electrochemical oxidation, the formation of protective oxide film on the surface of metal products by electrolysis. In the anodic product is immersed in the electrolyte, combined with a positively charged electrode current source (the anode). The film thickness of 1 to 200 microns protects metal from corrosion, has insulating properties and is a good base for paint. Anodizing is used for decorative articles made of aluminum and its alloys, and also used for corrosion protection of magnesium alloys, improving the antifriction properties of titanium alloys for coating parts of electronic equipment of niobium, tantalum, etc., in aircraft, missile and instrument, Electronics Engineers.
Immediately after machining aluminum reacts with oxygen in the air, so under normal conditions, the surface is always covered by a thin oxide film. The structure of the film and its composition depends on the atmospheric effects. But the aluminum oxide film always has a thickness of 2-3 nm. This film protects the metal from further oxidation and has excellent electrical conductivity. The oxide film formed on pure aluminum at room temperature and has an amorphous structure (not crystalline) and is therefore not a good corrosion protection.
Protection against corrosion of aluminum is realized through the establishment of a crystal on its surface oxide film thickness of 20-30 microns. At the next stage of the process of anodizing, this film may be colored or may retain the natural color.
Anodizing of aluminum can also get a variety of decorative effects, such as mirror, matte and semi-matte surface, imitation of polished and brushed stainless steel.
Before you begin the process of anodizing aluminum surface must be cleaned of contaminants and to remove the oxide film. To do this, spend degreasing and etching processes.
The process by which, are formed on the surface of highly porous metal oxide layers of aluminum. The process of anodizing is an electrochemical.
There are two types of oxide films, which are formed in the anodizing process - and the porous barrier.
Barrier - oxide film grows in neutral solutions in which aluminum oxide is difficult to dissolve. Mostly this is borate, ammonium phosphate or tartrate.
The porous-oxide film grows in acidic electrolytes, in which the oxide can not just settle, but to dissolve. The most commonly used dilute sulfuric acid, H2SO4. You can also use oxalic acid and phosphoric acid.
In the first seconds of anodization on the aluminum barrier layer is formed, initially formed in the active sites on the metal surface. Of these embryos grow hemispherical lens-shaped microcells, then connate in a continuous barrier layer. In case of contact with the six surrounding cells form a hexagonal prism shape with a hemisphere at the bottom. Under the influence of local effects of electrolyte ions in the barrier layer nucleate pores (center cell), whose number is inversely proportional to the voltage. In the pore width of the barrier layer decreases, and as a consequence, increases the electric field, while the ion current density increases with the speed of oxidation. However, since increasing the temperature in the channel pore, promotes etching pores comes dynamic equilibrium, and the thickness of the barrier layer remains practically unchanged.
In this process of anodizing finishes, we get a cover with remarkable optical and processing properties. Anodized products can serve for decades without changing their decorative properties. The anodic corrosion protection is so effective that can protect the details of the most aggressive actions. These remarkable properties have long appreciated the car manufacturers, builders, military, aircraft manufacturers.