At the end of the manufacturing process, the next stage is applying the surface finish. There are different methods, but in aluminum materials or other materials, the anodizing process is a reputable method. The reason is not farfetched, as it leads to the production of a durable yet beautiful product. Anodizing goes by processes that must be understood, and two important parts are knowing how to color aluminum and the anodized aluminum colors to use.
Anodizing colors are important since they say a great deal about the properties of the end product. In this article, you will learn about anodizing, common anodized aluminum colors, and how to color aluminum products using anodizing. Let’s get right on it so you can understand how the process works.
What is Anodized Aluminum?
Anodized aluminum is simply aluminum that’s been treated with an electrochemical process to create a tough, protective oxide layer. The process involves dipping the aluminum into an electrolyte bath and running an electric current through it, which makes the surface harder and more resistant to corrosion—about two to three times harder than regular aluminum.
Since the anodized layer is bonded to the aluminum, it won’t chip, peel, or flake like other coatings. This makes it great for applications where both looks and durability matter, like in architecture, automotive parts, and consumer products. Plus, the porous surface is easy to dye, so it can come in a wide range of colors and finishes.
The Process Of Anodizing Aluminum
Before discussing the various anodized aluminum colors or how to color aluminum, we should prickle your brain on what the process is all about. Unless you have a science background or are already deep-rooted in the process, it won’t be surprising that you wonder about the aluminum anodizing process. Therefore, in this section, we will summarize the basic things about the process.
Aluminum anodizing is an electrochemical process in which aluminum products’ surfaces are coated using a wear-resistant oxide layer. Therefore, the products exhibit properties that improve quality and aesthetics. For example, they are durable, and resistant to wear and corrosion. They also have this beautiful feel which fulfills the aesthetic requirement of most users.
Anodizing is an electrochemical cell reaction with the aluminum part acting as the anode, the cathode being an inert material, and an acidic electrolyte. Below are the electrochemical reactions at the electrode:
- Anode: 2Al + 3H2O = Al2O3 + 6H+ + 6e-
- Cathode: 6H+ + 6e- = 3H2
- Resulting anodizing reaction: 2Al + 3H2O = Al2O3 + 3H2
Types of Anodizing Process
There are three types of the anodizing process, each being different in terms of coating intensity. The difference is due to the electrodes, electrolytes, and energy used in each process.
· Type 1 Anodizing Process
The type 1 anodizing process is also known as the “light” type, and it involves the use of chromic acid as the electrolytes and the aluminum part as the anode. When current is made to pass through the electrolyte, positive particles from the anode are ejected, forming microscopic grooves on the surface. The microscopic grooves are then oxidized to form an oxide layer. Compared to normal aluminum products without finishes, products made via this process are better in heat and corrosion resistance.
· Type 2 Anodizing Process
The type 2 anodizing process uses sulfuric acid instead of chromic acid. Sulfuric acid is more potent, leading to a better ejection of positive aluminum particles than what is seen in type 1’s. Therefore, the microscopic groove formed is deeper, and the oxide layer is thicker. These two properties are responsible for the better paint retention properties exhibited by Type 2 aluminum parts.
· Type 3 Anodizing Process
The type 3 anodizing process is the ideal one for making heavy aluminum products. Unlike the other types of the anodizing process, it uses a higher voltage and a strong acid (sulfuric acid).
Advantages of Coloring Anodized Aluminum
Coloring anodized aluminum enhances its aesthetic appeal and provides practical benefits. Here are some advantages to knowing:
- Esthetic Pleasing: The dyeing of anodized aluminum to obtain several colors allows the production of aesthetic products that stand out in the market. Colors can be adjusted to fit branding requirements or design guidelines.
- Strong and durable: being part of aluminum instead of merely a top coating, the colored anodized layer is less prone to chipping, fading and wear. It means that products look good for a long time, even in extreme conditions.
- Functional Benefits: The use of colors may include the tracking of the components in industries or safety where visualization gives an advantage.
- Environmentally Friendly: Unlike typical paint or powder coatings, the anodizing process does not involve any harmful chemicals. Often, the dyes are non-toxic and sustainable.
Methods of Aluminum Anodizing Colors
Anodizing colors are different from the colors of other techniques such as powder coating or paint. In terms of achieving color consistency, there are difficulties due to the many variables involved in anodization.
While the anodizing process is open to all colors of the rainbows, the anodized aluminum colors used depend heavily on many factors such as size, grade, and finish tapes. Below are the steps to take on how to color aluminum products using an anodizing process.
Cleaning and Etching of the Aluminum Part
The first step starts with cleaning the aluminum products in detergent and rinsing tanks. After washing, the part is etched to give a polished and shiny surface. The etching is done to remove trace amounts of metals that can lead to an error during the process.
Building the Film Layer
After cleaning, the anodizing process takes place. You can anodize using any of the three types of anodizing discussed above. However, there are other things to look out for.
- The metal alloy controls the size and shape of the pores.
- The tank temperature, solution conc, and voltage control the depth of the pore.
Adding Color
There are four ways of adding anodized aluminum colors. Below is an explanation of the four methods.
- Electrolytic coloring. This method involves the immersion of the aluminum parts in a solution containing some metallic salts. On filling the pore, they provide a coating strong enough to resist UV rays. However, there is a limitation on the number of anodizing colors you can use, with bronze or black color being the most common.
- Dip coloring. In this method, the part is placed in a tank containing a dye. The dye fills up the pore, and the surface is boiled in de-ionized water to end the reaction. You can use dip coloring with many color variants. However, they are not that UV resistant.
- Integral Coloring. Integral coloring combines anodizing and coloring to color aluminum products in bronze and black shades. The products also become more abrasion-resistant.
- Interference Coloring. In interference coloring, the pore structure becomes enlarged. Therefore, the deposition of metals in the pores leads to light-fast colors ranging from blue, green, and yellow to red. These colors occur as a result of optical interference effects instead of light-scattering effects.
Sealing
Sealing is the last stage of anodization. Here, the dye molecules that are on the pore are trapped in the pores. Sealing prevents the absorption of undesired molecules in the pores.
Sealing is done in hot water at a temperature of 200ºF (93ºC). Crystals of hydrated aluminum oxide formed in hot water are responsible for the sealing of the pores. Metal salts can also be deposited after being dissolved in hot water at 180ºF (86ºC).
Color Matching in In Aluminum Anodizing
Anodizing colors, unlike in paints, is subtractive and not addictive. If you understand the concept of light, you will understand the distinction that this brings. Normally, the color shown by any material is what is reflected against what is absorbed. Therefore, if white light shines on a blue bucket, the bucket absorbs the other colors and reflects blue. This makes us perceive it as a blue bucket. The process is the same in anodizing color.
However, it has a slight addition. Instead of reflecting the light, the anodized film formed on the surface transmits light to the aluminum at the base surface. The base metal then reflects it to the film and outside. Therefore, the anodized layer acts as a filter instead of a reflector, which is important in color matching.
Matching the right color is not easy, especially if they don’t belong to the same batch. To choose the right matching method, you must understand the concept of color matching explained above and the important factors determining the anodizing colors’ appearance. Below are what you should look out for:
· Aluminum Grade
This is the most critical factor to look out for during color aluminum anodizing. Every aluminum grade has its color and shades, and they affect color matching.
· Finish Type
The finish (part of the film) plays a huge part in the aluminum product’s reflective properties. Therefore, for the best in terms of color matching, using a sample with the same finish is preferable.
· Number of Dyes That Make the Colour and Layering
The type of dyes used also plays a huge role in the dozing color variation. There is an increase in color variation when there is a need for more dyes to match color, color variation increases. Also, layering is important since there can be color variations from each layer.
· The Crystalline Structure of the Products
The crystalline structure of the metal that makes up the product also influences the reflective properties of an aluminum product. From an angle, the colors might seem to match. However, a different reflective angle can show otherwise. This condition is “color flip” and plays a huge role in color matching.
Applications for Color-anodized Aluminum
Color-anodized aluminum is widely used in various industries. The unique combination of beauty and strength makes it a good choice for a variety of applications, including:
- Architecture: Ideal for facades and window frames for a blend of beauty and weather resistance.
- Consumer Products: Appealing in both form and function for consumer products like water bottles or electronics.
- Automotive Components: Used more in trim and wheels and offer lightweight and customizable options.
- Industrial Uses: Suitable for gears and equipment due to its low friction properties.
- Sporting Goods: Found in bicycles and sports equipment, enhancing performance and style.
Factors to Consider When Selecting Anodized Aluminum Colors
There are many important aspects to consider when selecting anodized aluminum colors. The following are the things to consider:
Uses for Aluminum Components
Aluminum anodizing colors vary between different industries. For instance , vibrance is used for electronics and black is used in cookware to absorb heat. Light colors can get a high reflection of light and therefore you need them for the construction material; while green, yellow, and red are examples of colors used by some safety signs for better visibility.
Coating Thickness
The anodized layer thickness is very much related to its appearance and life span. The coatings of sizes around 10 nm are electro-colored and the bigger ones structurally up to 50 µm are developed with dyes, providing the electrochromic and structural colors respectively in shades of blue, red, etc.
Purpose of Anodizing
A thin anodized layer is enough for decorative applications (up to 5µm to 25µm). Industrial or marine must have thick (25µm to 50µm) coating for corrosion or abrasion protection.
Cost
Costs can go up with special dyes or metallic salts, but batch production makes it possible to maintain uniformity and keep overall costs down.
Conclusion
Anodizing is an electrolytic process of high importance in surface finishing, used to improve the surface finishing process in terms of aesthetics, optical relativity, etc. This article talks about the anodizing process and how to color aluminum parts. It also talked about the factors that play a huge role in color matching. No doubt anodizing process might seem complex. However, to get the best in terms of quality and low cost, you can trust us at RapidDirect.
FAQ
Yes. However, this requires some steps since you cannot anodize over-dyed anodized aluminum and color it again. You can check the article on what to do to change the color of anodized aluminum
Yes, even though they seem permanent, anodized aluminum does wear off. The time it takes for them to wear off depends on the type of anodizing process used. You can also remove it by using an acid wash.
Removing the color depends highly on the nature of the dye you used and the anodized products’ condition. You can only remove the dye for sealed anodized products using chromic/phosphoric stripping solution (if you want the aluminum intact). However, if you don’t mind a slight degradation of the aluminum part, you can use an alkaline etch.
For unsealed anodized products, you can use 10-15% nitric acid to remove the dyes. However, this only works for most dyes, not all.
The following methods should work to change the color of anodized aluminum:
Re-Anodizing：This process includes cleaning the aluminum, removing the old anodized layer using chemical stripping or mechanical abrasion, and then applying a new anodized finish.
Dye removal: If the anodized layer has not been sealed, a 10–15% nitric acid solution can be used to remove many dyes. The processes need to be done with care, or it will damage the aluminum.
Mechanical abrasion：Mildly sanding or polishing can decrease the glory intensity or remove surface stains. Avoid scratching the anodized layer beneath it.
Chemical Stripping: To achieve a deeper etch, chromic or phosphoric acid solutions emerged to strip anodized coatings. It does clean dye and oxide layer as well, but special care has to be taken while cleaning.
Anodized aluminum is highly dyeable. It is available in many colors, including blue, red, green, black, silver, gold, yellow, copper, pink, orange, and purple. At RapidDirect, you can also get custom colors by sending a sample.
Yes, although anodized aluminum is meant to last, fading in all colors eventually occurs through exposure to UV light and wear from other environmental factors, even with the best anodizing process.
Yes, anodizing does enhance aluminum’s corrosion resistance. This anodized layer forms a protective film that is resistant to oxidation and damage, and therefore ideal for extreme environments.
Type 3 anodizing (hardcoat anodizing) almost always produces black and bronze colors. Although a thicker, more durable oxide layer is formed, it also leaves room for colours.
Aluminum that has been anodized before can absolutely be anodized again. That said, surface preparation will need to be on-point to avoid poor adhesion of the new anodized layer.
Where the main differences lie is in the thickness and properties of the oxide layer. Type 2 anodizing creates a thinner coating which enhances appearance and provides some corrosion protection. Type 3 anodizing, in contrast, creates a thicker and harder coating which provides much better wear resistance and durability.
