Aluminum alloys usually require surface treatment to meet different needs. Common surface treatments of aluminum alloys include anodization, chemical oxidation, electrophoresis, electroplating, electrochemical plating, spraying, passivation, coloring, etc. Mechanical pretreatment includes wire drawing, sand spraying, and polishing.
Author: Marisa
Aluminum alloys usually require surface treatment to meet different needs. Common surface treatments of aluminum alloys include anodization, chemical oxidation, electrophoresis, electroplating, electrochemical plating, spraying, passivation, coloring, etc. Mechanical pretreatment includes wire drawing, sand spraying, and polishing. The following focuses on several aluminum alloy surface treatment processes.
1. Anodizing: It is the process of plating a thin layer of other metals or alloys on certain metals surfaces using the principle of electrolysis. Brush plating is used for partial plating or repair. Rolling plating is suitable for small parts, such as fasteners, washers, pins, etc. Through electroplating, decorative protective and various functional surface layers can be obtained on mechanical products, and wear and error-produced workpieces can also be repaired. The electroplating solution has acidic, alkaline and acidic and neutral solutions with chromium mixture. No matter what plating method is used, the plating tanks, suspension tools, etc. that come into contact with the product to be plating and the plating solution should have a certain degree of versatility.
2. Chemical oxidation: The oxide film is thin, with a thickness of about 0.5 to 4 microns, and is porous, soft, and has good adsorption. It can be used as the bottom layer of an organic coating, but its wear resistance and corrosion resistance are not as good as the anodic oxide film;
The chemical oxidation process of aluminum and aluminum alloys can be divided into two categories according to their solution properties: alkaline oxidation method and acid oxidation method.
According to the properties of the film layer, it can be divided into: oxide film, phosphate film, chromate film, and chromic acid-phosphate film.
3. Electrochemical oxidation, the chemical oxidation treatment equipment of aluminum and aluminum alloys is simple, convenient to operate, high production efficiency, no electricity consumption, wide application range, and is not limited by the size and shape of parts. The thickness of the oxide film is about 5 to 20 microns (the thickness of the hard anodized film can reach 60 to 200 microns), with high hardness, good heat resistance and insulation, higher corrosion resistance than that of the chemical oxide film, porous, and good adsorption ability.
4. Conductive oxidation (chromate conversion film) - used in situations where both protection and conductivity are required.
5. Sandblasting: The main function is surface cleaning. Sandblasting before coating (painting or plastic spraying) can increase the surface roughness and contribute to the improvement of adhesion, but the contribution is limited, which is not as good as pre-treatment of chemical coating.
6. Coloring: There are two main processes for coloring aluminum: one is the aluminum oxidation coloring process, and the other is the aluminum electrophoresis coloring process. Various colors are formed on the oxide film to meet certain usage requirements, such as optical instrument parts are often black, commemorative medals are golden yellow, etc.
7. Spraying: Used for external protection and decoration of equipment, usually based on oxidation. Aluminum parts should be pretreated before coating to make the coating and workpiece firmly bond. There are three general methods: 1. Phosphate (phosphate method) 2. Chromium (chromium-free chromium), and 3. Chemical oxidation.
8. Chemical polishing Chemical polishing is a chemical processing method that uses the selective autolysis effect of aluminum and aluminum alloy in acidic or alkaline electrolyte solutions to level and polish annual surfaces to reduce their surface roughness and pH. This polishing method has the advantages of simple equipment, no power supply, no limitations on the size of the parts, high throwing speed and low processing cost. The purity of aluminum and aluminum alloys has a great influence on the quality of chemical polishing. The higher its purity, the better the polishing quality, and vice versa.
9. Passivation is a method to convert the metal surface into a state that is not easily oxidized and delay the corrosion rate of the metal.
A phenomenon in which the chemical activity of an active metal or alloy is greatly reduced and becomes a precious metal state is called passivation.
If the corrosion product generated by the action of the medium has a dense structure, a thin film (often invisible) is formed, which closely covers the surface of the metal, changes the surface state of the metal, causing the metal electrode potential to jump greatly in the positive direction, and becomes a blunt state of corrosion resistance. For example, when Fe→Fe++ is standard potential is -0.44V, and after passivation, it jumps to +0.5~1V, and shows corrosion-resistant precious metal properties. This film is called a passivation film.