Aluminum Properties and Corrosion Resistance
Aluminum Properties and Corrosion Resistance
Aluminum alloys are used as a material of construction because of their many desirable qualities
1. High strength-to-weight ratio
2. Ready availability at a reasonable cost
3. Good corrosion resistance in many environments
4. High thermal and electrical conductivities
5. Readily formed and joined by conventional fabrication
processes
6. Amenable to surface treatments that enhance
corrosion resistance and impart colors
7. Nontoxic, wtilte or colorless, and nonstaining salts
8. A wide range of properties including high strength
achievable by ~loying, work hardening, and thermal
treatment
9. Application as a surface coating to other metals by
a variety of processes.
The commonly used designation for wrought aluminum alloysis a four-digit number. The first digit indicates the principal alloying element.
1XXX Commercially pure aluminum
2XXX Copper
3XXX Manganese
4XXX Silicon
5XXX Magnesium
6XXX Magnesium-silicon
7XXX Zinc
The other three digits have no specific relationship to alloying elements but do relate to an alloy series. For example, alloy 7175 is the first definable change in the 7075 alloy series. But the final two digits, 75, do not relate to specific alloying components in that series.
A three-digit designation is used for casting alloys. The major alloying element is indicated by the first digit.
Prefix letters are used to distinguish between compositions that differ in impurity or secondary alloying element contents. For example, A356 contains 0.2% iron maximum, whereas 356 does not contain iron.
Aluminum powders and particles are identified descriptively. However, aluminum-aluminum oxide alloys made from aluminum are distinguished as XAP or SAP alloys followed by a threedigit number.
The XAP alloys are made from flake powder, and the SAP alloys are made from granules. The threedigit numbers are not related to a composition code.
Prealloyed powders are used to make powder metallurgy products that cannot be made by conventional means or that do not develop the desired microstructure because of incompatibility of the alloying agents with aluminum.
These powders are
iden@ed descriptively or by a proprietary designation.
The wrought and cast aluminum alloy designations are
followed by an indication of the condition or temper of
the alloy.
Aluminum Corrosion Resistance
Although aluminum is an active metal, as indicated in the electromotive force series, it is resistant to corrosion in many environments.
The comosion resistance of aluminum results from the formation of a passive oxide film, which is 0.005-to 0.010-microm thick in air.
A thin protective film is also formed in water at ambient temperatures. As temperature increases, the film becomes thicker and more protective.
However, the protective fdm does not form in water or steam above approximately 230° C (446° F).
Aluminum is an amphoteric metal, i.e., it corrodes under both acid and alkaline conditions.
Exceptions are acetic acid and sodium disilicate. Other exceptions are ammonium hydroxide above about 30% concentration by weight, nitric acid above 80% concentration by weight, and sulfuric acid of 98 to 100% concentration by weight.
The resistance of aluminum alloys to general corrosion
is impaired by the presence of about 0.25% copper as an
alloying element. In moisture the copper forms ions that
the aluminum reduces.
During this process, aluminum is
oxidized and metallic copper plates out on the alloy
surface and establishes a galvanic cell.
Acid waters containing chlorides are especially corrosive to aluminum. Although sulfate-containing waters of low pH are also corrosive to aluminum, they are less corrosive than chloride-containing acid waters.
Aluminum and its alloys are resistant to attack by most
organic chemicals, but some organic chemicals will react
with aluminum if they are water free and at elevated
temperatures, usually near their boiling points. Also some
halogenated organic compounds will react with aluminum
at elevated temperatures near their boiiing points.
Some halogenated organic refrigerants will react with aluminum if sufficient water is present to cause the hydrolysis necessary to form hydrochloric acid.
Many cases of corrosion in the presence of organic chemicals can be traced to the presence of heavy metal contaminants such as copper, lead, nickel, and mercury.
Contaminants are more likely to be present in commercial grade products than in laboratory reagent-grade chemicals.
Also contaminants are more often found in used chemicals than in fresh batches.
see also:
ALUSelect: a computer-based reference database containing technical information on the most widely used aluminium alloys. This freely accessible web-site allows the user to obtain information about the mechanical, physical and chemical properties of aluminium alloys. At present, 35 wrought alloys and 12 casting alloys are indexed.
AluMatter: aluMATTER is a freely-accessible, award-winning website that provides innovative and interactive e-learning resources for aluminium science and technology.
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