Sealing Chemicals and Processes for Anodized Aluminum Components

Process: –

To seal the pores of the Anodized Aluminum oxide coating by Hydration, Hot /Cold sealing is done after anodizing. Applicable to all anodized aluminum parts (natural & colored) processed in the anodizing line. thereby improving

Increase corrosion resistance.

Surface hardness and durability.

Improves colour fastness (for dyed anodized parts).

Reduces contamination and surface staining.

Principle of Anodizing Sealing: –

After Anodizing, the sealing process is based on the hydration of aluminium oxide (Al₂O₃) in Boehmite (AlO(OH)) in the presence of hot water or sealing chemicals.

Al₂O₃ + H2O                    2AlO(OH)

• The anodic aluminium oxide film is amorphous and porous.
• When dipped into hot water at >96°C, it absorbs aluminium hydroxide molecules into the pore and forms a seal. This process needs 45 to 60 minutes for proper sealing.
• This causes a hydration reaction forming Boehmite, a hydrous crystalline oxide. Boehmite    occupies more volume.  It swells, fills and closes the pores.
• Once the pores are closed, the surface becomes dense, smooth, and impermeable to moisture and contaminants. Prevent corrosion by blocking electrolyte access to the metal.

Type of Sealing: –

Type	Medium	Principle
Hot water Sealing	DI water (96-100°C)	Hydration of Al₂O₃ → 2AlO(OH)
Hot Sealing Chemicals	Nickel based (85–95°C)	Hydration + nickel salt deposition to block pores
Cold Sealing Chemicals	Ni based complex  (25–35°C)	Chemical precipitation of  Nickel complex molecule into the pores
Mid-temperature sealing	Additive 205	Partial Hydration +  precipitation

Process Parameters:-

• Temperature: from ambient to 100 degree depends on the type of sealing chemicals.
• Time:   2 -3 min per micron coating thickness
• pH:      5.5-6.5
• Agitation and filtration:  Gentle continuous.

Quality Control Tests: –

• Dye stain test : Apply dye ( e.g. eosin) and check absorption  No visible stain after            2 min.
• Acid Dissolution test : Immerse in 10% phosphoric acid + 10% chromic acid for 15 min. calculate the weight loss<30mg/dm².
• Visual check : Inspect for smut, white spots, or colour fading. Material should be clean uniform, glossy appearance.
• Corrosion Resistance: Salt spray test (ASTM B 117) meet as per specification.

Common Defects & Causes and Solution:-

• Smutor white bloom: High pH , hard water, poor rinsing.

Adjust pH, use DI water.

• Poor corrosion resistance: Incomplete sealing (too short or low temp).

Increase sealing Temp & Time

• Colour fading: Over sealing or too hot for too long.

Reduce Time / temp

• Nickel stains: Contaminated or old sealing bath.

Filter or replace bath

Equipment & Safety Precaution: –

• Sealing tank should be made up of SS-316, Titanium, or PP/PPH.
•  heating should be done through hot water generator or an immersion heater.
• All equipment like (Thermostat & pH meter should be calibrated)
• Use gloves and face shield (boiling water hazard)
• Ensure exhaust for steam and fumes.
• Keep electrical equipment away from splashes.
• Neutralize wastewater before discharge.