Abstract
Aluminum foil specimens were immersed in neutral phosphate solutions with Can=10-4-10-1 M and doulbly distilled water (DDW) for 60 min at 99.5°C, and the formation behavior of hydroxide films on the surface was examined by gravimetry, chemical analysis, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and impedance measurements. In DDW, two-layer hydroxide films were observed with film formation efficiency, η, of 1.0, and in phosphate solutions, thin single-layer hydroxide films were observed with η=0.7-0.9. At Can=10-2 kmol/m3, the film thickness showed a minimum and η a maximum. The XPS results showed that phosphates are incorporated in the outer part of the films formed in phosphate solutions, and that the phosphate incorporation increases with Can. For all the films impedance measurements suggested the existence of an oxide layer with ca. 2.5 nm thickness at the interface between the hydroxide film and the metal substrate. The role of phosphate ions in the formation of hydroxide films is discussed in terms of dissolution and stabilization of the film with aluminum-phosphate complexes.
