Aluminum and aluminum alloys will be polluted to varying degrees during their production, transportation, and storage. Grease and dust will adhere during processing; welding slag will remain on welded parts; annealed aluminum will partially form an uneven oxide film. At the same time, the surface of the aluminum alloy sheet is too smooth. Therefore, the untreated aluminum and aluminum alloy surfaces do not meet the requirements for painting, nor can they meet the requirements for dyeing, electroplating, etc.
When decorating aluminum and aluminum alloys, it is generally professional to perform mechanical cleaning, degreasing, rust removal, and remove the original oxide layer, and form a new layer on the surface by chemical or electrochemical methods, which has a certain roughness and anti-corrosion ability. The oxide layer is used for further painting and dyeing. The thickness of the chemical oxide film is about 0.5 microns to 4 microns. It has low hardness, no wear resistance, and low corrosion resistance. It cannot be used as a protective layer alone. But it has strong adsorption capacity and can be used as a good bottom layer for paint coating. (Related instrument: adhesion Tester) Compared with electrochemical anodization, chemical oxidation has the characteristics of high production efficiency, low cost and wide applicability. Commonly used chemical oxidation methods include alkaline solution oxidation, phosphate-chromate oxidation and chromate oxidation. Electrochemical oxidation of aluminum and its alloys is also called anodization. Because the workpiece aluminum alloy is used as the anode during electrochemical oxidation, the resulting oxide film is also called anodized film. The thickness of the anodized film is generally between 5 microns and 20 microns, and the thickness can even reach 200 microns. It has high hardness, wear resistance, and higher corrosion resistance than chemical oxide films. However, its brittleness is too high. When it is subjected to a large impact load and bending deformation, the oxide film will first produce network cracks and crack. The cost of anodizing is also higher than that of chemical oxidation, and the anodizing of some large parts and assemblies (such as spot welds, cutting parts, slender pipes, etc.) will be greatly restricted. Anodized films of aluminum and aluminum alloys are also satisfactory primers for paint. Can be painted directly after anodizing. The anodized film is also porous. The pores on its surface can absorb materials such as various dyes, salts, lubricants, paraffin, drying oils and resins. Therefore, the anodization of aluminum and its alloys is often the pre-treatment of its dyeing, lettering, pattern making and other decorations. The anodizing methods of aluminum and its alloys mainly include sulfuric acid anodizing, oxalic acid anodizing and chromic acid anodizing.
The general process of sulfuric acid anodization is: degreasing→installation→chemical degreasing→hot water washing→flowing, cold water sinking→flowing cold water washing→alkali corrosion→hot water washing→flowing cold water washing→lighting→flowing cold water washing→sulfuric acid anode Chemicalization→flowing cold water washing and hot water washing→sealing treatment→hot water washing→flowing cold water washing→drying→disassembly of parts→inspection.
General process procedures ·
a. Degreasing. The alkali resistance of aluminum and its alloys is not as good as that of steel, so it cannot be degreased with strong alkali. It can be degreased with organic solvent first, and then degreased with weak alkali solution such as trisodium phosphate, sodium carbonate, water glass (aqueous solution of sodium silicate).
b. Lighting, alkali corrosion and then lighting is to prepare a new metal surface free of oil and rust (a compound) for chemical or electrochemical oxidation. The first light is to remove the salts and corrosion products remaining in the degreasing solution on the surface. Alkali corrosion and the second light emission are to remove the oxide on the surface, so that the surface becomes a new metal surface that is both oil-free and rust-free, and meets the needs of chemical oxidation or electrochemical oxidation.
C. Examples of process specifications for alkaline solution oxidation
Anhydrous sodium carbonate (N m ) 50 g/L, sodium chromate (Na2Cr04) 15 g/L, sodium hydroxide (NaOH) 2.5 g/L; the solution temperature is 80°C~100°C, and the chemical oxidation time is 5 ~8 points. The oxidizing ability of the oxidizing solution will gradually weaken over time. In the oxidation treatment, the temperature and time can be appropriately increased to make up for it. When it can no longer be oxidized, the oxidizing solution should be prepared again.
d. Examples of process specifications for sulfuric acid anodizing
The solution composition is industrial sulfuric acid (specific gravity 1.84) 180-200 g/liter k. The temperature is 13°C~26°C, the current density is 0.8~2.5 amperes/dm2, the voltage at the upper end of the tank is 13~22 volts, and the anodization time is 20~40 minutes. During anodization, the above factors can affect the formation of anodized film and the performance of the film.
e. Closed treatment.
The film layer after chemical oxidation and electrochemical oxidation becomes a porous structure. In order to improve the anti-corrosion ability of the film layer and avoid pollution, the film layer must be sealed, also called supplementary treatment. Membranes to be stained can be blocked with snot water or steam. The workpiece after the potassium dichromate sealing treatment must be cleaned, otherwise the residual potassium dichromate will penetrate into the coating surface during the baking of the coating. If the primer can be applied within 24 hours, no sealing treatment is required.
The sealing treatment after sulfuric acid anodization can be sealed in 40~55g/L potassium dichromate solution at 90℃~95℃ for 15~25 minutes. (Related instruments: constant temperature Water Tank)
After chemical oxidation, it can be supplemented with 50 g/L potassium dichromate solution at 90°C~98°C for 10 minutes.
f. Processing requirements before electrosensitization.
Before electroplating, the surface of the substrate is required to be free of oil and oxides. For aluminum and aluminum alloys, the treatment principles of chemical oxidation, degreasing and deoxidation before electrochemical oxidation in the above process are also applicable. However, with the development of surface treatment technology and the particularity of electroplating, the one-step process of degreasing and rust removal has been widely used in the pre-plating treatment of aluminum and its alloys.
