Anodized Aluminum: Anodization Process of Coloring Aluminum!

Late 19th century: Charles B. Dudley, a British scientist, discovered that anodizing aluminum in a strong acid improved its corrosion resistance by creating an oxide layer of uniform thickness. This is the earliest discovery of anodizing.

With the continuous development and progress of science and technology, the anodic oxidation process and method have been improved and perfected, and has become a widely used surface treatment method.

What Is Anodic Oxidation?

Anodizing is an electrochemical treatment process that forms an oxide layer by immersing a metal material, usually aluminum, in an acidic solution and applying an electric current. This oxide layer has corrosion resistance, high hardness, wear resistance, good insulation and other characteristics, so it is widely used in aerospace, automotive, construction, electronics and other fields.

The Process of Anodic Oxidation

Anodic Oxidation

Anodizing the aluminum is based on an electrolytic reaction where the material to be coated is used as the anode (positive terminal) while the material with which coating needs to be applied forms the cathode (negative terminal). The electrolyte (the liquid part of the cell) is mainly acidic that facilitates the process, and speeds up the process.

In anodizing, aluminum acts as the anode and is connected to the positive electrode, while the cathode is a conductive material, usually lead or copper. When an electric current is passed through the electrolyte solution, an oxide layer begins to form on the surface of the aluminum. The thickness of the oxide layer depends on the treatment time and current intensity. The longer the treatment time and the higher the current intensity, the thicker the oxide layer will be.

When electricity is passed through the terminals and into the electrolyte, ionic reactions occur, and the coating is applied on the material’s surface at the anode.

Introduction To Anodizing Colors

The anodizing colors are made with different compounds to efficiently serve the purpose, i.e., improving surface strength, durability, and color stability. There are many options for anodizing the metal surface in an aesthetically pleasing manner. However, the process of applying the color coats isn’t easy. It involves a series of steps and needs precision and accuracy for better results.

Therefore, proper care is taken when choosing the colors based on aluminum texture, grade, end expectations, resistance against corrosion, and more. If there is a slight mistake associated with the choice, the results won’t be pleasant, and it might destroy the material also. 

Types and Coloring of Anodizing

Anodization is done in three different processes, based on the requirements of the end-users and the features to be included. In this section below, we have briefly defined these three processes for your clarification.

Process Type 1: Chromic Acid anodization

anodizing process

Oxide Layer Applications

This is a common type of anodizing that is commonly used to make decorative aluminum products such as doors, Windows, furniture, auto parts, etc. The thickness of the oxide layer produced by it is relatively thin, generally between 10-20 microns.

Coloring Available

The color of the oxide layer depends on the treatment time and current intensity, and can be light silver, dark silver, golden yellow, black, etc.

Process Type 2: Sulfuric Acid Anodization

Oxide Layer Applications

This type of anodizing is commonly used to manufacture industrial parts and tools with wear and corrosion resistance, such as pistons, bearings, gears, etc. The oxide layer is thicker than sulfuric acid anodizing, generally between 25-150 microns.

Coloring Available

The color of the oxide layer formed by hard anodizing is usually dark black or dark gray, because the pores in the oxide layer are smaller, so the surface is relatively smooth, less reflected light, and looks dull. However, the color of hard anodizing is also affected by the composition of the electrolyte and the treatment conditions.

The color and quality of the oxide can be controlled by adjusting the electrolyte concentration, current density and treatment time. Sometimes, dyes are added during hard anodizing to produce different colors, such as gold, copper, black, and red.

Process Type 3: Hardcoat Anodization

Oxide Layer Applications

This type of anodizing is commonly used to make aluminum products with high corrosion resistance, such as ship parts, condensers, etc. It produces an oxide layer thicker than sulfuric acid anodizing, generally between 20-30 microns. Chromic acid anodizing treatment solution has some harm to the environment and human body, so it is not commonly used.

Coloring Available

Chromic acid anodizing results in a variety of colors, which can be adjusted by controlling factors such as electrolytic time, current density and electrolyte composition. Generally speaking, chromic acid electrolyte oxidation ability is relatively strong, can form a thick oxide layer in a short time. The following are the colors that may appear in an oxide layer formed by anodizing chromic acid:

  • Colorless or gray: When the oxide layer is thin, its color is colorless or gray.
  • Black: When the oxide layer is thick, its color is black.
  • Brown or brownish red: When aluminum products are anodized by chromic acid after copper plating, the oxide layer is brown or brownish red.
  • Orange or golden: When the oxide layer is thick, it can be made orange or golden by adding organic or metallic dyes.

Process of Anodizing Aluminum

1. Electrolysis

In this process, an acid salt is used in an aqueous solution as the electrolyte, which increases the absorption rate of the molecules. This helps seal the pores and offers UV-resistant properties so that the product can easily withstand the sun rays. However, the main problem is that the electrolysis process limits the use of color shades. Usually, you can use black, bronze, gold, nickel, chromium, and other colors that can be applied over the product surfaces. 

2. Dipping Method

The dipping method is used to apply color coats with different shades, both brighter to darker ones. Here, the product is placed inside a tank where dye solution is present. Once the chemical reaction is triggered, the dye molecules fill up the pores until they are sealed. The main drawback of this technique is the absence of UV-resistant properties from the surface of the end product. 

3. Interference Coloring

The interference coloring technique is used when the light rays interact with the dye molecules and suffer inference. The pores increase in size during the process and, therefore, accommodate more dye molecules to seal the entire surface. 

4. Integration method

In this method, the anodizing colors are integrated with the chrome molecules present inside the metal alloy. Therefore, the colors obtained are mainly black and bronze. One of the main benefits of the integration coloring method is the excellent abrasion resistance of the surface against scratches and other damages. 

Matching process of the color with aluminum

Anodized Aluminum

The same color can create different visuals as the anodizing layer acts as the filter. For example, a light coating will create darker color while the hardened coat will lighten the intensity. Therefore, it is important to choose a color that will ideally match the anodized layer and the base material features. 

In the below section, we have described a few factors that need to be considered for appropriate color matching. 

  • Grade Of The Base Material

As it is the base metal absorbing the light and reflecting the rest, you need to focus on its color and tone. These two features differ according to the grade of the metal. For example, you can find the metal alloy having light silver shade, bronze metallic shade, dark grey shade, and more. So, you have to pick the color based on the grade so that the perceived color is similar to the expected one.

  • Finish Type

If the finish of the aluminum base surface is smooth and clean, the light won’t be scattered, and therefore, regular reflection will occur. This will cause your eyes to gather most of the radiation, intensifying the color. On the other, if the base material has an uneven surface, it will cause irregular reflection or scattering of the chrome rays. Again, this will reduce the color intensity. 

  • Dyes Used and Their Types 

The final color that your eyes will perceive will depend on the layers of dye used and the number of the chromes. For example, multiple dyes need to be used to match the color intensity or create unique shades. Similarly, if there is more than one layer of the dye, the reflection will happen at every stage, and therefore, the intensity will differ. 

Does The Anodized Aluminum Wear Out?

Although anodizing can enhance the hardness and wear resistance of the aluminum surface, the oxide layer may still wear away. This is because there are still tiny pores and bumps on the surface of the oxide layer, and these areas are less hard and easily worn away.

In addition, in some cases, wear may result if the oxide layer is not properly treated and protected. For example, if the surface of the oxide layer is subjected to chemical corrosion or is wiped by objects with high friction, it may cause the oxide layer to wear away.

Therefore, in order to reduce the wear of the oxide layer, the following measures can be taken:

  1. Appropriately increase the thickness of oxide layer to enhance its hardness and wear resistance.
  2. The oxide layer is properly sealed to fill the pores and reduce surface roughness.
  3. Select the appropriate coating or surface treatment method to improve the wear and corrosion resistance of aluminum.
  4. Avoid using chemicals with strong acids or bases on the surface of the oxide layer.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.