Anodizing Sealing Process

Anodized aluminum meaning

Anodized aluminum refers to the formation of an oxide film on the surface of aluminum through the anodizing process. This oxide film not only enhances the corrosion resistance of aluminum, but also improves its wear resistance and allows further surface treatment, such as coloring.

Does anodized aluminum need to be sealed?

The oxide film formed during the anodizing process of aluminum is porous, with a porosity between 5% and 30%.

Unsealed anodized aluminum means that the pores in the oxide film have not been sealed. Although these pores are helpful for some applications, such as allowing dyes to enter for coloring, they also bring some problems.

• Reduced corrosion resistance: Due to the presence of pores, moisture and contaminants can penetrate the aluminum surface, causing corrosion.
• Poor wear resistance: Contaminants in the pores will accelerate the wear process and reduce the wear resistance of aluminum.
• Appearance degradation: Pores can accumulate dirt and dyes, causing the surface to darken or discolor, affecting the appearance.
• Chemical adsorption: Pores can adsorb chemicals in the environment, which may affect the performance of aluminum.
• Mechanical property degradation: The presence of pores reduces the strength and stiffness of the oxide film, affecting its mechanical properties.

Therefore, unsealed anodized aluminum requires additional protection measures to ensure its long-term stability and aesthetics.

In order to avoid the uncertainty brought by unsealed anodized aluminum, sealing treatment is particularly important, which can significantly improve the performance of the film layer.

Overview of sealing methods

Hot water sealing: Through high-temperature water treatment, the oxide film reacts with water to form hydrated aluminum oxide (boehmite) , and the volume expands to close the pores.

Steam sealing: Through steam treatment, the oxide film reacts with steam to form hydrated aluminum oxide (boehmite) .

Metal salt sealing: Use inorganic salt solution to seal under heating conditions, and fill the pores through hydration and salt hydrolysis products.

Organic coating sealing: Use organic coatings such as transparent paint, resin, etc. to seal, providing good wear resistance, corrosion resistance and decorative properties.

Next, we will discuss each method in detail.

Hot water sealing (HWS)

Hot water sealing is a method of sealing pores by reacting anodized aluminum with hot water to convert aluminum oxide into hydrated aluminum oxide (boehmite) .

The basic principle of hot water sealing is to contact the oxide film with water at high temperature to generate hydrated aluminum oxide (boehmite) , which expands in volume and seals the pores.

The reaction formula is as follows:

Al₂O₃ + nH₂O → Al₂O₃·nH₂O

where n can be 1 or 3. When n=1, the volume of the generated monohydrated aluminum oxide (boehmite) increases by about 33%; when n=3, the volume of the trihydrated aluminum oxide (boehmite) increases by nearly 100%. This significant increase in volume effectively seals the membrane pores.

Implementation details:

• Medium: hot water
• Temperature: hot water sealing needs to be above 95°C
• Time: hot water sealing for 20-40 minutes
• Water quality requirements: use deionized water or distilled water to avoid impurities affecting the performance of the membrane layer.
• Features: hot water sealing equipment is simple, easy to operate and low cost;

Steam sealing

The principle of steam sealing is similar to that of hot water sealing. It is a method of sealing pores by reacting anodized aluminum sheet with steam to convert anodized aluminum into hydrated aluminum oxide (boehmite) .

• Medium: steam
• Temperature: The steam sealing temperature is about 100-115°C.
• Time: steam sealing for 20-30 minutes
• Features: Steam sealing has better quality and the colored film is not easy to flow color, but the steam sealing equipment is complex and the cost is high.

Metal salt sealing (MSS)

Metal salt sealing (MSS) achieves the sealing effect by immersing anodized aluminum in a heated metal salt solution and filling the pores with hydration and salt hydrolysis products. Commonly used metal salts include dichromates, molybdates, silicates, etc.

Dichromate sealing

Dichromate sealing is a process that seals the pores on the surface of anodized aluminum by treating with a dichromate solution

• Principle: Alumina reacts with dichromate to form hydrated aluminum oxide (boehmite) and aluminum chromate, which seal the pores.
• Reaction formula: 2Al₂O₃ + 3K₂Cr₂O₇ + 5H₂O → 2Al(OH)₃CrO₄ + 2Al(OH)₃Cr₂O₇ + 6KOH
• Features: Improves corrosion resistance, but makes the film appear light yellow, which is not suitable for products with high decorative requirements.

Hydrolysis salt sealing

Hydrolysis salt sealing is a sealing technology for anodized aluminum, especially for anodized films after dyeing.

By heating in a neutral solution, a hydrolyzable metal salt (such as nickel sulfate or nickel acetate) is used to form hydroxide precipitation in the pores of the oxide film, thereby effectively filling and sealing the pores.

• Principle: Use a nearly neutral salt solution, such as nickel sulfate, to hydrolyze in the membrane pores to generate nickel hydroxide and other precipitations to fill the pores.
• Reaction formula: NiSO₄ + 2H₂O → Ni(OH)₂·H₂O + H₂SO₄
• Features: It does not affect the color of the film layer, is suitable for decorative films, and can enhance the stability of dyes.

Cold sealing process

Cold sealing of anodized is a sealing treatment method performed at room temperature. Specifically, it is to place the anodized film in deionized water containing a sealing agent for treatment

Cold sealing technology mainly achieves sealing by depositing filler materials in the micropores of the oxide film, rather than relying on the volume expansion of boehmite generated by hydration reaction.

This method is usually carried out at room temperature of 20-25℃. Compared with traditional boiling water sealing, cold sealing can save energy and time.

• Principle: At room temperature or slightly above room temperature, a sealing solution containing metal salts such as nickel and cobalt is used to generate hydroxides through hydration and hydrolysis of metal ions to seal the pores.
• Features: Friendly operating environment, low energy consumption, low environmental pollution, suitable for a variety of aluminum and aluminum alloy products.

Organic coating sealing

Organic coating sealing achieves sealing and decorative effects by coating organic coatings such as transparent paint, resin, etc. on the surface of aluminum and aluminum alloys to form a dense paint film.

Implementation details:

• Method: Electrostatic spraying, powder coating or electrophoretic painting.
• Advantages: Good film quality, wide color range, high hardness, excellent wear resistance and corrosion resistance, and high brightness.

Application Examples

Take Worthwill as an example. Its process flow of sulfuric acid anodizing, stannous salt AC electrolytic coloring and room temperature sealing is as follows:

1. Pretreatment: degreasing, alkaline etching, and polishing.
2. Anodizing: carried out in sulfuric acid solution, temperature 25-35°C, current density 1.0-1.5 A/dm², time 40-50 minutes.
3. Electrolytic coloring: carried out in a solution containing stannous sulfate, sulfuric acid and stabilizer, voltage 12V, frequency 50Hz, time 2-3 minutes.
4. Room temperature sealing: use sealing liquid with nickel acetate, sodium fluoride and other ingredients, temperature 25-55°C, time 10-15 minutes.
5. Performance test: The good corrosion resistance and coloring stability of the product are verified by hydrochloric acid and sodium hydroxide immersion test and neutral salt spray test.