, The general principle of anodic oxide film formation
The process of placing aluminum or aluminum alloy coupling products as anode in an electrolyte solution and using electrolysis to form an aluminum oxide film on the surface is called anodizing of aluminum and aluminum alloy.The cathode in the device is a material with high chemical stability in the electrolytic solution, such as lead, stainless steel, aluminum and so on.The principle of aluminum anodization is essentially the principle of water electrolysis.When an electric current passes, hydrogen gas is released on the cathode; on the anode, the precipitated oxygen is not only molecular oxygen, but also atomic oxygen (O) and ionic oxygen, which are usually represented by molecular oxygen in the reaction.The aluminum of the coupling as the anode is oxidized by the oxygen precipitated on it to form an anhydrous aluminum oxide film. Not all the oxygen generated interacts with the aluminum, and a part of it is precipitated in a gaseous form.
2. Selection of anodizing electrolytic solution
A prerequisite for the growth of the anodic oxide film is that the electrolyte should have a dissolving effect on the oxide film.But this is not to say that anodic oxidation in all electrolytes with dissolving effect can produce oxide film or the properties of the oxide film produced are the same.
3. Structure and properties of anodic oxide film
The anodic oxide film consists of two layers. The thick, porous outer layer is grown on the dense inner layer with dielectric properties. The latter is called the barrier layer (also called the active layer).Observed and studied with an electron microscope, almost all the longitudinal and longitudinal surfaces of the film showed tubular holes perpendicular to the metal surface, which penetrated the outer layer of the film to the barrier layer at the interface between the oxide film and the metal.With each pore as the main axis, dense aluminum oxide forms a honeycomb hexagonal prism, called a unit cell, and the entire film layer is composed of countless such unit cells.The barrier layer is composed of anhydrous aluminum oxide, which is thin and dense, has high hardness and prevents the passage of electric current.The thickness of the barrier layer is about 0.03-0.05μm, which is 0.5%-2.0% of the total film thickness.The porous outer layer of the oxide film is mainly composed of amorphous alumina and a small amount of hydrated alumina, and also contains cations of the electrolyte.When the electrolyte is sulfuric acid, the sulfate content in the membrane is 13%-17% under normal conditions.Most of the excellent properties of the coupling oxide film are determined by the thickness and porosity of the porous outer layer, which are closely related to the anodizing conditions.
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