Following the manufacturing process, several applications require surface finishing. Among various methods, anodizing stands out for aluminum and other materials because it produces durable and attractive products. Aluminum anodizing creates a protective layer on the surface of aluminum products, preventing corrosion and improving their durability. However, you can further enhance the aesthetic appeal of anodized aluminum by introducing various colors to the surface layer.

Anodized aluminum colors include blue, red, green, black, silver, gold, orange, purple, pink, etc. These colors are permanently applied through the anodization process to improve stability, durability, surface finish, and abrasion resistance.

This article explores the color options, techniques, and factors influencing the selection of anodized colors for aluminum. You will also learn how to adequately match colors for custom applications in your industry.

A Brief Overview of the Aluminum Anodizing Process

Let’s start by understanding the process of anodizing aluminum. Aluminum anodizing is an electrochemical process that adds a wear-resistant oxide layer to the surface of aluminum products. This enhances their quality and appearance, making them durable, resistant to wear and corrosion, and aesthetically pleasing.

Aluminum anodizing process
Aluminum anodizing process

The anodizing process involves an electrochemical cell reaction, where the aluminum part acts as the anode. An inert material serves as the cathode, and an acidic electrolyte is used. There are three types of anodizing processes, each varying in coating intensity. These differences arise from variations in electrolytes, electrodes, and energy used during the process.

Type 1 Anodizing

Type 1 anodizing uses chromic acid as the electrolyte and aluminum as the anode. Passing current through the electrolyte causes positive particles from the anode to create microscopic grooves on the surface. These grooves are oxidized to form an oxide layer, resulting in enhanced heat and corrosion resistance compared to unfinished aluminum products.

Type 2 Anodizing

In type 2 anodizing, sulfuric acid replaces chromic acid, providing a more potent solution. This results in a more effective ejection of positive aluminum particles than type 1. Consequently, the microscopic groove formed is deeper, and the oxide layer is thicker. These characteristics contribute to the improved paint retention of type 2 aluminum parts.

Type 3 Anodizing

Type 3 anodizing is well-suited for heavy aluminum products, using higher voltage and a stronger sulfuric acid. This process distinguishes itself from other types by its ability to create a thicker and more durable oxide layer, providing enhanced wear resistance. The resulting aluminum products are particularly suitable for heavy-duty applications in challenging environments.

Why Do You Need to Color Anodizing Aluminum?

Color anodizing aluminum involves creating an artificial oxide layer and adding color to the aluminum surface to enhance functional and aesthetic properties. Unlike coatings such as painting and plating, colored anodizing doesn’t chip off or fade over time. 

Various aluminum anodized parts
Various aluminum anodized parts

The benefits of color anodizing include:

Enhanced Aesthetics

Color anodizing aluminum offers a range of options, making it appealing to those who prioritize color in their purchases. It enhances aesthetics without compromising the metallic appearance of aluminum parts. The ability to create diverse colors and finishes distinguishes and supports branding and marketing efforts. If aesthetics matters in your application, this is surely your best bet. 

Increased Durability

The extended lifespan of anodized aluminum products brings economic benefits by reducing maintenance and operating costs. The added layer of protection from anodized aluminum colors enhances resistance to wear and corrosion. This is especially beneficial in harsh environments exposed to corrosive substances.

Excellent Corrosion Resistance

Anodizing is valuable because it forms a well-structured oxide layer around the metal, preserving its metallic quality and protecting it from corrosion. The added color layer enhances corrosion resistance, making it particularly beneficial in challenging environments.

Reduced Friction

Adding color through anodization to aluminum surfaces not only enhances their appearance but also reduces friction. This makes it suitable for applications where low friction is desirable. This property contributes to the versatility and functionality of anodized aluminum colors.

What Are the Various Aluminum Anodizing Colors Available?

The anodization process allows for a broad spectrum of colors on aluminum surfaces, including blue, red, green, black, silver, gold, bronze, orange, purple, pink, and many more. Aluminum alloys obtained during manufacturing processes often exhibit a wide variety of colors. Hence, the long list of available colors for anodized aluminum. 

Anodized aluminum color chart
Anodized aluminum color chart

Aluminum anodized colors range from light to dark. Factors such as anodized layer thickness, immersion time, dye concentration, and power source rating influence color intensity. Sulfuric acid as an electrolyte produces brown and black shades, followed by red, blue, gold, and bronze.

Additionally, clear anodizing creates a transparent, even film that enhances durability and stability. Common choices for industrial products include brown, black, and clear (silver) colors, constituting around 80% of anodized applications. The following anodized aluminum color chart gives a brief look at some of the colors and their constituents. 

Anodized Aluminum ColorsMajor Constituent
BlueCobalt
BrownMagnesium
GreenMagnesium/Silicon
SilverClear anodizing
Bronze and shades of redCopper
GrayManganese

The table below also shows how color anodizing is categorized based on the thicknesses of the anodized aluminum parts.

Color Anodizing CategoryThickness of Layer
Architectural Class I0.0007 inches or higher
Architectural Class II0.0004 inches – 0.0007 inches
Protective and Decorative0.0004 inches

Steps Involved in Applying Aluminum Anodizing Colors

Here are the steps involved in coloring aluminum products through the anodizing process.

Colored aluminum anodized parts
Colored aluminum anodized parts

Step 1: Cleaning the Aluminum Workpiece

This step is to thoroughly clean the aluminum workpiece surface, eliminating oils and grease deposits. This ensures optimal deposition of oxide ions, resulting in a uniform oxide layer. Additionally, addressing moisture content prevents the formation of white spots in the anodized layer. This will help maintain the workpiece’s quality.

Step 2: Etching the Aluminum Workpiece

Following the surface cleaning, the workpiece undergoes immersion in an etching solution to eliminate natural oxides. This step removes a fine layer to ensure a uniform, impurity-free surface. Subsequently, the part is rinsed to eliminate any residues from the etching solution.

Step 3: The Anodizing Process

After achieving an impurity-free surface, the workpiece is placed in a sulfuric acid electrolytic solution. Adding one part sulfuric acid to three parts water is crucial while preparing the solution to avoid hazardous gas production. The aluminum workpiece, serving as the anode, is connected to facilitate oxide ion deposition. 

Passing electricity through the solution forms an oxide layer, indicated by the appearance of air bubbles near the anode. If no bubbles appear, the power supply should be switched off to check connections. Initially, the formed aluminum oxide layer is thin and uniform (the barrier layer). The current intensity is increased after its formation to create pores on the surface, enabling the bonding of the colored layer. 

Maintaining this current for around 45 minutes changes the anodized layer’s color to brown and then yellow. Following the anodizing process, thorough rinsing is done to eliminate any residue from the electrolyte.

Adding colors to anodized aluminum
Adding colors to anodized aluminum

Step 4: Adding Color to the Workpiece

Once the anodizing process is finished, you have several methods to add color to your workpiece. Four ways to color anodized aluminum exist, and each method is explained below.

  • Electrolytic Coloring: This method involves immersing anodized aluminum parts into colored electrolytic baths with metals like tin, cobalt, and nickel. Unlike plating, this method uses an AC power source for a uniform coating with minimal defects. Various color tones are achieved based on the duration of electric current through the workpiece. However, the range of aluminum anodize colors is limited, with black and bronze being common.
  • Integral Coloring: Also known as the “duranodic process,” it combines anodizing and coloring to achieve bronze and black shades on aluminum products. This single-step method enhances abrasion resistance, incorporating aspects of immersion and electrolytic coloring.
  • Dip Coloring: In dip coloring, the part is immersed in a dye-filled tank. The surface is then boiled in de-ionized water to conclude the reaction. This method offers various color options but has lower UV resistance.
  • Interference Coloring: It enlarges the pore structure to facilitate the deposition of metals that result in light-fast colors like green, blue, red, and yellow. These colors arise from optical interference effects rather than light scattering effects.

Step 5: Sealing

Sealing is a crucial anodizing step, where dye molecules on the pores are trapped to prevent the absorption of undesired molecules. Sealing is conducted in hot water at 93ºC, forming hydrated aluminum oxide crystals that close the pores. Metal salts can also be deposited after dissolution in hot water at 86ºC. Sealing enhances corrosion resistance and more desirable coating with improved material properties.

Color Matching in Anodized Aluminum

Unlike paints, anodizing colors are subtractive rather than additive. In standard color perception, materials reflect what they don’t absorb. Anodizing introduces a slight difference: the formed film transmits light to the aluminum base, reflecting it back through the film. This transforms the anodized layer into a filter rather than a reflector. This concept is crucial for accurate color matching.

Color matching in anodized aluminum
Color matching in anodized aluminum

Achieving color consistency is challenging, especially between batches. Successful anodized aluminum color matching requires understanding the mentioned concept and the key factors influencing anodizing colors’ appearance. To ensure color uniformity, consider the following factors during the anodizing process.

Aluminum Grade

The aluminum grade is the most crucial aspect to consider in aluminum anodizing color matching. Each grade has distinct colors and shades, significantly influencing the color-matching process. Considering the aluminum grade is vital for achieving accurate and consistent anodized colors, ensuring the desired aesthetic outcome.

Finish Type

An essential consideration in anodized color matching is the surface finish type, as it significantly impacts the reflective properties of the aluminum product. To achieve optimal color matching, using a sample with the same finish is preferable. This ensures consistency in the final appearance.

Dye and Sealing Solution

The choice of dyes significantly influences anodized color variation. Increased color variation occurs when more dyes are required for matching. Layering is also crucial, as color variations can emerge between layers. To maintain consistency, it’s essential to use dyes of the same quality and concentration across different parts. Also, consistently using the same sealing solution for the entire anodized batch is crucial to preserve the colored anodized layer. Otherwise, variations in colors may occur.

Electrolyte

The quality and thickness of color anodizing vary with the chosen electrolyte type and input current. Selecting consistent input parameters for the process is crucial to minimize color mismatch among parts.

Process Duration

Maintaining the color shade in dyeing or color anodizing parts is influenced by processing them for the same duration. A longer duration ensures a more uniform coating thickness and accurate color shade. Therefore, precise regulation of the duration is essential for consistent results.

Metal’s Crystalline Structure

The reflective properties of an aluminum product are influenced by the metal’s crystalline structure. Colors may appear to match from one angle, but a change in reflective angle can reveal otherwise. This phenomenon is known as “color flip,” which significantly affects color matching.

Considerations for Choosing Anodizing Aluminum Colors

If you look at the anodized aluminum colors chart, there are several options to choose from. However, not every color is fit for every application. Therefore, you must consider some factors before choosing the one for your project. 

Choosing anodized aluminum colors
Choosing anodized aluminum colors

Here are the top factors for your consideration:

Application of the Aluminum Parts

Different colors are chosen for anodized parts based on their intended application. Electronic gadgets come in various colors and finishes, aiming for aesthetic appeal. Cooking utensils are often anodized in black for better heat absorption and durability. 

Some construction applications demand anodized aluminum with good reflection properties and weather resistance. So, they use light colors with excellent reflectivity. Safety signs requiring longer-wavelength colors typically employ green, yellow, and red.

Anodized aluminum colors find common use in various products, such as

  • aircraft components 
  • automotive engine parts and wheels
  • medical devices like surgical instruments and implants 
  • consumer goods like sporting equipment and jewelry 

Coating Thickness

The visual appearance of a part is influenced by the thickness of the color coat on the anodized surface. For instance, electro-coloring may have a thickness of around 10 nm, impacting light reflection properties. This method can produce shades such as brown, black, blue, and grey. By using dyes, the thickness can increase up to 50 µm. Additionally, incorporating metallic salts like cobalt and copper results in blue and red shades.

Purpose of Anodizing

Anodizing aims at improving aesthetics and can result in various colors, regardless of coating thickness. For decorative purposes, the suggested coating thickness ranges from 5µm to 25µm. A thick oxide layer is necessary in applications where the base metal needs protection from corrosive and abrasive elements. For instance, hard anodizing is suitable for marine use due to constant exposure to corrosive agents. Such applications require an oxide layer thickness between 25µm and 50µm.

Cost

The chosen color type and the quantity of workpieces influence the cost of coloring. Special dyes for unique colors can increase the overall process cost. Opting for batch production is a preferred approach to reduce the coloring cost of an anodized product when maintaining color uniformity is essential.

How To Remove Color from an Anodized Aluminum Part

Custom anodized aluminum parts
Custom anodized aluminum parts

Several factors can lead to unexpected outcomes in anodized colors, necessitating color removal. The process depends on the dye type used and the current condition of the anodized product. Chromic or phosphoric stripping solutions are effective for removing dye from sealed anodized coatings. They often do not cause any significant damage to the aluminum.

If slight damage to the aluminum is acceptable, alkaline etching provides a more thorough color removal. 10 to 15% nitric acid can be used for unsealed anodized parts, but not all dyes are compatible with this method.

Get Custom Anodized Aluminum Colors At Zintilon

There is an extensive array of available anodized colors for aluminum parts. However, you must consider your part’s application and the thickness of the coating layer before choosing a color. Moreover, adequate color matching is crucial to help you achieve the required functional and aesthetic results. 

For optimal quality and cost-effectiveness, Zintilon stands out as your trusted choice. We excel in aluminum anodizing and other surface finishing services. Our experts combine their expertise and our cutting-edge technologies to provide a comprehensive solution for diverse applications. With a commitment to quality and cost efficiency, Zintilon ensures customer satisfaction through its expertise and reliable services. Contact us today, and let’s get to work!

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