Anodizing is an electrochemical process used to improve the corrosion resistance, surface hardness and appearance of aluminum and other nonferrous metals. Unlike an applied coating, it converts the metal surface itself into a dense oxide film that remains integral to the substrate. The process is widely used in precision manufacturing, where finish durability and consistency are critical.
Anodizing versus electroplating
Anodizing is not a simple coating. Electroplating deposits a metal layer onto a substrate, while anodizing uses an electrochemical reaction to convert the base metal into a dense oxide film. That film is part of the metal surface and is valued for its strong adhesion and durability.
How the process works
Anodizing takes place in an electrolytic cell. The basic steps include surface preparation, electrolytic oxidation, and coloring and sealing.
- Surface preparation: Mechanical grinding, sandblasting or chemical cleaning removes oil, grease and other contaminants and helps ensure a uniform substrate.
- Electrolytic oxidation: The workpiece is immersed in an electrolyte, such as sulfuric, chromic or phosphoric acid, and serves as the anode. Current drives an oxidation reaction that forms the oxide film.
- Coloring and sealing: The porous film can be dyed or electrolytically colored, then sealed to improve corrosion resistance and wear resistance.
Each step affects the uniformity, thickness and performance of the final film.
Common types and applications
Anodizing is classified by electrolyte and film properties. The main types differ in thickness, hardness and intended use.
| Type | Electrolyte | Characteristics and applications |
|---|---|---|
| Type I | Chromic acid | Thin film used for decorative and specific functional requirements. |
| Type II | Sulfuric acid | Most common type, offering good corrosion resistance and decorative effects. |
| Type III | Sulfuric acid (hard) | High density and high hardness for applications requiring strong wear resistance. |
| Phosphoric acid anodizing | Phosphoric acid | Mainly used for surface pretreatment to improve adhesion of subsequent coatings. |
Key process controls
Finish quality depends on several variables, including electrolyte concentration, temperature, pH and current density. Contaminants that accumulate in the bath, such as metal ions above 20 g/L, can significantly reduce process efficiency. Bath condition therefore must be monitored closely and maintained through filtration and chemical neutralization.
Consistent control of these parameters is essential for reproducible film thickness, corrosion resistance and surface appearance.
