Saturday, January 24, 2009

Anodizing Process

Anodic oxidation or anodizing, is the electrochemical method of providing an artificially thickened oxide film on aluminum alloys. It is carried out by making the aluminum alloy anodic (positive) in a solution of chromic, sulfuric, oxalic or less commonly, phosphoric acid. In any of these solutions the effect of the current very soon insulate the metal and stop the current is to convert the surface of the metal to a compact adherent oxide film. This would very soon insulate the metal and stop the current. If the electrolyte did not simultaneously attack the oxide and render it porous. As a result the current can continue to pass and the film can continue to grow in thickness, to the point where the speed of dissolution equals the speed of formation. Chromic acid anodizing was the first of these processes to be developed. A 3% solution is used at 50 oC and the voltage increased gradually up to 50 V. After a few minutes at this voltage the growing resistance of the film practically stops the current. The resulting oxide film is dark and greenish in color. It has considerable protective systems. Chromic acid anodizing can be applied to the strong copper containing alloys such as are used in aircraft construction. Residues of the electrolyte trapped in joint, etc. are not harmful and are inhibitory to corrosion. For these reason the chromic acid anodizing process is very important in the vital protection of aircraft structures and components.

In solutions of sulfuric acid, preferably of about 20% concentration, anodizing can be carried out at room temperature, indeed the temperature must not be allowed to rise above 20 oC or the film redissolved. A current density of 15 to 20 A/ft2 is used which requires 16 to 18 V. under these conditions the film continuous to grow in thickness and 0.001 in is formed in about an hour. On super purity aluminum, or its alloys with magnesium or zinc, the film is completely transparent. Other alloying elements and impurities, especially iron, render the film milky. Copper containing alloys cannot be treated. In 8% oxalic acid solution the process is similar, but a voltage of 50 to 60 V is necessary. The film produced in sulfuric or oxalic acids are porous when first formed, at this stage they are very receptive of dyes from aqueous solutions, or inorganic pigments can be precipitated in the pores. The porosity is subsequently sealed by the relatively simple process of immersing the anodized parts in pure water at 96 to 100 oC for 20 to 30 minutes.

In this way coatings which enhance and protect the silvery luster of the metal, or provide strongly colored metallic effects are produced. These are used for a multitude of consumer goods, but also for the protection and decoration of aluminum parts on the outside of buildings. The film produced in sulfuric acid is very hard. The throwing power of all anodizing process is excellent. The thickness of the anodic film is similar all over the article, regardless of its shape, even inside quite tubes.

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