Anodic oxidation of machined parts, electrochemical oxidation of metals or alloys. Under the corresponding electrolyte and specific process conditions, aluminum and its alloys form an oxide film on the aluminum product (anode) due to the action of the applied current. Anodizing usually refers to sulfuric acid anodizing unless otherwise specified.
Anodic oxidation of parts, electrochemical oxidation of metals or alloys. Under the corresponding electrolyte and specific process conditions, aluminum and its alloys form an oxide film on the aluminum product (anode) due to the action of the applied current. Anodizing usually refers to sulfuric acid anodizing unless otherwise specified.
In order to overcome the defects of surface hardness and wear resistance of aluminum alloys, expand the scope of application and prolong the service life, surface treatment technology has become an indispensable part of the use of aluminum alloys, and anodizing technology is the most widely used and most successful.
The process of using an aluminum plate as an anode and placing it in an electrolyte solution and using electrolysis to form an aluminum oxide film on the surface is called anodizing treatment of an aluminum plate. The cathode in the device is a material with high chemical stability in the electrolytic solution, such as lead, stainless steel, aluminum, etc.
The principle of aluminum anodization is essentially the principle of water electrolysis. When the current is passed, on the cathode, hydrogen gas is released; on the anode, the evolved oxygen is not only molecular oxygen, but also atomic oxygen (O) and ionic oxygen, usually expressed as molecular oxygen in the reaction.
The aluminum used as the anode is oxidized by the oxygen precipitated on it to form an anhydrous aluminum oxide film. The generated oxygen does not all act on the aluminum, and a part is precipitated in the form of gas.
A prerequisite for the growth of the anodic oxide film is that the electrolyte should have a dissolving effect on the oxide film. However, this does not mean that anodizing can generate oxide films or that the properties of oxide films generated are the same in all electrolytes with dissolution.
Anodizing is divided into: direct current anodizing, alternating current anodizing, and pulse current anodizing according to the current form. According to the electrolyte, it is divided into: sulfuric acid, oxalic acid, chromic acid, mixed acid and natural color anodizing with sulfoorganic acid as the main solution.
According to the nature of the film, it is divided into: ordinary film, hard film (thick film), porcelain film, bright modification layer, semiconductor barrier layer and other anodizing. Commonly used anodizing methods and process conditions for aluminum and aluminum alloys are shown in Table-5. Among them, the application of direct current sulfuric acid anodizing method is the most common.
The anodic oxide film consists of two layers, a porous thick outer layer is grown on a dense inner layer with dielectric properties, the latter is called the barrier layer (also called the active layer). Observation with electron microscope shows that almost all the longitudinal and transverse planes of the film layer are tubular holes perpendicular to the metal surface, which penetrate through 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 alumina forms a honeycomb hexagonal body, called a unit cell, and the entire film layer is composed of countless such unit cells.
The barrier layer is composed of anhydrous alumina, thin and dense, with high hardness and the effect of preventing the passage of current. The thickness of the barrier layer is about 0.03-0.05μm, which is 0.5%-2.0% of the total film. 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 layer is normally 13%-17%. Most of the good properties of oxide films are determined by the thickness and porosity of the porous outer layer, which are closely related to the anodizing conditions.
In addition to metals, the oxidation caused by other substances as anodes is also called "anodizing".
In the actual process, there are many anodizing of aluminum alloys, which can be applied in daily life. Because of the characteristics of this process, a hard protective layer is formed on the surface of aluminum parts, which can be used to produce kitchen utensils and other daily necessities. However, the anodizing effect of cast aluminum is not good, the surface is not bright, and it can only be black. Aluminum alloy profiles are better.
Improve the adhesion to organic coatings
Improve the bonding force with the inorganic coating
Additional features in development
1) Anodizing is carried out under the condition of high voltage, it is an electrochemical reaction process; while conductive oxidation (also called chemical oxidation) does not require electricity, just soak in potion, It is a pure chemical reaction.
2) Anodizing takes a long time, often tens of minutes, while conductive oxidation only takes a few tens of seconds.
3) The film produced by anodizing is several microns to dozens of microns, and is hard and wear-resistant; the film produced by conductive oxidation is only 0.01-0.15 microns, and the wear resistance is not very good, but it is both conductive and resistant. Atmospheric corrosion, that's its advantage.
4) The oxide film is originally non-conductive, but because the film formed by conductive oxidation is really thin, it is conductive.