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The In-depth Guide on How to Anodize Aluminum

anodized aluminum

In manufacturing industries, anodizing is one of the simple yet outstanding solutions to improve the durability and aesthetics of final products. Several metals are compatible with this process, but aluminum is usually considered best.

Typically, anodized aluminum is indicated by a porous, durable, and uniform surface that standard aluminum doesn’t feature. If you’re here to learn how aluminum anodizing can be done successfully, you’re at the right place. The following blog article will take you through the whole process of aluminum anodizing and its benefits, applications and ways to paint it.

 

What is Anodizing and Anodized Aluminum?

The aluminum treated with an electrochemical process to develop a durable shiny finish is called anodized aluminum, and the process to do so is known as anodizing. During the procedure, aluminum is dipped in a series of tanks, each offering a different function. Typically, the anodizing layer, i.e., aluminum oxide, originates from the aluminum substrate.

The anodized layer not only enhances visual effects but also hardens the aluminum. Plus, there is no worry about peeling, chipping, and wearing under normal conditions. Instead, its natural beauty, texture, and luster remain. Apart from that, anodized aluminum is almost 60% lighter than other metals, such as copper and stainless steel.

Anodizing1

 

How to Anodize Aluminum? Different Steps to Follow

Regarding anodizing the aluminum, the first and foremost stage is to load the part onto the carrier racks. These carrier racks are electricity conductors, thus offering high conductivity through the aluminum part. Further, the anodizing process consists of five steps, which are as follows:

1. Cleaning

Cleaning the aluminum is a pre-treatment process that helps to remove any grease, oil, or dirt on it, as these impurities may affect the anodizing. In order to ensure the cleaning, the part is immersed in an acid or alkali-based detergent container.

2. Etching

The second step is an etching, where the aluminum part is made free from spotting or uneven etching. To do so, the aluminum material is rinsed in a pool containing a caustic or acid etch. As a result, a thin uppermost layer of aluminum is removed, leaving behind a uniform surface. Typically, this step offers a smooth matte finish to the product making it ideal for anodizing.

3. De-smutting

After passing the aluminum from the acid etch, it is again rinsed multiple times for the final preparation before moving toward the anodizing step. This rinse is called de-smutting and is responsible for removing leftover residues from the etching process.

4. Anodizing

Anodizing is the most crucial process. Here the aluminum part is submerged in an anodizing tank that comprises the sulfuric acid electrolyte. An electrical power source supplies electrical current to the medium. The cathode is set on the inner side of the tank, whereby the aluminum extrusions act as an anode.

When it comes to working, the oxygen ions released from the electrolyte bond with the aluminum atoms at the surface. This leads to the growth of the anodic coating.

Moreover, you need to follow some considerations when anodizing the aluminum, like

  • Normally, the anodizing process generates high heat; hence, refrigerate the electrolyte to maintain the temperature.
  • To achieve the desired thickness, vigilantly monitor the dispersion time.

Afterwards, anodized aluminum is rinsed in deionized water to ensure the removal of all minerals, as they may cause spotting. Furthermore, if you want to color the anodized part, it is the right time because it is porous at this stage, so you may introduce various dyes into these pores.

5. Sealing

The final step is to seal the pores using a nickel acetate solution. Once you have locked the pores, you’ll get a pretty durable surface.

Anodize Aluminum

 

Introduction and Comparison of Different Types of Anodizing Processes

Several types of anodizing processes exist, but we have discussed only three major ones below. So, let’s have a look!

Chromic Acid Anodizing – Type 1

This type of anodizing is suitable for applications that require the thinnest anodic coatings, approximately between 1 to 10 Microns. Being the thinnest anodizing coat among the three major types, it still offers superb corrosion resistance to the components.

However, when dyed, you can’t use this process for color anodizing due to its grayer appearance and poor color absorption ability. In order to provide a protective coating on housings, you may dye them black. Yet, black dyeing is not done often as it requires raised temperature of the chromic acid to accept it.

Typically, chromic acid anodizing is considered best for tight tolerance objects. Also, it’s ideal for bonding, welded parts, and assemblies.

Chromic Anodizing2

Sulfuric Acid Anodizing – Type 2

Sulfuric acid anodizing differs from chromic anodizing in terms of chemical composition, hardness, and voltage application method. The coating thickness in the case of sulfuric acid anodizing ranges from 5 to 25 Microns.

Additionally, it offers colored surface finishes on aluminum alloys. This is because aluminum oxide is efficient in absorbing the dyes and is inclined to bleach in exposure to prolonged sunlight. The most common coating colors include black, urban grey, blue, red, green, gold, and brown.

Sulfuric acid anodizing is more budget-friendly than other anodizing types. It’s mainly used for anodizing military weapons, mechanical hardware, optical devices, and electronic enclosures.

Sulfuric Anodizing6

Hardcoat Anodizing – Type 3

Hard anodizing is known for its thickness and density, which is pretty much from sulfuric and chromic acid anodizing. Generally, it forms an oxide layer between 20 to 100 Microns thick. It’s usually preferred for applications where high electrical insulation, abrasion resistance, and durable coating are required.

Most commonly, hardcoat anodizing is limited to aluminum parts subjected to corrosive environments and high-wear applications. It also improves the surface of components for slide applications.

You can use hard-coat anodizing for pistons, gears, swivel joints, cams, blast shields, valves, etc.

Hardcoat Anodizing3

 

How to Choose the Suitable Type of Anodizing?

Each anodizing has its specifications and benefits, so you can choose according to the required conditions of the product you want to design. Here are the key distinctive features of all three types:

  • Chromic acid anodizing is best for applications requiring corrosion resistance.
  • Sulfuric acid anodizing is perfect for manufacturing kitchenware, aerospace parts, consumer goods, and architectural components.
  • Hardcoat is ideal for objects with extreme temperatures and chemical exposure.

Properties

Chromic Acid Sulfuric Acid Hard Anodize

Applications

Decorative Decorative

Corrosion resistance

Corrosion Resistance

Good Good

Excellent

Wear Resistance

Poor Moderate

Excellent

Porosity

Low Moderate to high

High

Environment Friendly

No Yes

Yes

Appearance Primary Coat Clear or Dyed

Hard clear or Hard black

Chromic Anodizing3

 

How to Paint or Color the Aluminum After Anodizing?

Here are the few steps you should follow while painting an anodized aluminum part:

1. Wash and Clean the Anodized Aluminum

Although anodized aluminum is pretty adhesive due to its porous surface, still its adhesion can be masked by the presence of contaminants, like, oil, grease, or dust particles. Thus, the less anodized aluminum is adhesive, the lower the probability of durable painting. So, you must ensure to clean the part thoroughly before moving towards further steps.

The ideal way to clean is to use a dish soap specified for both the removal of bacteria and grease from the surface. Then wash off the soap by scrubbing thoroughly. As soon as the anodized aluminum dries, jump for the next painting steps without delay. Otherwise, the impurities may cause a hurdle.

2. Use Etching Primer

For this step, firstly, pour the etching primer into a small container. Then coat the whole surface of the part with etching powder. Take care that not a single spot gets rid of the primer, as it is crucial for perfect adhesion.

The moment the primer is applied to the anodized aluminum, it is wet; therefore, you need to let it dry properly. It may take two to three hours for complete drying.

3. Removal of Bumps

The application of etching primer often results in slight bumps and snicks. In order to face such a situation, you should use sandpaper to rub the surface gently to remove any bumps on it. Doing so will finally end in a sleek finish of the anodized aluminum part.

However, you must use the sandpaper carefully to ensure that there is no removal of primer.

4. Apply the Paint

Finally, it is time to paint the workpiece. You are suggested to prefer the oil-based paint because it better absorbs inside the pores. Well, always paint the surface with a smooth thick layer. Sometimes, it may require additional coats, especially when the color is light.

In such a case, you again need to pause to ensure the drying of the paint. Afterward, go for the next coat in the same way.

Anodized Aluminum2

 

Color Options for Anodized Aluminum

Another significant factor when anodizing aluminum is selecting several color options. The standard anodizing colors for aluminum parts are transparent, champagne, black, and bronze. Apart from these, all rainbow colors are open to the anodizing process.

However, color matching is tricky, especially if they are from different batches. The following are some factors you must consider in this regard:

  • Aluminum grade
  • Finish type
  • Number of dyes needed to make the color and layering
  • The crystalline structure of the workpiece

 

Design Tips For Anodizing Aluminum

The following tips will make the anodizing process smoother for you:

Edges and Corners

The most crucial point to remember when anodizing aluminum is to ensure at least 0.5 mm radii of all edges and corners of the workpiece. In addition, there must not be any burrs.

Go for Other Finishing Processes

It’s always recommended to use some other finishing process beforehand, i.e., bead blasting and mechanical finishing. If the aluminum object goes directly to anodizing, there is a greater probability of the appearance of scratches or machine marks.

Tolerances

You should also consider the tolerances while applying the anodizing process. If you want tight tolerances, prefer type 1 and type 2 anodizing.

Work with Batches

To color the aluminum parts, you must anodize them in small batches. As a result, you’ll experience high color uniformity.

Sulfuric Anodizing3

How Can You Identify if the Anodizing Process is Successful?

Once you have completed the anodizing process, you may need to ensure whether the process has been successful or not. Below are some ways to let you know the success of the process:

Simple Scratch Test

Take a coin and scrape it on the surface of anodized aluminum. Then, look for the scratch; if it is visible, it’s not anodized yet; instead, it’s just been polished. This is because anodized products are always scratch-resistant.

Matte Finish

If the matte finish appears, you can identify that anodizing is successful.

Color Dispersal

A successful anodizing process results in a uniform surface finish with consistent color dispersal.

Sulfuric Anodizing1

 

How is Anodizing Beneficial?

The following are some advantages of anodized aluminum that make it a perfect fit for your applications:

  • Durability

No doubt, aluminum itself is a harder material, but after its anodizing, it becomes super tough and durable. Typically, anodized aluminum is three times harder than standard aluminum. It offers a longer lifespan with an excellent finish and doesn’t flake or peel even when colored.

  • Budget-Friendly

Anodized aluminum is a budget-friendly alternative to many other materials offering the same visual effects, for instance, zinc, copper, steel, etc. Also, it features a high strength-to-weight ratio that lowers its total cost per square foot. In addition, its lightweight benefits you in reducing the shipment cost while transporting it to other areas.

  • Recyclable

Fortunately, only aluminum is 100% recyclable among all metals. The anodizing of aluminum is also an environmentally friendly process as it doesn’t release any harmful components into the environment. Additionally, the recyclability and renewability of aluminum are not disturbed during its anodizing. Therefore, unlike spraying and electroplating, anodizing is a fairly safe process.

  • Ease of Maintenance

Another remarkable benefit of aluminum anodizing is that no extra effort is required to maintain its durable finish. You just need to wash it with soap or mild detergent to remove the dirt. Ultimately, you’ll get clean and dirt-free anodized aluminum parts ready for use.

  • Corrosion-resistance

When anodized aluminum is exposed to air or other oxidizing agents, a thick layer of aluminum oxide is formed. This layer makes the aluminum corrosion-resistant. Thus it is less likely for it to undergo rust and damage.

  • Offers Good Machinability

When it comes to CNC machining, anodized aluminum is ideal for it. It can be easily chipped, making it a perfect choice for machine operators to manufacture the parts with great accuracy and speed.

  • Appearance

In addition to a uniform surface finish, aluminum anodizing offers quite a fascinating appearance to the parts that serve to increase their beauty. Naturally, aluminum features a bright silver color, but you can give it a matte or a luminous finish while painting. On top of that, you may also provide different shades and textures to anodized aluminum parts to make them more unique and attractive.

 

Applications for Anodizing Aluminum

Let’s discuss some of the significant applications of anodized aluminum:

Anodize Aluminum Parts

  • Medicine and Health

In the medical sector, anodized aluminum is extensively used for making dental and medical equipment. Mostly, the devices and equipment used in emergency and operating rooms are made of anodized aluminum. It helps to prevent the instruments from rusting.

  • Electronics

Anodized aluminum has endless applications in the electronics industry. It offers the manufacturing of hundreds of electronic products that show excellent temperature resistance. Some examples include computers, gaming systems, telephones, and solar panels.

  • Food and Packaging

As anodized aluminum is waterproof and easy to clean, it’s ideal for the food and packaging industries. Plus, it doesn’t flake and provides fungus resistance to the food. The typical applications in the food and packaging industries involve the formation of grills, pans, cookers, ice makers, bakeware, and other kitchen equipment.

  • Aerospace Industry

The aerospace industry requires all material properties to be in anodized aluminum. For instance, low density, corrosion resistance, durability, flaking resistance, and environmentally friendly.

 

Anodizing and the Environment

The best feature of anodizing is its environmentally friendly nature. The process uses simple water-based chemicals that don’t give rise to the formation of harmful by-products. Other by-products of this process are usually recycled. On the other hand, aluminum hydroxide and aluminum sulfate by-products act as filters in the secondary treatment of sewage. Also, the U.S. Environmental Protection Agency has declared the anodizing process harmless to the environment.

 

Conclusion

Hopefully, in the end, you are there with enough knowledge about how to anodize aluminum and its advantages. Generally, anodized aluminum is widely used in aerospace, automotive, homeware, and architecture industries for producing highly durable parts with uniform finishing.

At Jiahui, our technical experts have hands-on experience in anodizing different aluminum parts. If you’re seeking to anodize your aluminum products or prototypes at competitive prices, contact us now and get outstanding services.

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