Aluminum and its alloys owe their excellent resistance to corrosive by the atmosphere, waters, foodstuffs, etc. to an oxide film on the surface of the metal, which is very rapidly reformed when the naked metal is exposed by cutting or deformation. This film is very protective and relatively pore free, but it is very thin about one millionth of an inch. It is therefore logical to devise means of artificially thickening it to provide even better resistance to corrosion. There are two main type of methods for doing this, one of which is purely chemical, the other electrochemical.
MBV Treatment
The modified Bauer Vogel process, known as the MBV process, consists of immersing the aluminum alloy parts for10 to 30 m is an almost boiling aqueous solution of 5% sodium carbonate and 1% sodium chromate. The oxide film so formed is light grey to almost black in color, depending on the aluminum alloy, but is smooth and glossy; it is only a few hundred thousandths of an inch thick. Nevertheles it has considerable corrosion resistance, and is absorbent to oil or paint so that it is an excellent preparatory process for painting aluminum alloys.
Chromium Process
Another chemical process for increasing the thickness of the oxide film employs an acid solution containing phosphoric and chromic acids and sodium floride. It is used at more moderate temperatures, (40 to 50oC) and for shorter times than the MBV process. The coating is green, brown or golden in color, quite thin and rather soft. It is principally employed as a preparatory process for painting aluminum alloys. Owing to the chromic acid content it should not be used on articles intended for use with foodstuff. This type of process is mostly used used proprietary names, such as alodine in the USA, and in GB Alocrom.
Previous Treatment: Polishing
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