligible. The dissolution rate of aluminum in sulfuric acid, phosphoric acid, and other acids can be increased considerably by addition of water‐soluble fluorides. The behavior of aluminum in hydrofluoric acid, as well as in hydrochloric acid, could be explained qualitatively using the concept of local currents in a manner similar to that proposed earlier.