Anodizing is a surface treatment procedure that enhances the durability and appearance of metals, especially aluminum. One key effect of anodizing is that it adds a thin, protective oxide layer to the surface. This layer resists corrosion and wear and provides a base for coloring and improved aesthetics. Understanding how much thickness anodizing adds is essential for precise manufacturing, as even a small increase can affect fit, tolerance, and performance.
This comprehensive guide explains anodizing, the different types, and the thickness it adds to anodized metal.
What Is Anodizing?
Anodizing is an electrochemical process that changes a metal surface into a beautiful anodic oxide finish. The anodic oxide structure consists of a layer of aluminum oxide on the surface, slightly increasing the metal’s thickness.
Aluminum oxide does not adhere to the surface in the same way as paint or plating does. It is connected with the underlying aluminum substrate, which prevents chipping and peeling. Its porous structure enables subsequent operations like coloring and sealing. Anodizing involves soaking aluminum in an acid electrolyte bath and sending an electric current.
A cathode is positioned inside the anodizing tank. The aluminum acts as an anode, producing oxygen ions from the electrolyte that join with the aluminum atoms on the surface of the anodized metal. Anodizing is a form of carefully controlled oxidation.
Producing the intended result requires both specialized talent and an accurate grade of metal, making it a difficult technique to master! As a result, many producers avoid the approach and only offer plated finishes despite the restricted variety of finishes available through this process.
Types of Anodizing
There are several types of anodizing, each with its specific application and impact on thickness.
Sulfuric Acid Anodizing
This is the most widely used anodizing process for aluminum and other non-ferrous metals. Sulfuric acid anodizing creates an oxide layer by immersing the metal in a sulfuric acid solution and applying an electric current. The layer formed is relatively porous, allowing for easy dyeing and coloring, a significant advantage for decorative applications. The anodised metal layer improves the metal’s resistance to corrosion, wear, and fading, making it ideal for parts exposed to harsh environments, such as architectural components, automotive, and aerospace.
Hardcoat Anodizing
Hardcoat anodizing uses a similar sulfuric acid process but operates at higher voltages, producing thicker oxide layers. This thicker oxide layer is harder, making it much more resistant to scratching, wear, and environmental damage. As a result, hard coat anodizing is often used in applications that require superior durability, such as military, aerospace, and industrial equipment. It also improves the wear resistance of aluminum parts in heavy-duty machinery or tools.
Hardcoat anodizing also produces a more electrically insulative surface, which can benefit specific electronic applications. The hard oxide layer is usually gray or black, which is also desirable for some industrial uses, although it can be dyed in some cases.
Chromic Acid Anodizing
Chromic acid anodizing is a gentler process than sulfuric acid anodizing. The treatment uses chromic acid as the electrolyte and operates at lower voltages and temperatures. This type of anodizing is favored in aerospace and military applications where maintaining the metal’s mechanical properties is critical.
The thinner oxide layer produced by chromic acid anodizing distorts the base material less, preserving its strength and dimensional accuracy. It also produces a less visible coating, useful when the goal is minimal visual impact or when working with high-precision components.
While the layer is thinner, it still offers good corrosion resistance, though not as robust as hard coat anodizing. Chromic acid anodizing is especially used in applications where weight is a concern, and the part needs to maintain its structural integrity.
Phosphoric Acid Anodizing
Phosphoric acid anodizing involves using phosphoric acid as the electrolyte and producing a relatively thin oxide layer. The process creates a non-reflective, dull finish, which makes it ideal for military, aerospace, and defense components.
One of the main benefits of phosphoric acid anodizing is its ability to enhance bonding and paint adhesion. The oxide layer is more chemically active, allowing for better paint, adhesives, or primer bonding. As a result, this type of anodizing is often used in applications where the metal will be further coated or painted.
Additionally, phosphoric acid anodizing improves corrosion resistance and provides a suitable surface for coatings that withstand high temperatures or extreme conditions. However, it doesn’t provide the same wear resistance as sulfuric or hard coat anodizing, so it is more suited for parts that do not undergo heavy mechanical stress.
Organic Anodizing
Organic anodizing is a variation of the sulfuric acid anodizing process where organic dyes are added to create vibrant, colorful finishes. The oxide layer created during organic anodizing is typically used for decorative purposes, such as in consumer products, jewelry, and architectural finishes. The colors can range from gold to blue, red, and even black, and the process allows for excellent color consistency and durability.
While organic anodizing offers an aesthetically pleasing finish, it doesn’t provide as much wear resistance as hard coat anodizing. However, it still offers good corrosion resistance, and the process is often used when appearance is a primary consideration.
How Much Thickness Does Anodizing Add
The thickness anodizing adds varies depending on the type of anodizing process used. The oxide layer on the surface of the metal can range from a very thin layer for decorative purposes to a much thicker layer for industrial applications. Here’s an overview of how much thickness anodizing usually adds:
Sulfuric Acid Anodizing
In standard sulfuric acid anodizing, the oxide layer thickness typically ranges from 0.0002 to 0.001 inches (5 to 25 microns). This process is commonly used for decorative applications, such as automotive and architectural industries, and provides moderate corrosion resistance.
Hardcoat Anodizing
Hardcoat anodizing produces a much thicker oxide layer, typically ranging from 0.002 to 0.004 inches (50 to 100 microns). This process is used in applications requiring high wear resistance, such as in aerospace, military, and industrial equipment. The thicker layer also increases hardness, improving the part’s durability.
Chromic Acid Anodizing
Chromic acid anodizing creates a thinner oxide layer, usually ranging from 0.0001 to 0.0003 inches (2.5 to 7.5 microns). This type of anodizing is ideal when you need a balance of corrosion resistance without altering the material’s mechanical properties too much. It’s commonly used in aerospace components where weight and precision are critical.
Phosphoric Acid Anodizing
The oxide layer manufacture by phosphoric acid anodizing typically ranges from 0.0002 to 0.001 inches (5 to 25 microns). This type of anodizing is used when parts will be further coated, painted, or bonded, as the layer enhances adhesion and provides corrosion resistance. It’s popular in military and aerospace applications.
Organic Anodizing
Organic anodizing, often used for decorative purposes, results in an oxide layer about 0.0005 to 0.001 inches (12.5 to 25 microns) thick. The process adds a layer of color through organic dyes, making it a good choice for consumer products, jewelry, and architecture.
Here are the factors that Influence Thickness
- Electrolyte type: Different anodizing processes use different chemicals, affecting the oxide layer thickness.
- Voltage and temperature: Higher voltages and temperatures generally lead to thicker oxide layers.
- Duration: The longer the anodizing process, the thicker the oxide layer.
- Current density: A higher current density can increase the oxide layer thickness, particularly in hard coat anodizing.
Conclusion
Anodizing is an effective and versatile surface treatment that enhances the durability and appearance of metals, especially aluminum. The thickness of the anodized layer varies depending on the process used. Anodizing provides a reliable solution, whether seeking improved corrosion resistance, enhanced wear properties, or aesthetic appeal. This guide will help you learn the thickness variations in different anodizing processes, ensuring that the right approach is chosen for your specific requirements.
