Hard anodizing provides greater resistance to corrosion through processes such as longer oxidation times. Common hard anodizing colors are black, gray, and bronze. This article will explore how this process works and why there are color limitations..
What is Hard Anodizing?
Hard anodizing, also known as Type III anodizing(hard coat anodizing), is an electrochemical process that forms a protective oxide film or layer on aluminum (and other non-ferrous metals).
The process requires specific conditions to be carried out, such as keeping the bath temperature where the metal is immersed between 28°F to 32°F. The direct current (DC) voltage applied starts at a lower value (around 25V) and then gradually increases to a higher range (between 60-100V). If done correctly, the surface hardness achieved can reach up to 60 Rockwell C. Anodized parts can become highly durable, offering excellent corrosion resistance.
Color Range for Hard Anodizing
As mentioned earlier, hard anodizing produces darker, more subdued colors compared to decorative anodizing. The range of colors achievable in hard anodizing includes:
- Black
- Dark Grey
- Bronze
These colors are typically used in industrial applications where the focus is more on performance and durability than on aesthetics. However, they still provide a clean and professional look, making hard anodized parts suitable for visible components in consumer products, such as electronic devices or outdoor equipment.
How Does It Work?
The first step in this process is cleaning the aluminum (or any other metal being used) with a solution of sodium hydroxide (also called a caustic solution), diluted with distilled water. The natural oxide layer already present on the aluminum needs to be removed.
The metal is then connected to an electrolyte (usually a 50/50 mixture of water and sulfuric acid), immersed in the solution for around 45 minutes. Afterward, the anodized metal is removed from the sulfuric bath (in a well-ventilated area), dipped into a hot dye bath for 15 minutes, and sealed with a chemical sealant.
Types of Anodizing
Apart from Type III anodizing, there are also Type I and Type II anodizing. The following table explains their key differences:
| Type | Electrolyte | Color Range | Characteristics | Applications |
|---|---|---|---|---|
| Type I | Chromic acid | Limited (e.g., grey, black) | Thin non-conductive oxide layer, better adhesion for painting, high fatigue strength, maintains substrate integrity | Aerospace, military equipment needing corrosion resistance |
| Type II | Sulfuric acid | Gold, bronze, blue, green, red, silver, black | Thicker layer, leaves porous anodized finish, allows dye absorption before sealing | Consumer goods, home equipment, automotive parts |
| Type III | Sulfuric acid | Dark tones (black, grey, dark) | Thickest of the three, wear-resistant, suitable for harsh environments | Industrial parts, heavy machinery, outdoor equipment |
Properties of Hard Anodizing:
- Thickness: Hard anodizing results in anodized aluminum thicknesses ranging from 25 to 125 microns. Standard anodizing (Type II) only produces a thickness of 5-20 microns, much thinner than hard anodized aluminum. These thicker layers on aluminum surfaces make it ideal for pistons and gears, which are high-friction parts.
- Surface Hardness: Hard anodized surfaces can achieve a surface hardness of 60-70 Rockwell C, close to the hardness of hardened steel!
- Anodized Layer: The process takes place at freezing temperatures, ensuring a more uniform and denser anodized coating.
- Color Range: Due to the density of anodized aluminum, the color options are limited to darker shades, such as black, grey, and bronze. These colors are not as vibrant as those produced by Type II anodizing.
Conclusion
Hard anodizing(hard coat type iii anodize) is a special anodizing process where a thicker oxide layer provides greater durability, wear resistance and corrosion protection for aluminum parts. The color range is limited, usually black, gray, bronze, but hard anodizing is well suited for industrial and high-performance applications.
References:
Wikipedia: Anodizing
Anodizing.com: Aluminum Anodizing Interference Coloring
