ANODIZING (TYPE II or DECORATIVE)
Type II anodic coatings are used to protect and color aluminum parts for decorative purposes only, where there are no requirements for enhanced durability. Typically, these are thin, highly porous, clear coatings that will easily receive dyes. These coatings can be dyed in a wide spectrum of colors.
Decorative anodic coatings increase the thickness of aluminum&s naturally occurring surface oxide layer. These natural oxide layers are generally 2 to 3 nanometers thick; Type II anodizing creates oxide layers that range in thickness from 1.8 μm* to 25 μm (0.00007" to 0.001"). When applied properly, decorative anodic coatings are very regular and uniform across the surface of the treated object.
Type II anodizing is an abbreviation derived from the military standard Mil-A-8625 Type II category.
* μm = micrometer (1000x the length of a nanometer)
In Type II anodizing, the oxide layer is created by passing direct current (DC) through an electrolytic solution of diluted sulfuric acid. The aluminum object serves as the positive electrode (or anode, from which the term "anodizing" originates). The electrical current causes the release of hydrogen at the negative electrode (cathode) and oxygen at the surface of the aluminum object, creating a buildup of aluminum oxide. Decorative anodic coatings are formed using high voltage and low current density (<15 amps/ft2).
Type II coatings can be sealed by hydrothermal or precipitation processes, or left unsealed. Choice of seal depends on the object’s final application, dye selection, and how it will be used.
Decorative Type II anodizing is used for applications where:
- Corrosion protection is required
- For low coefficient of friction.
- Coloring the aluminum is desired
- MIL-A-8625 Type II
- MIL STD 171, 7.2.1 & 7.2.2
- ASTM B580, Type B
- AMS 2471 (Undyed) / 2472 (Dyed)
- BS EN ISO 12373
Note that each specification has specific processing requirements.
- Are their limits to the color range for Decorative anodizing?
- No. The range is nearly infinite, although in practice, the number of dyes is limited as are the dye set up conditions. All colors are available, except white.
- What Seals Can Be Used for Decorative Anodizing?
- Both hydrothermal and precipitating seals. Choice of seal depends on end application.
- What Is the Coating Thickness for Decorative Anodizing?
- Decorative anodizing coating thickness can be 2 to 25 μm (.0001” to .001”).
- How Are Parts Racked for Decorative Anodizing?
- Type II rectification normally requires < 15 Amps per square foot current density . This in turn requires a rack that can hold the current density and hold the part securely to prevent arching. Typically conventional anodizing will result in a less visible rack mark than rack marks after Type III anodizing, where double current density is used.