[China Aluminum Industry Network] 1. Preface
After anodization and coloring of aluminum profiles, its aluminum-based surface is a very thin layer of porous anodized film, which has strong adsorptivity, corrosion resistance, and abrasion resistance. In the process of surface treatment of architectural aluminum profiles, sealing holes as a post-treatment process is the key to determining the surface quality, decorative effect and service life of aluminum. There are many micropores in the anodized film of aluminum and aluminum alloy. If it is not closed or incomplete in time, it will seriously affect the corrosion resistance, wear resistance, light resistance and decorative effect of the aluminum profile surface.
2. Sealing principle at room temperature
Anodic oxide films of aluminum and aluminum alloys are mainly sealed by high-temperature and low-temperature methods (excluding electrophoretic painting), and the principle of sealing is not the same. The principle of high temperature sealing method is to use the hydration of the oxide film at high temperature to generate a stable γA12O3?H2O deposited in the film hole, thereby closing the film hole. Since the speed of the hydration reaction and the stability of the product are related to the temperature, the principle of the low temperature sealing method is not only the result of hydration. In general, it is a combination of the following three effects.
2.1 Hydration
Low-temperature sealing (also known as normal temperature sealing) uses an aqueous solution and utilizes its hydration. Because the temperature is low, the hydration reaction rate is very slow, and the hydration product is reversible and therefore unstable. Therefore, substances that promote the hydration reaction, such as Ni2+, Cr3+, Co2+, Li+, and the like, need to be added in the low-temperature sealer.
2.2 The role of chemical conversion film forming aluminum
The chemical reaction of certain substances in the sealant with the aluminum oxide film produces a stable chemical conversion film on the surface, for example, the use of a chromate-forming passivation film, a phosphate-forming precipitation film, and a red complex salt to form the surface Chrome compounds and so on.
2.3 Generate Metal Hydroxide, Blocking Film Holes
Some of the metal ions in the sealer diffuse into the pores of the membrane and hydrolyze at a certain pH, precipitate in the pores of the membrane in the form of hydroxide, or some active particles in the sealer react with the aluminum oxide film to produce OH-. Then, it reacts with the metal ions that diffuse into the pores of the membrane to form a hydroxide precipitate that blocks the pores of the membrane.
The composition of the sealant varies. The size of the above three effects is different, but it is always the combined effect of the three effects.
3. Nickel-fluoride system sealing at low temperature
Low-temperature sealing is a more end-to-end process for the anodizing process of aluminum profiles. At present, the basic methods adopted in Japan are the metal fluoride-polar solvent sealing method that was invented in Japan in the early 1980s, and its main components consist of nickel salt and fluoride ions.
The mechanism of low-temperature sealing of the nickel-fluorine system is: (1) fluoride ion promotes the hydration reaction of the oxide film; (2) fluorine ion reacts with amorphous alumina to form a complex, and hydroxide ions are released at the same time to make the film pores The pH value increases; (3) Nickel ion in the oxide film is hydrolyzed, and hydroxide precipitates. The main chemical reactions are as follows:
Al2O3+12 F-+3H2O →2 AlF6 3-+6OH-
AlF6 3-+ Al2O3+3H2O →Al3(OH)3F6+3OH-
Ni2++2 OH-→Ni(OH)2
In these dissolution and deposition reactions, the reactants (packing substances) are mainly hydrated Ni(OH)2, Al(OH)3, AlF3 mixtures, and additionally AlOOH (Al2O3) (which is the reaction of F- with an oxide film. Al3+ produces hydrated Al (H2O) 63+, and when its ion concentration reaches a certain value, the ions undergo association, hydrolysis, and condensation, which later turns into a stable phase of AlOOH (Al2O3). Visible, F-to-low temperature sealing It has played an important role in promoting.Anodic Oxidation Sealing after the low temperature, the sealing material is mainly concentrated in the outer layer of the oxide film 5 ~ 8μm area.
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