Aluminum Anodizing: Everything You Need to Know

Aluminum anodizing involves placing aluminum or its alloy workpieces in a suitable electrolyte, with the aluminum acting as the anode and another metal material as the cathode. Under an applied electric current, an oxide film forms on the surface of the aluminum. 

By selecting different types and concentrations of electrolytes and controlling process conditions during oxidation, anodizing oxide films with varying properties and thicknesses ranging from tens to hundreds of micrometers can be achieved. These films exhibit significantly improved corrosion resistance, wear resistance, and decorative qualities compared to chemical oxide films.

The electrolyte used for anodizing of aluminum and its alloys is an acidic solution with moderate dissolving capacity. During the anodizing process, two simultaneous reactions occur. On one hand, the aluminum workpiece, acting as the anode, reacts with oxygen atoms generated from water hydrolysis to form an oxide film (Al₂O₃) through an electrochemical reaction, as shown below:

H₂O – 2e ^– → [O] + 2H⁺

2Al + 3[O] → Al₂O₃

On the other hand, the electrolyte continuously dissolves the newly formed Al₂O₃ film, with the reaction:

Al₂O₃ + 6H⁺ → 2Al³⁺ + 3H₂O

The growth of the oxide film is a dynamic process of continuous formation and dissolution. A thicker oxide film is achieved only when the formation rate exceeds the dissolution rate. A porous, honeycomb-like structure with a two-layer composition characterizes the surface of the anodizing oxide film on aluminum and its alloys. The inner layer, adjacent to the substrate, is a dense Al₂O₃ film with a thickness of 0.01–0.05 μm, commonly referred to as the barrier layer, which exhibits high hardness. The outer layer is a porous oxide film composed of Al₂O₃ with crystalline water, which is less hard but has excellent adsorption properties.

Properties of Aluminum Anodized Films

The micropores in a porous anodizing oxide film are regularly arranged, perpendicular to the metal surface, forming a tubular structure. For example, in a sulfuric acid anodizing oxide film with a thickness of 10 μm, the pore diameter is typically less than 20 nm, making the pore length over 500 times its diameter. These pores resemble elongated, straight tubes. The pore density can reach up to 76 billion pores per square centimeter—the number of pores on an area the size of a thumbnail is about ten times the global population.

The porous structure and morphology of anodizing oxide films are typically observed directly using high-resolution scanning electron microscopy, which reveals the film’s characteristics from various angles.

Aluminum Anodizing Process

The typical process for anodizing of aluminum and aluminum alloys includes the following steps:

Surface leveling → Racking → Chemical degreasing → Rinsing → Neutralization → Rinsing → Alkaline etching → Rinsing → Anodizing → Rinsing → Dyeing or electrolytic coloring → Rinsing → Sealing → Mechanical polishing → Inspection

This is a standard process, but specific steps may be adjusted or omitted based on the requirements and the specific anodizing method used.

Table: Key Parameters for Sulfuric Acid Anodizing

Step	Process	Solution Composition	Process Parameters
1	Degreasing	2% Na3PO4 1% Na2CO3 0.5% NaOH	Temperature: 45–60°C Time: 3–5 min
2	Hot water rinse	Tap water	Temperature: 40–60°C Time: Until clean
3	Alkaline etching	40–50 g/L NaOH	Temperature: Room temperature Time: 1–5 min
4	Cold water rinse	Tap water	Temperature: Room temperature Time: Until clean
5	Neutralization	10–30% HNO3	Temperature: Room temperature Time: 3–8 min
6	Sealing	Pure water	Temperature: ≥ 90°C Time: > 20 min pH: 4–6

Table: Post-treatment Specifications for Chemical Oxidation

Process	Solution Composition (g/L)	Process Parameters	Remarks
Filling treatment	Potassium dichromate: 30–50	90–95 °C 5–10 min	For acidic oxidation
Passivation treatment	Potassium chromate: 20	Room temperature 5–15 s	For alkaline oxidation

Sealing for Aluminum Anodized Films

Due to the porous structure and strong adsorption properties of anodizing oxide films, the surface is prone to contamination, particularly by corrosive media that can enter the pores and cause corrosion. Therefore, after the formation of the anodizing oxide film, whether dyed or not, sealing treatment is necessary to close the pores, enhancing corrosion resistance, insulation, and wear resistance while reducing the adsorption of impurities or oils. Common sealing methods include hot water sealing, steam sealing, dichromate sealing, hydrolysis sealing, and filling sealing.

(1) Hot Water Sealing

Hot water sealing involves treating a freshly formed anodizing oxide film in boiling or near-boiling water for a specific duration. This deactivates the film, preventing further dye adsorption and stabilizing any applied color. The principle relies on the hydration of amorphous Al₂O₃

Al₂O₃+ nH₂O → Al₂O₃ · nH₂O

Here, n is typically 1 or 3. When Al₂O₃ hydrates to form monohydrate (Al₂O₃ · H₂O), its volume increases by about 33%. For trihydrate (Al₂O₃ · 3H₂O), the volume nearly doubles. This hydration causes the pores to close as the volume of Al₂O₃ on the surface and pore walls expands.

Hot Water Sealing Process

• Water temperature: 90–110°C
• pH value: 6–7.5
• Time: 15–30 min

The water used must be distilled or deionized, as tap water can reduce the transparency and luster of the oxide film.

(2) High-temperature Steam Sealing

High-temperature steam sealing operates on the same hydration principle as hot water sealing, causing the porous film to close due to volume expansion. Its advantages include:

1. Faster sealing and higher efficiency compared to hot water sealing.
2. Less dependence on water quality and pH.
3. Reduced the occurrence of white ash, a common issue in hot water sealing.
4. Lower risk of dye leaching or fading, making it suitable for dyed anodizing oxide films.

The key to steam sealing lies in equipment design and airtightness to ensure the required temperature and humidity. The temperature must exceed 100°C, typically 115–120°C, with steam pressure controlled at 0.7–1 atm (1 atm = 10⁵ Pa). Condensation on the surface must be avoided. From chemical kinetics, a 10°C temperature increase can boost the reaction rate by approximately 30%. However, rapid heating (ideally within 5 minutes) and good insulation are critical, requiring sophisticated equipment. The high cost of constructing and operating steam sealing systems limits their widespread use.

(3) Dichromate Sealing

Dichromate sealing is performed in a strongly oxidizing potassium dichromate solution at elevated temperatures. The reaction between the Al₂O₃ on the pore walls and potassium dichromate (K₂Cr₂O₇) is as follows:

2Al₂O₃ + 3K₂Cr₂O₇ + 5H₂O → 2AlOHCrO₄ + 2AlOHCr₂O₇ + 6KOH

The resulting basic aluminum chromate, basic aluminum dichromate, and hydrated alumina collectively seal the micropores. The process details are as follows:

Sealing Solution	Potassium dichromate	50–70 g/L
Process Parameters	Temperature	90–95°C
	Time	15–25 min
	pH value	6–7

The treated oxide film appears yellow and offers excellent corrosion resistance, making it suitable for protective anodizing but not for decorative dyed films.

(4) Hydrolysis Sealing

Hydrolysis sealing is widely used in China, particularly for sealing dyed oxide films, as it overcomes many drawbacks of hot water sealing. The process involves adsorbing easily hydrolyzed cobalt or nickel salts into the oxide film’s pores, where they undergo hydrolysis to form hydroxide precipitates that seal the pores:

Ni₂ + 2H2O → Ni(OH)₂↓ + 2H⁺

Co₂ + 2H₂O → Co(OH)₂↓ + 2H⁺

The resulting nickel and cobalt hydroxides, which are nearly colorless and transparent, seal the pores without affecting the film’s original color, making this method ideal for dyed films.

(5) Filling Sealing

In addition to the above methods, anodizing oxide films can be sealed with organic materials such as clear varnish, molten paraffin, resins, or drying oils. For example, silicone oil can enhance insulation in hard anodizing oxide films, silicone grease can create dust-free surfaces, and fatty acids or high-temperature greases can be used for infrared reflectors to prevent absorption losses in the 4～6 μm wavelength range. Various organic sealing agents have been developed for specific applications.