Everything You Need to Know About Aluminum Anodizing

Aluminum anodizing is a finishing process that produces wear and corrosion-resistant aluminum oxide coating on the surface of aluminum parts. This coating can be colored after anodizing for a decorative finish.

Aluminum anodizing is an electrolytic process that takes place in an electrolyte such as dilute sulfuric acid. A current is passed through the part, causing negatively charged oxygen ions from the electrolyte to be attracted to the positive charge aluminum atoms produced at the metal surface. The oxygen anions react with aluminum cations to create a strongly adherent aluminum oxide layer. There are three different anodizing processes: Type I (chromic acid anodizing), Type II (sulfuric acid anodizing), and Type III (hard coat anodizing).

This article will describe the anodizing process in detail, explaining its advantages and disadvantages, as well as how to tell if an aluminum part has been anodized. Figure 1 is an example of anodized aluminum parts:

What is Aluminum Anodizing?

Aluminum anodizing is an electrolytic process used to increase the thickness of the tightly adhering oxide layer that forms naturally on any aluminum surface exposed to air. The anodized layer has a porous, ordered structure. This porosity makes it easy to perform secondary processes on anodized parts, such as coloring them with dye or sealing their surfaces. The anodized oxide layer acts as a barrier to protect the aluminum better from corrosion and wear compared to aluminum’s natural oxide. Aluminum anodizing is a widely used finishing process because it is inexpensive, durable, and does not require special skills or equipment.

What is the Purpose of Aluminum Anodizing?

The purpose of anodizing aluminum is to increase its wear and corrosion resistance. Aluminum is a popular metal for manufacturing products from cookware to car parts, because it is strong, yet still lightweight. However, aluminum is also highly susceptible to corrosion and wear when the corrosive potential of the environment increases like exposure to seawater and other extreme conditions. To prevent this, manufacturers often anodize the metal, which creates a thin oxide layer that protects against corrosion and wear. Anodized aluminum is also often used for its aesthetic properties, as the anodization process can produce a variety of colors. 

Where is Anodized Aluminum Used?

Aluminum anodizing is used wherever aluminum components may be exposed to corrosive or hard-wearing applications, such as automotive parts, bicycles, and outdoor furniture. Anodized aluminum can be easily dyed to produce a scratch-resistant colored surface. It is therefore used for many consumer products, to both improve their appearance and increase their durability. Examples of applications include architectural cladding, aluminum canoes, boats, and kitchen utensils. Anodizing will also turn the aluminum into an insulator since the oxide coating is not conductive.  

How Does Aluminum Anodizing Work?

Aluminum anodizing builds on the natural tendency of aluminum to form a thin oxide coating on its surface. The anodizing process makes the coating thicker and more consistent, improving its protective qualities in applications with more demanding requirements for resisting corrosion and wear. Before aluminum can be anodized, it must be cleaned, and the naturally occurring oxide coating must be etched away. 

To anodize the clean aluminum surface, it is immersed in a tank containing an electrically conductive solution. The electrolyte completes the circuit between the aluminum anode and an inert cathode (made of a material such as carbon), which can conduct electricity but will not react with the electrolyte. The electrolyte is typically sulfuric or chromic acid depending on the type of aluminum anodizing and serves to aid the rate of addition of the anodic layer.

The anodizing process involves an anodizing tank, an anode (positive electrode), and a cathode (negative electrode). A direct current is passed through the anodizing tank. The aluminum gives up electrons from its surface, leaving positively charged aluminum ions. Electrons leaving the cathode participate in producing negatively charged oxygen ions, which migrate to the aluminum surface and combine with the aluminum ions to form a thin layer of aluminum oxide. The thickness of this layer can be controlled by adjusting the current density, time, temperature, and electrolyte solution concentration. 

The first layer of the formed oxide, called the barrier layer, will be continuous, with no pores. However, as the oxide layer continues to build up, it will impede the flow of current. A series of attachment points will then form on the barrier layer which ultimately forms a series of cylindrical pores which are oriented perpendicularly to the barrier layer. The current will be distributed radially outwards from the center of the pore, meaning the subsequent oxide layer will radiate outward until it reaches the oxide layers of the surrounding pores as shown in Figure 2: 

How To Tell if Aluminum is Anodized

There are a few simple tests that can be performed to determine whether an aluminum part has been anodized or not:

1. Scratch Resistance Test: Try to scratch the surface of the metal with a sharp object. If the surface is anodized, it will be harder to scratch than uncoated aluminum. 
2. Eddy Current Test: Using an eddy current thickness tester is a good method to not only check whether a part is anodized but to also measure the thickness of the coating.
3. Conductivity Test: All that is required to conduct this test is a basic voltmeter.

What Are the Types of Aluminum Anodizing Processes?

There are three types of aluminum anodizing processes. They are described in more detail below:

1. Type I - Chromic Acid Anodizing: This process uses chromic acid as the electrolyte and produces the thinnest coating of all the methods, 2.5 μ (0.0001 in). Despite the reduced thickness, this process produces comparable corrosion resistance to the other two processes. The coating produced tends to be darker and does not accept color as well due to its reduced thickness and reduced porosity.
2. Type II - Sulfuric Acid Anodizing: This process uses dilute sulfuric acid as the electrolyte. It is the most commonly used technique. The coating thickness ranges from 5.1 to 30.5 μ (.0002-.0012 in). A common industry specification Xometry and many other manufacturers follow is MIL-A-8625 Type II, Class 1 (non-dyed) or Class 2 (dyed). This coating is harder than the one produced by chromic acid anodizing. Parts anodized using the Type II process can easily be colored with various dyes, however, it should be noted that colors typically cannot be matched to specific Pantone or RAL colors due to variability in the process. Sulfuric acid is a relatively low-cost electrolyte when compared to chromic acid.
3. Type III - Hardcoat Anodizing: This process makes use of sulfuric acid as the electrolyte but is used to produce much thicker coatings than Type II anodizing — typically 12.7 to 50.8 μ (0.0005 to 0.002 in) due to a higher voltage, longer bath immersion time and lower bath temperature. This coating is harder than tool steel and is used where high levels of wear resistance are required. Although unpigmented, a clear hardcoat anodized finish will darken the part surface due to the higher thickness of the coating compared to standard anodizing. Depending on various factors, the natural color can vary from light to darker gray or brown.

What Are the Benefits of Aluminum Anodizing?

Anodized aluminum provides a number of benefits, including improved corrosion resistance, wear resistance, and electrical insulation. Anodized aluminum can also be dyed to create a variety of colors. Anodized aluminum is easier to clean and maintain than non-anodized aluminum because the aluminum surface is sealed with a relatively non-reactive surface layer and does not react with substances that can stain the untreated aluminum surface. 

What Are the Limitations of Aluminum Anodizing?

The aluminum anodizing process has some limitations. For example, there is a chance that slight composition differences between lots of the same grade of aluminum can result in different surface finish appearances. These different surface finishes can make it difficult to color-match parts. While all types of aluminum can be anodized, not all react well to anodizing.  The 5,6, & 7xxx series aluminum alloys are considered the best for anodizing. 

How to Anodize Aluminum

Before aluminum can be anodized it needs to be cleaned and etched to remove any dirt, cutting fluid, or grease. General cleaning is usually performed using a strong degreaser followed by thorough rinsing. 

Next, the parts must be etched or brightened. This process removes any naturally formed oxide layer and creates a clean, uniform surface to which the anodized oxide layer can bond. After etching, the parts are rinsed, then placed in a neutralizing solution to remove any potential residue from the etching step, and then rinsed again.

At the end of the process, an anodic layer will have formed that is chemically bonded to the base aluminum. After the required thickness is achieved, the parts are rinsed and can then be colored. 

Coloring anodized aluminum works by allowing metallic salts or chemical compounds that will produce specific shades to enter the pores produced during the anodizing process. After coloring, the pores must be sealed for optimal performance. The sealing can be performed with either of the processes below:

1. Hydration: Hot water or steam causes the oxide layer to hydrate, which causes expansion of the oxide layer. Expansion of the oxide layer closes up the pores.
2. Impregnation: Parts are immersed in a tank with deionized water and mineral salts that are deposited in the pores and react chemically with them, causing the pores to close up. 

What Are the Materials Needed to Anodize Aluminum?

The materials need to anodize aluminum are: an acid-resistant tank to hold the electrolyte, a DC power source to provide current, conductive wire to complete the circuit from the power source to the cathode and anode, and cathode (typically in the form of a lead sheet), cleaned & etched aluminum parts to serve as the anode, degreaser, etchant, and dye for coloring part after anodizing.

What Happens to Aluminum When It Is Anodized?

When aluminum is anodized, it forms an aluminum oxide layer on its surface that improves its abrasion and corrosion resistance. This coating can also be dyed as desired.

What Are the Anodizing Colors for Aluminum?

Anodized aluminum can be dyed virtually any color, however as mentioned previously, precise color-matching is typically not feasible. Expect some degree of variation in color consistency with anodized parts. If the part is bead-blasted before anodizing, the rougher surface will produce a matte finish. There are two methods of color addition: electrolytic coloring and dip coloring. Electrolytic coloring makes use of metal salts that are bonded to the oxide layer, and dip coloring refers to the process of dipping the anodized part into a dye bath. Electrolytic coloring produces a more UV (ultraviolet) resistant finish better suited to long-term outdoor exposure. 

How Long Will Anodized Aluminum Last?

An anodized coating can last between 10 and 20 years. This depends on the application, the thickness of the coating, and whether or not the surface was sealed after anodizing. Type II colored coatings may bleach or fade over time under prolonged direct sunlight or long-term exposure to UV light.

Is Anodized Aluminum Prone to Rust? 

No, anodized aluminum is not prone to rust. Rust is typically used to describe the formation of a flaking oxide layer on ferrous metals that eventually destroys the base metal. The oxide layer on aluminum also forms due to oxidation but in the case of aluminum, it adheres to the surface and protects the base metal from further oxidation.

Summary

This article presented aluminum anodizing, explained what it is, and discussed the different types and benefits of using it. To learn more about aluminum anodizing, contact a Xometry representative.

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