In this article we will discuss about:- 1. Production of Aluminium 2. Properties of Aluminium 3. Anodic Coating Treatment 4. Electro-Brightening or Electro-Polishing 5. Uses and Engineering Applications.
Production of Aluminium:
First stage in the production of aluminium is the production of alumina by chemical refinement of bauxite. Aluminium is then produced by the electrolysis of alumina dissolved in a bath of molten cryolite. It is available in the market as wrought and cast products in the form of ingots or notched bars for remelting. It is possible to obtain over 99.97% pure aluminium commercially.
Properties of Aluminium:
i. Physical Properties:
Aluminium is a silver white metal. Its outstanding properties are lightness (its density is one-third that of iron) good electrical and thermal conductivity. It is a good reflector of light and a good radiator of energy. It is non-magnetic.
It is resistant to atmospheric attack. Oxide film that is set up upon exposure to air insulates it against continued attack. It has good tensile strength in the form of alloys. Due to its ductility it can be easily worked. By itself it is very weak and ductile and melts at 660°C. It’s tensile strength is 600 kg/cm2 but it can be increased by cold working.
Aluminium is generally 99.9% pure as obtained by Hall-Heroult process and impurities of iron and silicon present from alloy with aluminium. Pure aluminium is silvery white in colour but commercial aluminium due to impurities has got a bluish tinge. It is used mainly as the base of reasonably strong and light alloys.
ii. Chemical Properties:
It is resistant to atmosphere due to the formation of a protective oxide film. This oxide film is very thin, less than 0.02 micron (urn’ is thickness but is impervious and highly protective. On heating, this film increases in thickness.
Heat of combination of aluminium with oxygen is very high. Finely divided powered aluminium burns in air.
Anodic Coating Treatment of Aluminium:
By anode treatment of aluminium oxide, film of sufficient thickness resistant to abrasion can be formed in certain electrolytes which are acidic in character. Anodic coatings are amorphous in nature and they have got firm adherence to the metal surface.
Electrolyte used generally is 15% sulphuric acid. Thickness of oxide film is between 0.002 to 0.02 mm depending upon the quantity of current employed. Chromic acid and oxalic acid are also used a electrolyte.
As the anode coating is minutely porous, it should be made non-absorptive to prevent staining etc. The scaling of pores is done by dipping the anodised surface in hot water. Such sealed surfaces are not stained by coffee or other coloured liquids.
Anodised surfaces may be impregnated with corrosion inhibitor by treating with chromate solutions. Chromate gets absorbed in coating and protects the coating from any corrosive attack. Coatings which are sealed with chromate solutions form an excellent paint base and are used very much for protecting aircraft.
Anodic coatings can be coloured by impregnation with organic dyes and with mineral pigments. Coatings coloured by organic dyes are permanent indoors but not outdoors in sunlight because they fade in sunlight. Coatings which are impregnated with mineral pigments do not fade even in sunlight.
Oxide coatings can also be formed on aluminium by chemical treatment. Such coatings are inferior in thickness, hardness and abrasion resistance as compared to anodic coating. A hot solution of sodium carbonate and potassium or sodium dichromate will produce a greyish green oxide coating.
Electro-Brightening or Electro-Polishing of Aluminium:
Brightening is carried out by anodic treatment in fluoboric acid as electrolyte. Such brightened surface has as high reflectivity as 90%. Such electro brightened surfaces can be protected by anode oxide coating.
Aluminium can be electroplated with other metals by electrolysis. The surface is first dipped in sodium zincate solution.
Uses and Engineering Applications of Aluminium:
1. Chemical and Food Industries:
Aluminium is resistant to many mineral and organic acids, salt solutions, organic compounds, sulphur and many other substances. Aluminium is available in different fabricated forms and it can be assembled and finished by different processes.
Due to all these reasons it is used for fabricating equipment for chemical and food processing industries. For these very reasons it is used for making cooking utensils. Cookers and steam jacketed kettles, etc. are produced from aluminium.
2. Metallurgical Industry:
Aluminium is used in the metallurgy of iron and steel as it is a powerful deoxidizer and reduces the dissolved and combined oxygen content of molten steel.
Metallic aluminium is also used to reduce oxides of metals such as iron, chromium, vanadium and molybdenum.
Aluminium is a fine alloying metal in ferrous metallurgy, in steel for nitriding and in iron alloys where certain electrical, magnetic and oxidation resistant properties are desired.
3. Structural Applications:
Due to its light weight and high tensile strength it is used for the construction of aeroplanes, buses, tracks, trains and ships.
Resistance of aluminium to weather makes it possible to use aluminium in architecture for constructing such parts as roofing, sheathing, windows, etc. Star rails and furniture, etc. are also made out of aluminium.
4. Electrical Industry:
It is used in the manufacture of cables. In cables, steel wire core is surrounded by aluminium conductors that carry the current. The strength of the core and the light weight of the cable permit long spans.
Aluminium bus bars are used for the distribution of power in and around factories.
Induction motors are produced with cast aluminium windings. The rotor is made with laminated steel core and the aluminium winding is cast in slots-extending through the laminations.
Aluminium conductors are used in the rotors of high speed turbine generators.
5. Brewery Industry:
In brewery industry, aluminium is used extensively as it does not react chemically with the beverage and also does not alter the taste of the beverage. For example, beer is produced, stored and is shipped in aluminium tanks.
6. Cryogenic Applications:
Cryogenic means science of low temperature. In modern times there is a great worth of missile and rocket industry. Cryogenic applications have close connection with missiles, rockets and space navigation. Aluminium alloys have the unusual property of remaining ductile and resistant to stock loading at extremely low temperatures.
As the temperature decreases, their tensile and yield strengths improve. So they have got many advantages when used as a material of construction for cryogenic equipment. For example, aluminium is used in liquid fuelled missiles and rockets as storage material for liquid oxygen and forms the integral part of the missile or rocket.
7. Process Equipment:
Process equipment in plastics, rubber, rayons, synthetic-resins and petroleum industry are usually made of aluminium.
IS : 617 deals with the specifications, designation, characteristic, and uses of aluminium and aluminium alloy ingots and castings for general engineering purposes, and the similar information for wrought aluminium and its alloys is given in IS : 737.
According to IS : 617, all aluminium and its alloys ingots are designated by letter A’ followed by numbers from 0 to 24 depending on the variety of ingot; for example number ‘0’ is for aluminium ingots of 99% aluminium used for re-melting, ‘1’ for aluminium alloy ingots, ‘2’ for aluminium alloy ingot mainly used for pressure die casting, ‘4’ for sand casting ‘5’ for gravity casting, ‘6’ for special die casting, etc.
Aluminium castings are designated in same way with a last letter M, P, W, or WP depending on the method of casting and treatment; letter M being used for aluminium alloy castings, letter P if casting is precipitation treated, W if solution treated. WP if fully heat treated.
According to IS : 836, wrought aluminium is designated by letter I, followed by letters A, B and C as there are three grades A, B and C. Aluminium alloys for normal duty are designated by letter N followed with number 2 to 8, and medium strength aluminium alloys by letter H followed by numbers 9, 11, 12, 14, 15, 18, 19, 20 and 30.