Anodized Aluminum Colors: Knowing Anodizing Color Matching

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An appropriate surface finish to the product is essential after the manufacturing process. Many manufacturers choose anodizing as one of the ideal surface finish processes. Anodizing is an electrochemical technique that imparts a decorative, durable, and corrosion-resistant finish to a metal surface.

Anodizing provides a micro-thin aluminum oxide layer completely bonded to the aluminum substrate. The aluminum oxide layer is relatively clear and colorless, but it has a highly porous structure that can be colored and subsequently coated to generate a colorful metallic shine.

In the process of anodizing, it is important to know how to add color to your aluminum parts and choose the right anodized aluminum color. This article will take you through anodizing colors and color matching so that you can more easily create anodized aluminum colors that are both consistent and attractive.

Brief Introduction of Aluminum Anodizing

Anodizing involves an electrochemical process that changes the outer surface of metal parts into an extremely scratch-resistant, durable, and corrosion-resistant layer. It is a typical finishing method used on various nonferrous metals such as aluminum, titanium, and magnesium. 

Steps of Anodized Aluminum

Steps of Aluminum Anodizing

Although anodizing appears complicated, the steps are simple. The following are the general steps of anodized aluminum.

Step 1: Before anodizing, clean the aluminum part first. This is vital for removing contaminants that may impede the anodizing process.

Step 2: Place the material in an electrolytic solution bath and apply a direct electric current. This generates a positive charge in the aluminum and a negative charge in the electrolyte plates of the solution. The resultant electrochemical reaction forms pores on the surface of the aluminum part, allowing the aluminum substrate to combine with negatively charged oxygen ions in the solution to produce aluminum oxide. The formation of an anodic oxide barrier on the part’s surface is more durable than the underlying aluminum.

Types of Anodizing Process

There are three widely used types of anodizing processes, each producing different functional attributes and aesthetic properties. Let’s take a deep closer at the three types of anodizing processes.

Type I – Chromic Acid Anodize 

The type I anodizing process, often known as the “light” type, employs chromic acid as the electrolytes and aluminum part as the anode. When the current run through the electrolyte, positive particles from the anode are ejected. Thus, you will get microscopic grooves on the surface. After that, the microscopic grooves are oxidized to produce an oxide layer.

Type I produces the thinnest anodized layer on the surface of metal parts (up to 0.0001 inches). Products created with this process have superior heat and corrosion resistance than normal aluminum products. However, it takes in the least amount of color when dyed.

Type II – Sulfuric Acid Anodize

Type II anodizing is also called sulfuric acid anodizing. Since sulfuric acid is more potent than chromic acid, positive aluminum particles can be ejected better. As a result, Type II forms deeper microscopic grooves and a thicker oxide layer than Type I.

Type II sulfuric acid anodize has a thickness between 0.0002 and 0.001 inches. It is more suitable for dyeing parts. And aluminum parts created by Type II exhibit better paint retention properties.

Type III – Hardcoat Anodize

Type III anodizing, often known as “hard anodizing,” uses a higher voltage and sulfuric acid. The type III anodizing process is very appropriate for producing heavy aluminum products.

Compared to Type II anodizing, Type III produces a thicker anodized layer (> 0.001 inches). Parts created by hardcoat anodizing provide the finest abrasion resistance and color dyeing ability. However, it may not be a perfect choice for parts with highly tight tolerances.

You can also learn more about the different types of anodizing and their advantages in our recently updated blog post or via the below video.

What Aluminum Alloys Can be Anodized?

Aluminum Alloys Can be Anodized

Not all aluminum alloys can be satisfactorily anodized. Different aluminum alloys contain various types and amounts of alloying metals, resulting in distinct properties of coatings. Below is the anodization of the aluminum alloy series from 1xxx to 7xxx.

1xxx Series

The 1xxx series alloys are 99% pure aluminum and only contain trace amounts of alloying elements. Although they can be anodized, the underlying aluminum structure is not robust enough for many uses.

2xxx Series

Because the 2xxx series is largely alloyed with copper, any anodized coating will have a yellowish hue. In addition, the coatings created on 2xxx series alloys do not offer the same high level of protection as other alloy series.

3xxx Series

The aluminum alloys of the 3xxx series are alloyed with manganese. Anodizing the 3xxx series alloys results in a gray or brown coating. Since it is difficult to match the coating color for projects involving multiple sheets, painted applications work best.

4xxx Series

The 4xxx series can be alloyed with silicon. It provides a dark gray and sooty appearance when anodized. Although the oxidized coating offers protection, its surface color is not attractive.

5xxx Series

Magnesium is present in alloys of the 5xxx series. When anodizing, the 5xxx series alloys will develop a strong and clear oxide layer if the alloying magnesium remains within a certain percentage range.

6xxx Series

The aluminum alloys of the 6xxx series contain magnesium and silicon. These alloys are anodized to provide a strong and clear protective layer, making them particularly suitable for architectural applications. 

For instance, Al 6063, or architectural aluminum, can be anodized to provide an aesthetically attractive and colored finish for architectural applications such as door frames, window frames, roofs, and sign frames.

7xxx Series

The aluminum alloys of the 7xxx series contain zinc. Anodizing 7xxx series alloys give a clear and protective coating, but too much zinc in the mix can cause the coatings to brown.

According to the Aluminum Anodizers Council (AAC), aluminum alloys of the 5xxx and 6xxx series are appropriate for architectural anodizing applications. For instance, the alloys 5005, 5657, 6063, and 6463 are designated as “Anodizing Quality (AQ)”. 

Even though not all 5xxx and 6xxx series alloys fulfill AQ criteria, they can be anodized successfully. For example, alloys 6061, 5052, 5252, or 6101 are not regarded as AQ.

How to Add Color to the Anodized Aluminum Parts?

Aluminum oxide’s porous nature allows it to absorb colors effectively, and subsequent sealing helps to avoid color loss in service. Anodized aluminum can be added in many colors, including black, red, blue, green, urban gray, coyote brown, and gold. 

Below are the steps you need to know about how to color anodized aluminum parts.

Cleaning and Etching 

Anodizing Parts Cleaning

The first step is to clean the aluminum parts in detergent and rinsing tanks. After cleaning, the part’s surface is etched to provide a polished and glossy finish.

Etching is performed to remove trace amounts of metals that might cause an error in the anodizing process. This changes the part’s surface chemistry at the molecular level. Many factors influence the amount of material removed, including temperature, time, solution pH, and the size and shape of the item.

Building the Film Layer

After cleaning and etching, the anodizing process takes place. The target part is suspended in a tank of the electrolyte, and an electric current is employed to create micropores on its surface. The metal alloy determines the size and shape of the pores. These pores are then filled with free oxygen ions from the electrolyte, resulting in a strong yet porous oxide layer.

The depth of the pores is determined by tank temperature, solution concentration, electric voltage, and time in the tank. More time in the tank or higher power will create deeper pores and a thicker film layer of aluminum oxide.

Adding Color

anodizing parts coloring

Coloring anodized aluminum parts have four methods – electrolytic coloring, dip coloring, integral coloring, and interference coloring.

Electrolytic Coloring

The aluminum parts are immersed in a solution containing metallic salts in the electrolytic coloring method. As the salt fills the pores, the surface of the part develops a strong coating that is sufficiently resistant to UV rays. However, the choice of anodizing colors is limited, with bronze and black being the most prevalent.

Dip Coloring

Dip coloring involves placing the part in a tank of dye. After the dye has filled the pore, the further reaction ends by boiling the surface in deionized water. Dip coloring comes in a wide range of color options, but the colors may not be very UV resistant.

Integral Coloring

Integral coloring combines anodizing with coloring. In this method, you can color the aluminum products in bronze and black shades. Additionally, the products become more abrasion-resistant.

Interference Coloring

The pore structure enlarges in the interference coloring process. Consequently, the deposition of metals in the pores produces light-fast colors ranging from blue, green, and yellow to red. These colors appear due to optical-interference effects rather than light scattering effects.

Sealing

Anodizing Parts Sealing

Sealing is the final stage in the anodization process. In this case, sealing can trap dye molecules in the pores and prevent absorbing the unwanted molecules.

Anodized aluminum parts can be sealed in a variety of ways. You can immerse the polished anodized parts in a solution, making many chemical plugs accumulate in the anodic film opening. After being anodized, the aluminum parts now have exceptional corrosion protection.

What Are the Considerations for Anodizing Color Matching?

color matching of anodized aluminum

Typically, the color exhibited by any material is what is reflected against what is absorbed. However, the anodized film on the surface transmits light to the aluminum at the base surface instead of reflecting it. The base metal then reflects the light to the film and the outside. As a result, the anodized layer functions as a filter rather than a reflector.

Finding the correct color match can be challenging, especially if they don’t come from the same batch. It is important to comprehend the concept of color matching and the crucial factors affecting how the anodizing colors appear. Thus, you can select a suitable matching method. 

Below are some considerations for anodizing color matching you should watch out for:

Aluminum Grade

The aluminum grade is the most important consideration to watch out for when coloring aluminum anodizing. Every aluminum grade has its shades and color, which impact color matching.

Aluminum for Anodizing

Finish Type

The finish (part of the film) has a significant role in the reflecting properties of the aluminum product. As a result, choosing a sample with the same finish is preferred for the best color matching.

Number of Dyes That Make the Color and Layering

The type of dyes used also have a significant impact on color variance. When there is a need for more dyes to match the color, color variation increases. Additionally, layering is also crucial since each layer might have color variations. 

The Crystalline Structure of the Products

The metal’s crystalline structure affects the reflecting properties of an aluminum product. The colors may appear to match if viewed from an angle. A different reflecting angle would indicate a different result. Therefore, check if the reflection angle is proper before color matching.

How to Remove the Color from an Anodized Part If It doesn’t Match?

Remove Colors for Anodized Parts

Many factors are why the anodized color occasionally doesn’t turn out how you expect it. The color of your anodized parts may thus need to be removed.

Color removal from anodized part depends highly on the type of dye used and the condition of the anodized product. You can use chromic or phosphoric stripping solution to remove the dye from a sealed anodized coating. This method can leave the aluminum intact following the removal of color.

However, if you can accept slight damage to the aluminum part, go with an alkaline etch. Alkaline etching generally removes the colors more thoroughly. You can consider using 10-15% nitric acid to remove the dyes from unsealed anodized parts. Unfortunately, while most dyes can be used with this approach, not all can.

Conclusion

Product developers favor anodized aluminum because of its appealing look and physically robust finish. During the anodizing process, a layer of oxide is put on the surface of the metal. This layer protects the metal from corrosion, scratches, and fading.

LEADRP combines professional knowledge with efficient manufacturing practices, which can assist you in maximizing the benefits of your anodizing process. We have the flexibility to apply anodized aluminum colors as well as anodizing color matching. Contact us immediately so we can discuss your project.

References

Anodizing – From Wikipedia

What Aluminum Alloys Can Be Anodized? – From Industrial Metal Supply Company

FAQ

Yes, anodized aluminum does wear off, even though it appears permanent. The type of anodizing process employed determines how long it takes for anodized aluminum to wear off. 

Over time, all anodized metal colors have the propensity to fade. However, if the anodizing process is improved, you can anticipate longer years with less color fading.

Anodizing is an electrochemical technique that forms a protective coating on the surface of a metal. The protective anodized aluminum coating improves corrosion resistance and enhances durability.

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