According to the China Aluminum Industry Network, aluminum surfaces develop a large number of pores in their anodic oxide films, making them highly adsorbent and giving them a sticky feel when touched. To enhance the anti-fouling and anti-corrosion properties of these films, sealing is a crucial step. There are two main methods for sealing anodic oxide films: heat sealing and cold sealing. In China, cold sealing is currently the most commonly used method. As the film thickness increases and the sealing quality improves, the consumption of cold sealing agents has also risen, from 0.8 to 1.2 kg per ton of aluminum profiles to 1.5 to 2.0 kg. Nickel fluoride is the primary component of cold sealing agents, so the performance of the agent largely depends on the quality of the nickel fluoride. While impurities like iron, zinc, and copper may not significantly affect the new sealing tanks, they can lead to inconsistent sealing quality over time due to long-term production.
Previously, it was believed that the pH of cold-sealed pores should be between 5.5 and 6.5. However, this value actually depends on the specific formulation of the cold sealing agent. For fresh bath solutions, the pH usually ranges from 5.3 to 7.0. In contrast, for older baths that use ammonium fluoride or ammonium hydrogen fluoride to control fluoride ion levels, the pH should be maintained between 6.5 and 7.1 due to the influence of ammonium ions. At this range, the sealing process is faster, fluorine consumption is lower, and the “whitehead†phenomenon is avoided. Ammonium ions in the sealing solution are less likely to cause dusting compared to sodium ions, but they do result in a slower sealing rate. To ensure high-quality sealing, several key process points must be followed:
(1) The anodizing temperature should generally remain below 23°C. If the temperature is too high, the cold sealing agent will be consumed more rapidly, and the surface may turn green. (2) After anodizing, the parts should be washed promptly. Leaving them in the oxidation tank can negatively impact the subsequent sealing process. Improper washing may lead to sludge pollution and increase fluorine usage in the sealing tank. (3) Any aluminum scraps or wires that fall into the sealing trough should be removed immediately, as they can accelerate pH rise and fluorine consumption. (4) After processing 20 tons of aluminum material per cubic meter of sealing bath, the tank should be cleaned at the bottom to maintain efficiency. (5) When adjusting the fluoride level using hydrofluoric acid, it's best to wait 5 to 10 minutes before starting the process. It’s recommended to use a diluted 10% solution for better results. (6) To improve sealing quality and speed up drying, it is advised to wash the parts with hot water at 55±5°C for 10–15 minutes after cold sealing. This is known as post-treatment and helps achieve a more uniform and durable finish.
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