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.

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.

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. 

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. Therefore, it is 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 conducive.

How Aluminum Anodizing Works

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.

Types of Aluminum Anodizing Processes

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

• 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.
• 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/MIL-PRF-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.

• 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 MIL-A-8625/MIL-PRF-8625 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.

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. 

Advantages and Disadvantages 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. 

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. 

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