How Does Aluminum Anodizing Work? Properties and Film Formation

Properties and Uses of Anodic Oxide Films

Anodizing is a process in which a metal serves as the anode in a suitable electrolyte and an applied current forms an oxide film on its surface. Different electrolyte types and concentrations and different process conditions produce films with different properties and thicknesses ranging from tens to hundreds of micrometers. By comparison, the natural oxide film on aluminum is 0.010 to 0.015 μm thick.

Porous Structure

An anodic film has a porous honeycomb structure. Porosity depends on the electrolyte and process conditions. The pores adsorb organic substances, resins, paraffin wax, inorganic substances, dyes, and paints. The film can therefore serve as a base for coatings or plating and can be dyed to improve appearance. Sealing is required to increase corrosion resistance.

Wear Resistance

The high hardness of the film improves surface wear resistance. Adsorbed lubricant can further improve wear performance.

Corrosion Resistance

The film is stable in the atmosphere and provides good corrosion resistance. Performance depends on film thickness, composition, porosity, substrate composition, and structural integrity. Anodized films are normally sealed or painted to improve corrosion resistance.

Electrical Insulation

An anodic film has high insulation resistance and breakdown voltage and can serve as the dielectric in an electrolytic capacitor or as an insulating layer on electrical products.

Thermal Insulation

The film is a good thermal insulator and remains stable up to 1500°C. On parts exposed briefly to high temperatures, it can help prevent the aluminum from melting. Thermal conductivity is generally 0.419 to 1.26 W/(m·K).

Adhesion

Because the film grows directly from the substrate, it bonds very strongly and is difficult to remove mechanically. Even when an oxidized sheet-metal part is bent until it breaks, the film remains attached. Its adhesion is therefore much stronger than that of a deposited coating.

Transparency

Anodic oxide films are highly transparent, and transparency increases with aluminum purity. Both material purity and alloy composition affect transparency.

Functional Properties

Functional particles can be deposited in the pores to create materials with electromagnetic, catalytic, sensing, separation, and other functions.

Formation Mechanism of Anodic Oxide Films

Aluminum and aluminum alloys are generally anodized in acidic solutions with moderate dissolving power. Lead can serve as the cathode and mainly conducts current. The following reactions occur at the anode:

H₂O → [O] + 2H⁺ + 2e⁻

2Al + 3[O] → Al₂O₃

The following reaction occurs at the cathode:

2H⁺ + 2e⁻ → H₂↑

The acidic electrolyte also dissolves the aluminum substrate and the oxide film chemically:

2Al + 6H⁺ → 2Al³⁺ + 3H₂↑

Al₂O₃ + 6H⁺ → 2Al³⁺ + 3H₂O

Film growth and dissolution occur simultaneously. Early in anodizing, growth is faster than dissolution, so film thickness increases. As the film thickens, its electrical resistance rises, current decreases, and growth slows. When growth and dissolution reach dynamic equilibrium, thickness stabilizes.

The voltage-time curve also explains the film-growth sequence.

1. Stage a: barrier-layer formation. In section AB, voltage rises sharply from zero to a maximum called the critical voltage during the first few seconds to tens of seconds. A continuous, thin, nonporous film forms on the anode and impedes further thickening. Barrier-layer thickness is proportional to formation voltage and inversely proportional to dissolution rate in the electrolyte. Thickness is 0.01 to 0.1 μm.
2. Stage b: porous-layer formation. In section BC, the thinnest locations dissolve first to form cavities. Electrolyte reaches fresh aluminum through these cavities and the electrochemical reaction continues. Resistance decreases and voltage drops by 10% to 15% of the maximum, indicating that the barrier film is dissolving and a porous layer is forming.
3. Stage c: porous-layer thickening. In section CD, after about 20 seconds the voltage enters a stable, slowly rising stage. As the barrier layer continually dissolves into a porous layer, a new barrier layer grows. Film formation and dissolution continue at the bottom of each cell. When their rates reach dynamic equilibrium, extending the anodizing time no longer increases thickness and the process should be stopped.