| This work focuses on surface analysis of the phosphate thin films formed on pure aluminum using surface-analytical techniques such as SEM/EDX, XPS and AES, and investigation of the effect of fluoride ion on the phosphating of aluminum.;The thin films formed on aluminum are principally composed of phosphorus, oxygen, zinc and a small amount of manganese. The phosphating of aluminum consists of at least three series steps: (1) spontaneous deposition of zinc on fresh aluminum surface within initial 30 second immersion; (2) transformation of the deposited zinc into zinc phosphate crystals, along with the rapid growth of individual crystals within 2 minutes; (3) growth of the crystals to a uniform and complete zinc phosphate thin film after 5 minute immersion.;Electrochemical investigation leads to understanding of key roles of fluoride ion in the phosphating of aluminum: (1) fluoride ion is able to rapidly remove the native aluminum oxide film, and in turn activate the aluminum sample within the initial 15 second immersion; (2) the presence of fluoride ions substantially enhances the driving force of the substitution reaction of zinc for aluminum; (3) fluoride ion is capable of significantly reducing the resistance of aluminum dissolution, thus accelerating the active dissolution of aluminum and increasing the pH value at the surface; (4) free Al;Data suggest that both uncoated and phosphated aluminum exhibit high corrosion resistivity in neutral aqueous solution. Phosphated aluminum repassivates faster than uncoated aluminum. In acidic solution, phosphated aluminum has an improved anticorrosion performance over uncoated aluminum with general corrosion characteristics. Both uncoated and phosphated aluminum undergo severe localized corrosion in Cl... |
