AA C-6:1994

$18.20

Technical Report: Aluminum and Its Alloys

Published By	Publication Date	Number of Pages
AA	1994	30

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Description

THE OXIDE LAYER
To a student of chemistry it might seem strange that aluminum, one of the most chemically active elements with a very strong affinity for oxygen, could be such a commonly used metal. And yet one sees aluminum every day being used, unprotected, in every common environment. In fact, aluminum is second only to steel in tonnage produced and used each year.
What makes aluminum so usable is that oxygen, a deadly enemy and destroyer of other reactive metals such as sodium, and even of less reactive metals such as iron, is a friend and protector of aluminum. Possibly every chemistry student has witnessed in a laboratory the spectacular reaction between sodium and moisture, rendering sodium unrecognizable and even more unusable. Certainly everyone has seen a rusty steel surface. Neither of these conditions occurs with aluminum. Indeed, a freshly produced aluminum surface reacts instantly with oxygen, but once a thin oxide layer has formed, it very effectively protects the base metal, not only from attack by other agents, but also from further oxidation.
The oxide coating invariably found on aluminum is so important to its usefulness that one must first consider this coating in order to understand many of aluminum's properties, especially corrosion resistance. Among the properties of the natural oxide coating on aluminum are:
1. It is extremely thin – normally about 50 to 100 Å thick. Once formed, the film tends to prevent further oxidation of the metal. The thickness of the oxidation is increased to over one mil by anodizing for greater resistance to corrosion and abrasion.
2. It is tenacious. Unlike iron oxide, which tends to spall from the base metal leaving a fresh surface to oxidize, aluminum oxide tightly adheres to aluminum.
3. It is hard. Aluminum oxide is one of the hardest materials known.
4. It is relatively stable and chemically inert. As a rule, the oxide is stable over a pH range of about 4.5 to 8.5, but is dissolved by most strong acids and alkalis. However, some chemicals will attack the oxide near the neutral point (pH7), while it is resistant to concentrated nitric acid at pH1 and to ammonium hydroxide at pH13.
5. It is transparent and does not detract from the metal's appearance.
Since the natural oxide coating is so thin, it has a negligible effect on the properties of aluminum, other than providing corrosion and oxidation resistance.
PROPERTIES OF PURE ALUMINUM
Everyone knows that aluminum is lightweight. Its specific gravity is only 2.7 times that of water and is roughly one-third that of steel. An easy number to remember is that one cubic inch of aluminum weighs one-tenth of a pound; a cubic foot weighs 170 pounds compared to 62 pounds for water and 490 pounds for steel. Pure aluminum melts at 1220°F, which is considerably lower than the melting point of most other structural materials. It is a n excellent conductor of heat and electricity. On a volume basis, the electrical conductivity of pure aluminum is roughly 65% of the International Annealed Copper Standard, but pound-for-pound aluminum is a better conductor than copper, and is surpassed only by sodium.
As far as mechanical properties are concerned, pure aluminum is a relatively low strength but highly ductile metal. Its modulus of elasticity is approximately 10 × 10(6) psi compared to 30 × 10(6)psi for steel which means that, for a given size and shape and under comparable loading, aluminum will elastically deform three times more than steel and absorb three times more energy. Aluminum is very tough, and, unlike steel and titanium, aluminum and its alloys do not lose ductility and become brittle at cryogenic temperatures.
Aluminum, like most metals, can be strengthened by strain hardening or cold working to a significant extent. However, much greater strengthening is provided by alloying aluminum with a variety of elements. While commercially pure aluminum (99+% A1) is used for electrical conductors, chemical equipment and sheet metal work, aluminum alloys are much more widely used, especially where strength is an important consideration.