The electrolyte used for anodic oxidation of aluminum and its alloys is generally an acid solution with medium solubility. As a cathode, lead only conducts electricity. When aluminum and its alloys are anodized, the following reactions occur at the anode:

The formation and dissolution of anodic oxide film are carried out at the same time. In the early stage of anodic oxidation, the formation rate of the film is greater than the dissolution rate, and the thickness of the film increases continuously; With the increase of the thickness, its resistance also increases, resulting in a slower growth rate of the film. The film thickness is a certain value until it is equal to the dissolution rate of the film.

The first section A: no pore layer is formed. Aluminum oxidation processing Chemical oxidation film is thin, about 0.5~4 μ m thick, porous, soft, with good adsorption, and can be used as the bottom layer of the organic coating, but its wear resistance and corrosion resistance are not as good as the anodic oxidation film. In section ab of the curve, the voltage increases sharply from zero to the maximum within a few seconds to dozens of seconds at the beginning of power on, which is called critical voltage. It shows that a continuous, pore free film layer is formed on the anode surface at this time. The appearance of this film prevents the continuous thickening of the film. The thickness of the non porous layer is proportional to the formation voltage and inversely proportional to the dissolution rate of the oxide film in the electrolyte.

Second section B: formation of porous layer. Although the polishing oxidation process and pure aluminum polishing oxidation process look similar, they are essentially different, because in terms of brightness, the latter is much higher than the former. In addition, the composition of raw materials is also different. As for polishing and oxidation, the main purpose of polishing is to improve the smoothness and feel of the product surface. In the bc section of the curve, after the voltage reaches the maximum value, it starts to drop, and the drop amplitude is 10%~15% of the maximum value. It shows that the non porous membrane begins to be dissolved by electrolyte, and a porous layer appears.

The third section C: the porous layer is thickened. In the cd section of the curve, after about 20s of oxidation voltage, it starts to enter a steady and slow rising stage. It shows that while the non porous layer is continuously dissolved to form a porous layer, the new non porous layer is growing, that is, the porous layer is continuously thickened, and the process of membrane formation and dissolution is carried out at the bottom of each membrane cell. When the formation rate and dissolution rate of the film reach a dynamic balance, even if the anodizing time is extended, the thickness of the oxide film will not increase again. At this time, the anodizing process should be stopped.

The anodic oxide film is composed of two layers. The porous thick outer layer grows on the dense inner layer with dielectric properties, which is called the barrier layer. Dye immediately after sand blasting and oxidation. If the workpiece is exposed to the air for too long after oxidation, the film gap will shrink, and it may be polluted by other factors, resulting in difficult dyeing. If it is impossible to dye, put the product to be dyed in clean water.

It is observed and studied by electron microscope that the vertical and horizontal surfaces of the film almost all present tubular pores perpendicular to the metal surface, which run through the outer layer of the film until the barrier layer between the anodic oxide film and the metal interface. With each pore as the main axis, the dense anodized aluminum forms a honeycomb hexagonal body, which is called the unit cell. The entire film is composed of countless such units.

The barrier layer is composed of anhydrous anodic aluminum oxide, thin and dense, with high hardness and the function of preventing current from passing. The thickness of the barrier layer is about 0.03-0.05 μ m. 0.5% - 2.0% of total membrane. The outer layer of the porous anodic oxide film is mainly composed of amorphous anodic aluminum oxide and a small amount of hydrated aluminum oxide, in addition, it also contains cations of the electrolyte.

When the electrolyte is sulfuric acid, the sulfate content in the film is 13% - 17% under normal conditions. Most of the excellent properties of anodic oxide film are determined by the thickness and porosity of the porous outer layer, which are closely related to the anodizing conditions.

There is a layer of oxide film on the surface of the aluminum alloy coil, which not only has an aesthetic effect, but also can protect the aluminum alloy coil from external damage. However, sometimes the color of the oxide film is uneven. What is the cause.

1. If the working area is too large and the operation is too large, it will swing in the slot, which is very different from the touch solution at the edge and center. Then the color generated in the oxide film is inconsistent. The oxidized workpiece has small rotary fluctuation, which can be handled quietly. However, when the solution temperature is too low, it is easy to appear map like piebald, which is unnatural.

2. When the damaged part of aluminum alloy roll processing coating is cut off, high-quality aluminum alloy is worn on the outside, and the inner layer is miscellaneous aluminum. Due to the large quality of water-cooled aluminum row, vitiligo like spots will be produced after oxidation.

3. Process operation problems: alkaline etching treatment of the workpiece is incomplete, and the original part of the oxide film and dirt cannot be removed; Dispose immediately after alkaline etching, and the surface is still alkaline; The workpiece contacts foreign matters during transfer.

Source: Changjiang Nonferrous Network