Anodizing Aluminum in Oxalic Acid Solution
For the anodizing of aluminum and its alloys. Solutions of oxalic acid containing 2 - 10 percent of the acid are used, sometimes with additions of other chemicals such as succinic acid, potassium permanganate, chromic acid etc. For anodizing conditions are: anodic current density, 10 - 20 ampere per sq. ft., solution temperature, 28 - 32 oC; bath voltage 40 - 60 volts; and anodizing time 30 - 40 minutes.
Anodizing of aluminum in oxalic acid produces colored film depending on the anodizing conditions and the thickness of the anodic oxide film. The table below gives the anodizing conditions suitable for producing films of there different colors.
Anodizing Conditions for Obtaining Various Colors.
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Color of Film Temperature Anodic Current Voltage Time of
Of Bath (oC) Density (A/Sq.Ft) (volts) Anodizing (min)
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Silvery 25 15 - 20 35 - 40 35 - 40
Brass 40 30 50 35 - 40
Bronze 50 40 - 50 50 35 - 40
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The color variation of film with the thickness of the film is as follows: White – 5 micron; yellow – 15 micron; golden yellow – 50 micron; and brownish yellow – 100 micron. Addition of a small amount of succinic acid bring about an improvement in the dielectric properties of the oxide films obtained.
A solution consisting of 4 - 5 percent oxalic acid, and 0.1 percent chromic acid is used for anodizing with alternating current. The anodizing conditions are: current density, 15 - 45 amps/sq.ft.: solution temperature, 25 - 30 oC; bath voltage, 40 - 60 volts; and anodizing time 30 -40 minutes. Because of the high electrical resistance of the film, the passage of the current during anodizing is accompanied by an appreciable heating up of the metal and of the solution. It is therefore, particularly important to watch out for attack on the film by solution and to counteract it by providing for the cooling of the parts and of the solutions.
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