The Basics of Aluminum Corrosion
Discovered in 1827 by Friedrich Wöhler, aluminum, though the most common metal on earth, is always found tightly locked in compounds. Efforts to use electrolysis to reduce it failed repeatedly, and for years it remained an exotic metal used in jewelry and for such special purposes as capping the Washington Monument. The race for a commercially viable route to aluminum was won in 1886 by two young men working independently:
Paul Héroult (1863–1914) in France
Charles M. Hall (1863–1914) in the United States
Hall was just six months out of Oberlin College; his sister Julia, who had also been a chemistry major at Oberlin, was of great assistance to him, helping with experiments, taking laboratory notes, and giving business advice.
Aluminum is the second most plentiful metallic element on earth. It has been estimated that 8% of the earth crust is composed of aluminum, usually found in the oxide form known as bauxite. Aluminum has become the most widely used non-ferrous metal on a volume basis. Whilst more expensive on a tonnage basis, it is the least expensive of metals other than steel on the basis of volume or area. Pure aluminum is used mostly as cladding material on aluminum alloys or as protecting coating for other metals.
How Corrosion Resistant is Aluminum?
High purity aluminum wires and ribbons are produced in a broad range of sizes under tightly controlled purity refining standards. Aluminum 1199 (99.99% min. purity), as well as 99.999% and 99.9999% aluminum are widely in specialty electronic and chemical applications. Applications include vacuum deposition of thin films and coatings used in the manufacturing of electronic devices, integrated circuits and optical products. Aluminum 1199 has excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties and good workability.
Information collected over the years from manufacturers and users has shown that aluminum structures will provide reliable service for periods in excess of 30 years. The factor which assures the long life of aluminum is its self forming microscopically thin surface layer of aluminum oxide. This layer is so thin that it is measured in atomic units. The air-formed film on new aluminum surfaces is about 2.5 nm thick, while the film on aluminum that is several years old may be 10 or more nm thick. The film is composed of two parts:
A thin, inner barrier layer.
And a much thicker bulk outer layer which is more permeable than the inner barrier layer.
Chemically, the film is a hydrated form of aluminum oxide. The corrosion resistance of aluminum depends upon this protective oxide film. which is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired. The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it.
The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode but by no means always does so. For example, aluminum is quite resistant to concentrated nitric acid. When aluminum is exposed to alkaline conditions corrosion may occur, and when the oxide film is perforated locally, accelerated attack occurs because aluminum is attacked more rapidly than its oxide under alkaline conditions. The result is pitting. In acidic conditions, the oxide is more rapidly attacked than aluminum, and more general attack should result.