Sealing is the final step in anodic oxidation (ELOXAL) or dyeing after anodization of aluminum. Sealing closes the open, porous oxide layer that is receptive to reaction and dyes.
There are two fundamentally different types of sealing: the widely used and well-known hot sealing (hot water sealing also known as warm water sealing) and the less well-known cold sealing.
When hot sealing is used, the aluminum oxide Al2O3 formed during anodization is swollen (hydrated) by the action of hot water (above 95°C or above 86°C). The oxide layer pores begin to swell at the outer points, the pores are compressed and close during prolonged treatment. Here, the compression process is associated with swelling, crystal water absorption and lattice transformation to Al2O3*1H2O and leads to closing of the pores. This hydration process reduces the hardness of the original Al2O3. With the closing of the pores, the final corrosion protection is created.
During the application of the cold sealing, hydrolysis products of the metals contained in the cold sealing baths, not the metals themselves, are deposited into the pores of the oxide layer. The pores are rapidly plugged. This completes the densification process. The metal used here is nickel. There is no swelling of the aluminum oxides. The original hardness of the material produced is retained. Furthermore, the risk of dye bleeding is lower with colored anodized layers. Here, too, as with hot-sealing, the appropriate corrosion protection is achieved by closing the pores.
Rieger Metallveredlung offers both sealing processes, depending on the surface requirements.
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