The rate of dissolution of aluminum in hydrofluoric acid

Pure aluminum has a relatively low strength, and its use is, therefore, rather limited. However, the resistance of pure aluminum to the attack of acids and many neutral solutions is higher than that of less pure aluminum or most of the aluminum-base alloys. For this reason, alclad alloy sheets, are made with a coating of pure aluminum on one or both sides of an aluminum alloy core. The coating is metallurgically bonded to the core over the entire area of contact. Thus the coating electrochemically protects the core from corrosive attack. Aluminum is more negative in the electrochemical series than most other common metals, hence its behavior in corrosive environments is greatly influenced by contact with many other metals which either form an alloy with aluminum or are in external contact with it. The resistance of pure aluminum against atmospheric attack, and also to the destructive action of some acids is explained by the presence of a surface oxide film that forms on aluminum and its alloys upon exposure to the atmosphere. The film, although thin, is adherent, highly protective, and resists corrosive attack under most conditions* A thicker and more protective film can be formed by a chemical or electrochemical treatment. This film is usually invisible to the unaided eye. The corrosion of aluminum in neutral, or nearly neutral solutions is accompanied by the formation of hydrated aluminum oxide, which usually sticks to the surface of the metal, and then protects it from further attack. For this reason, the attack by some solutions may be relatively rapid at first, but as soon as the insoluble products of the reaction adhering to the surface are formed, a continuous film covers the metal reducing the contact of the solution with the underlying metal. As a result, the corrosion stops or is reduced to a very low rate. In solutions which tend to dissolve the existing oxide coating or in solutions which tend to produce highly soluble corrosion products, the attack would be expected to be relatively greater than what it would be in solutions in which the film is spontaneously healed. Therefore, in many solutions the corrosion rate will be partially controlled by the solubility of the corrosion products. Extensive research has been carried out on the rate of dissolution of aluminum in various acids and bases. Many papers have been published regarding the mechanism of attack on aluminum by aqueous solutions. However, no work has been done on the mechanism and rate of dissolution of aluminum in hydrofluoric acid. With this purpose in mind, the present study --Introduction, pages 1-3