The anodic oxide layer formed on aluminium contains many small pores. These pores can absorb moisture, oils, and other contaminants. Sealing the anodized surface serves to close up these pores and protect the surface from corrosion. Additionally, the pores must be closed up in order to protect the colour on dyed anodized parts from bleeding or fading.
Why the Oxide Layer Needs Sealing
Anodising produces a porous aluminium oxide layer, rather than a solid non-permeable surface. All porosity must be closed in order to achieve corrosion protection and to lock in dyeing. The material must therefore be sealed after anodising.
Hot Deionised Water Sealing
By submerging the anodised article in near-boiling deionised water, the oxide forms a hydrated, swelled up layer which penetrates the pores. Hot water sealing is one of the oldest and most commonly used of the methods for sealing anodized parts. It is suitable for most general industrial and architectural work.
Cold Nickel Acetate Sealing
Cold Sealing – a chemical sealing process using nickel salts in an aqueous solution typically at room temperature (20 to 30 degrees C). The process is particularly suitable for large numbers of components and components sensitive to thermal expansion. Aqueous solutions of nickel salts are used to fill the pores of the anodized coating, thereby sealing them.
Mid-Temperature Sealing
Mid-temperature sealing: seals quicker than hot water sealing but doesn’t produce smut and surface bloom associated with cold sealing. A good all round process when speed of cycle and surface quality are critical.
What to Check When Specifying
Make sure that the chosen method for sealing the anodised coating is compatible with the used dye and the intended use. Components which are exposed to acids or alkalis during use must be checked against the relevant Standard. For Anodising, see https://www.poeton.co.uk/surface-treatments/anodising.
Sealing is as critical to the function of a coating as the anodising process itself. It should therefore receive the same attention when specifying.
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