Seal anodized aluminum by immersing it in boiling water for 30 minutes, or in steam for 20 minutes, which closes the pores of the oxide layer and makes it corrosion-resistant and hard. Dye the part first if it is to be coloured, because a sealed layer will not take dye, and use clean water so the surface stays clear.
Part of Aluminum Smelting
The final step of anodizing: hot-water or steam sealing of the porous oxide layer.
The Pores Must Be Closed After Dyeing
Fresh anodic oxide is porous. The carrier’s seal is boiling water for 30 minutes or steam for 20, giving a sealed layer that is hard, scratch-resistant and extremely corrosion-resistant. The Phase 5 article Aluminum Anodizing explains that boiling converts the oxide to a hydrated form that closes the pores, locks in dye, and recommends distilled water; it also lists “sealing done before dyeing” as a reason dye will not take. Dyeing, if wanted, therefore comes between rinsing and sealing.
What You Need
- A clean vessel (glass, glazed ceramic or stainless) and a heat source, with water that is distilled or rainwater (my suggestion; the Phase 5 article specifies distilled).
- Tongs or a rack of non-reactive material.
- A steam source if steam sealing is used.
- A timer.
Method
Step 1: Rinse the part in clean water straight after anodizing (Sulfuric Acid Anodizing).
Step 2: Dye if desired, per the Phase 5 article.
Step 3: Seal. Hold the part fully immersed in boiling water for 30 minutes, or in steam for 20.
Step 4: Dry and handle with clean gloves. A bloom or chalky film on the surface suggests impure water; I recall dissolved minerals cause this.
Step 5: Check. A drop of dye or ink should wipe off a sealed surface. This test is my suggestion; the corpus gives none.
Worked Example: Heat For A Bath
Heating 10 L of water from 20 °C to 100 °C needs 10 kg × 4.18 kJ/(kg·K) × 80 K = 3,344 kJ (specific heat of water recalled), which is 3,344 / 3,600 = 0.93 kWh. Holding it boiling for the 30-minute seal costs further heat that the corpus does not size.
Common Mistakes
- Sealing before dyeing.
- Cutting the time. Under-sealed layers stain and corrode.
- Using hard or dirty water.
- Dropping the part in cold. Heat it with the water.
Safety
Boiling water and steam
A vessel of boiling water scalds on contact and a steam source burns instantly. Keep the vessel stable, never lift it full while boiling, wear gloves and an apron, and keep the part clear of the surface when lowering it. Do not seal in a closed vessel.
What’s Next
- Learn next: Why Aluminum Matters - back to the metal itself
- Combine with: Aluminum Anodizing - dyeing and alloy choice