Aluminum and its alloys can easily form a very thin layer of aluminum oxide on their surfaces (with a thickness of approximately 0.01 to 0.02 micrometers). This layer has a certain anti-corrosion ability in the atmosphere. However, since this oxide layer is non-crystalline, it causes the surface of the aluminum parts to lose its original luster. In addition, the oxide layer is loose and porous, uneven, and has poor anti-corrosion ability, and is prone to staining. Therefore, aluminum and its alloy products usually require oxidation (anodic oxidation aluminum) treatment.
Aluminum and its alloy surface coloring:
1. Requirements for the oxide layer.
1. The oxide layer must have sufficient thickness, and the specific thickness should be determined according to the coloring tone.
2. The oxide layer must have sufficient pores and adsorption capacity.
3. The oxide layer should be uniform, and the color of the layer itself should be suitable for the coloring treatment.
2. Introduction to the anodic oxidation coloring process:
. Electrolytic coloring:
After aluminum and aluminum alloys are anodized, they are placed in solutions containing nickel salts, chromium salts, tin salts, or copper salts, and undergo alternating current electrolysis to deposit metals such as nickel, cobalt, tin, or copper at the bottom of the membrane, resulting in different colors. This process has the characteristics of simple process, low energy consumption, uniform coloring, and high production efficiency.
. Inorganic salt coloring:
Inorganic salt staining mainly relies on the physical adsorption effect of anodic oxidation staining. The salt molecules enter the pores and undergo chemical reactions to produce colored substances. Due to the limited variety of inorganic salt colors and the lack of vivid tones, it is no longer widely used nowadays.
Organic dye dyeing:
Due to the porous nature and strong adsorption capacity of the anodic oxide film, it can be dyed in various colors. The anodic oxide film that can be directly dyed is obtained from the sulfate solution. It forms a colorless transparent film on most aluminum and aluminum alloys, with an appropriate thickness, porosity, and adsorption capacity. The oxalic acid anodic oxidation process is more expensive than the sulfuric acid process and results in a yellow film. When the film layer exceeds 50 μm, a natural yellow or brown color is obtained. The chromic acid anodic oxidation process is not suitable for dyeing because the film is thin, has few pores, and it is itself gray. Generally, it is not recommended for dyeing. The requirements for the anodic oxide film for dyeing are appropriate film thickness, sufficient pores, good adsorption capacity, no trauma or contamination.
Overall dyeing:
Aluminum and aluminum alloys are placed in an electrolyte containing organic substances (such as cresol, benzenesulfonic acid, sulfosalicylic acid, etc.) for anodic dyeing treatment. The film is dyed simultaneously during anodic oxidation. The tiny particles are dispersed on the inner wall of the membrane pores. Due to the scattering of incident light, different colors are produced. The tiny particles come from the base metal or the decomposition products of organic substances in the electrolyte. The depth of the color is directly related to the thickness of the film. This anodic oxidation process requires high anode current density and high voltage, so it consumes a lot of energy.
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