What is anodizing

1, the general principle of anodized aluminum plate oxide film generation:

Aluminum plate as the anode in the electrolyte solution, the use of electrolytic action, so that the surface of the formation of alumina film process, known as the anodic oxidation of aluminum plate. The cathode of the device is the material with high chemical stability in the electrolytic solution, such as lead, stainless steel, aluminum, etc. The principle of anodizing aluminum is essentially the principle of hydroelectrolysis. When an electric current passes through, at the cathode, hydrogen gas is released; At the anode, the oxygen precipitated is not only molecular oxygen, but also atomic oxygen (O) and ionic oxygen, which is usually expressed as molecular oxygen in the reaction. As the anode, the aluminum is oxidized by the oxygen precipitated on it, forming an anhydrous alumina film. The generated oxygen does not all interact with the aluminum, and part of it is precipitated in the form of gas.

2, anodized aluminum plate oxidation electrolytic solution selection:

A prerequisite for the growth of anodic oxide film is that the electrolyte should dissolve the film. However, this does not mean that anodizing can form an oxide film or the properties of the oxide film are the same in all dissolved electrolytes.

3. Types of anodized aluminum plate oxidation:

Anodic oxidation is divided into direct current anodic oxidation, alternating current anodic oxidation, pulse current anodic oxidation. According to the electrolyte, it can be divided into sulfuric acid, oxalic acid, chromic acid, mixed acid and natural coloring anodic oxidation with sulfo-based organic acid as the main solution. According to the film layer, there are: ordinary film, hard film (thick film), porcelain film, bright modification layer, semiconductor barrier layer and other anodic oxidation. Common anodic oxidation methods and process conditions of aluminum and aluminum alloys are shown in Table 5. Among them, direct current sulfuric acid anodizing is the most popular method.

4, anodized aluminum plate oxide film structure, properties:

The anodic oxide film consists of two layers, a porous, thick outer layer that grows on top of a dense inner layer with dielectric properties, called the barrier layer (also known as the active layer). Observation by electron microscope shows that the vertical and horizontal surfaces of the film almost all show tubular holes perpendicular to the metal surface, which penetrate through the outer layer of the film until the barrier between the oxide film and the metal interface. Around each pore as the main axis is dense alumina formed a honeycomb hexaxed body, called the cell, the whole membrane layer is composed of countless such cells. The barrier is made up of anhydrous aluminum oxide, thin and dense, which has high hardness and prevents the passage of current. The thickness of the barrier layer is about 0.03-0.05μm, which is about 0.5%-2.0% of the total membrane. The porous outer layer of oxide film is mainly composed of amorphous alumina and a small amount of hydrated alumina, in addition to containing electrolyte cations. When the electrolyte is sulfuric acid, the content of sulfate in the membrane is 13%-17% under normal conditions. Most of the excellent properties of the oxide film are determined by the thickness and porosity of the porous outer layer, which are closely related to the anodized strip.