The natural oxide layer on the surface of aluminum products is soft and thin, with poor corrosion resistance and cannot serve as an effective protective layer. It is not suitable for coloring. Chemical oxidation and anodization are mainly used to create artificial oxide films. Oxidation is a process where certain less valuable metals of aluminum react in weakly alkaline or weakly acidic solutions, causing the natural oxide film on the surface to thicken or generating some other passivation films. Common chemical oxide films include chromic acid films and phosphoric acid films. These films are thin and absorbent, and can also be colored and sealed. Compared with anodized films, chemical oxide films are much thinner, have lower corrosion resistance and hardness, are not easy to color, and have poor light resistance after coloring. Therefore, the coloring and color matching of aluminum metal only involves the introduction of anodization treatment. 

The general principle of forming anodic oxide film: Place aluminum or aluminum alloy products as the anode in an electrolyte solution. The process of forming an oxide aluminum film on the surface through electrolysis is called the anodic oxidation treatment of aluminum and aluminum alloys. The cathode in the device is a material with high chemical stability in the electrolyte solution, such as lead, stainless steel, aluminum, etc. The principle of aluminum anodic oxidation is essentially the principle of electrolytic water decomposition. When the current passes through, hydrogen is released at the cathode; at the anode, the released oxygen is not only molecular oxygen but also atomic oxygen and ionic oxygen, which is usually represented as molecular oxygen in the reaction. The aluminum serving as the anode is oxidized by the oxygen deposited on it, forming a dry oxide aluminum film. Not all the generated oxygen reacts with aluminum; some oxygen precipitates in gaseous form. 
Types of anodizing: Anodizing based on current form can be classified as direct current anodizing, alternating current anodizing, and pulse current anodizing. The electrolyte solutions include sulfuric acid, oxalic acid, chromic acid, mixed acid, and natural coloring anodizing with organic sulfonic acid as the main liquid. According to the film layer, there are: ordinary film, hard film (thick film), ceramic film, bright decorative layer, semiconductor barrier layer, etc. Anodizing methods and processes commonly used for aluminum and aluminum alloys include DC sulfuric acid anodizing. 
Structure and properties of anodic oxide film: The anodic oxide film consists of two layers. A porous thick outer layer grows on top of a dense inner layer with dielectric properties, which is called the barrier layer (also known as the active layer). Electron microscope observations show that almost all vertical and horizontal surfaces of the film exhibit tubular holes perpendicular to the metal surface. These holes pass through the outer layer of the film and reach the barrier layer at the interface between the oxide film and the metal. The honeycomb-like hexagonal bodies with each hole as the main axis are called crystal cells. The entire film is composed of countless such crystal cells. The barrier layer is composed of anhydrous aluminum oxide, is thin and dense, has high hardness, and has the function of preventing current from passing through. The thickness of the barrier layer is approximately 0.03-0.05 μm, accounting for 0.5%-2.0% of the total thickness of the film. The porous outer layer of the oxide film mainly consists of amorphous aluminum oxide and a small amount of hydrated aluminum oxide, and also contains cations of electrolytes. When the electrolyte is sulfuric acid, the normal content of sulfate in the film is 13%-17%. The majority of the excellent properties of the oxide film are determined by the thickness and porosity of the porous outer layer, and the thickness and porosity of the porous outer layer are closely related to the anodic oxidation conditions.