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'AL' the Carnegie Institute Overview:
Pure aluminium is easy to form but lacks mechanical strength.
Melting aluminium with small amounts of other elements such as
silicon, magnesium, copper, zinc, and nickel in various combinations
increases the tensile strength without adding weight.
Aluminium differs from steel by oxidizing
immediately in air, developing a thin film that seals the surface
from further chemical or environmental reactions. Similar in
crystalline structure to tin and gold, aluminium is very easy
to form even when cold. Its softness in combination with its
low melting temperature (aluminum:
660 degrees Celsius; silver: 961; iron: 1,535)
allows it to be quickly formed into a multitude of shapes and
thicknesses.
Scientists discovered early in aluminum's
history that it was very sonorous. When struck, a bar of aluminium
produces a high musical ring like a crystal bell. The sound matches the simultaneous
musical notes A sharp, and D flat.*
Aluminium is too reactive in its natural state
to exist as a free metal like gold or silver, but like iron and
tin, it is found as a compound. More than 270 different minerals
contain aluminium, but the ore bauxite contains the highest concentration
of aluminium. Alumina powder, a compound of aluminium and oxygen,
is dissolved in molten cryolite in large carbon-lined pots. Electrical
current is passed through the mixture. The oxygen reacts with
the carbon, forming carbon dioxide, while the aluminium settles
to the bottom and is siphoned off. The ability to combine aluminium
with small amounts of other elements-silicon, magnesium, copper,
zinc, and nickel-in various combinations, increases its strength
and overall performance Aluminum
develops a resilient natural oxide layer over time.
*Anodizing
is an electrochemical process that accelerates this change in surface.
Formed aluminium is first cleaned with acid. The object is immersed
in an [Sulfuric?] acid bath through which an electrical current is
passed to develop the anodic film on the metal's surface. The
acceptance of colored dyes is a byproduct of the process. The
anodic surface is porous and accepts dyes easily. A salt solution
bath is used to tighten the pores around the color making it
part of the surface.
The various processes of corrosion are affected
by several factors. Among these are the type of material selected
for the application, the heat treatment of the material, the
environment of the application, and the presence of any contaminants* in the material
itself. The problem with
stress corrosion cracking of the thicker components manufactured
from older, [6000,-?] series aluminium
alloys has been a pervasive hidden corrosion problem,
causing unanticipated major expenses and substantial downtime
for the US military. [Some relevant examples:]
Filiform corrosion: (Appearance- bi-product) High humidity around fasteners,
skin joints or breaks in coating cause an electrolytic process.
Fine, meandering, thread-like trenches that spread from the source.
Similar to scale. Lifting of the coating.
Galvanic Corrosion: Corrosive condition that results from contact of
different metals. Uniform damage, scale, surface fogging or tarnishing.
Emission of mostly molecular hydrogen gas in a diffused form.
Stress Corrosion: Cracking Mechanical tensile stresses combined with
chemical susceptibility Micro-macro-cracks located at shielded
or concealed areas. Initially produces scale- type indications.
Ultimately leads to cracking.
(NIST), the authors write:
"Evidence of a severe high temperature corrosion attack
on the steel, including oxidation and sulfidation with subsequent
inter granular melting, was readily visible in the near-surface
microstructure. ... No clear explanation for the source of the
sulfur has been identified. The rate of corrosion is also unknown."
[I contend this
final statement is categorically false!]
*Physicists, Please See: "Cyclical Degradation Report"
More data for us ordinary "Techno-geeks"
Aluminium nearing its melting point, becomes
"hot short" and crumbles easily. As a pure metal, it
is quite soft, and must be strengthened by alloying with Cu,
Mg, Si or Mn before it can be used structurally.
Anhydrous AlCl3 is a compound which easily
sublimates, showing that it is not ionic, and is partially hydrolyzed
by water to release HCl gas. It cannot be formed by heating the
hydrated form to drive off water. This only produces the oxide
and HCl gas. It is now made commercially by heating aluminium
oxide with carbon and chlorine. Aluminium should displace hydrogen
from water because of its positive oxidation potential, but does
not normally do so because of the protection by a surface layer
of oxide. This oxide has the same density as the metallic aluminium,
so it does not crack or wrinkle when it is formed, a lucky thing.
A little HCl or NaOH that dissolves the oxide will permit the
evolution of hydrogen.
The thermite
reaction, discovered by Goldschmidt, is
also a displacement reaction, but here aluminium reduces iron.
The reaction is Fe2O3 + 2Al ? 2Fe + Al2O3, which liberates a
good deal of heat. The liquid metal produced is at about 2300°C,
which is very hot. Powdered aluminium and rust in the approximate
ratio of 1:3 are packed in a refractory crucible with a magnesium
ribbon, or a powder of magnesium and barium peroxide, to ignite
it. Either the red or black iron oxide can be used, giving "red
Thermit" or "black Thermit." A trade name for
the powder is Thermit. The vigorous reaction makes liquid iron
or steel, which flows out of a hole in the bottom of the crucible
into the mold and can be used for welding. The stock to be welded
is usually preheated with a gas flame playing through the mold.
The metal produced is about half the weight of the original mixture.
This reaction is also called aluminothermic, and can be used
for reduction of other metals, such as nickel, manganese or chromium.
Closely related to alumina is the hydroxide,
Al(OH)3, usually formed as a gelatinous precipitate when aluminium
compounds are hydrolyzed in water. If water is driven out of
this precipitate by heating, a light, foamy solid results called
activated alumina that will absorb moisture and other things,
and can be reactivated by heating. This hydroxide reacts with
both acids and bases according to the formula H+ + AlO2- + H2O
= Al(OH)3 = Al+++ + 3OH-. Adding an acid removes OH-, driving
the reaction to the right, while adding a base removes H+, driving
the reaction to the left. Since it can go either way, aluminum
hydroxide is called amphoteric, and is an excellent example of
the type.
Aluminium is not a very colorful element.
It gives no coloration to the flame, and its compounds are relentlessly white.
In the U.S., electricity for aluminium reduction
has been heavily subsidized to make the industry viable. A famous
deposit in the United States was in Arkansas, and other deposits
were in Alabama and Georgia. Although the U.S. produces about
7,500,000 metric tons of aluminiium yearly, half primary and
half secondary (recovered from scrap), all of the bauxite is
now imported. Alumina can also be made from alunite, mentioned
above and found in the western U.S., but this source is not used.
Aluminium flakes can be used as a pigment
in paint. This makes an excellent anti-corrosion paint for iron
and other metals.
All these uses contributed to a great increase
in the use of aluminium even before the Second World War. Aluminium
was the preferred structural material for aircraft after wood
and fabric were superseded, and all sides in the war foresaw
the need for increased aluminium capacity. The United States
had a severe problem with bauxite supply that every means was
taken to solve. New reduction plants were quickly built, mostly
with government money, and were then leased to private companies
for operation. New producers, such as Kaiser Aluminium and Reynolds
Aluminium, joined the established Aluminium Corporation of America
(Alcoa)
to operate the government plants and profit by the war economy,
which they did most handsomely. The
aluminiium demand was met, and in 1943 metal was even released
for civilian uses, showing how successful the program had been.
http://www.du.edu/~jcalvert/phys/alumin.htm
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