What is Aluminum Anodizing?
Aluminum anodizing is an electrolytic process used to increase the thickness of the tightly adhering oxide layer that forms naturally on any aluminum surface exposed to air.
The anodized layer has a porous, ordered structure. This porosity makes it easy to perform secondary processes on anodized parts, such as coloring them with dye or sealing their surfaces.
The anodized oxide layer acts as a barrier to protect the aluminum better from corrosion and wear compared to aluminum’s natural oxide.
Aluminum anodizing is a widely used finishing process because it is inexpensive, durable, and does not require special skills or equipment.
The purpose of anodizing aluminum is to increase its wear and corrosion resistance. Aluminum is a popular metal for manufacturing products from cookware to car parts, because it is strong, yet still lightweight.
However, aluminum is also highly susceptible to corrosion and wear when the corrosive potential of the environment increases like exposure to seawater and other extreme conditions.
To prevent this, manufacturers often anodize the metal, which creates a thin oxide layer that protects against corrosion and wear.
Anodized aluminum is also often used for its aesthetic properties, as the anodization process can produce a variety of colors.
Aluminum anodizing is used wherever aluminum components may be exposed to corrosive or hard-wearing applications, such as automotive parts, bicycles, and outdoor furniture.
Anodized aluminum can be easily dyed to produce a scratch-resistant colored surface. Therefore, it is used for many consumer products to both improve their appearance and increase their durability.
Examples of applications include architectural cladding, aluminum canoes, boats, and kitchen utensils.
Anodizing will also turn the aluminum into an insulator since the oxide coating is not conducive.
How Aluminum Anodizing Works?
It is possible to anodize an aluminum part no matter what manufacturing method. The anodizing aluminum process is a cost-effective and simple method, so it has a wide range of applications for various industries.
Before understanding how anodized aluminum works, we should first know the selection of anodizing materials and the preparations that need to be done before anodizing the parts.
Materials Needed To Anodize Aluminum
The materials you’ll need to anodize are pretty basic and not too expensive. If you think you’ll be doing this somewhat regularly, it makes sense to set up a home anodizing station.
Here’s what you’ll need:
- Sulfuric acid
- Distilled water
- Several tanks (containers) to hold the liquids
- A cathode
- Aluminum wire (titanium will also work)
- Degreaser
- Lye
- Acid neutralizer
- A power source
- Dye (if you want a colored part)
You will also need a well-ventilated area to set up your anodizing station, as well as personal safety gear (eye protection, gloves, respirator, etc.).
In addition to the necessary materials above, you may also want to purchase some optional items to make your work easier. These include the following:
- An agitator for the acid bath
- Scotch-Brite pads for cleaning parts before anodizing
- Ping pong balls (put in the tank to prevent acid mist)
- Cheap tea kettle to heat dye
- Fish tank heater to regulate tank temperature
- Cooking thermometer for checking temperatures
Tips for Surface Preparation Before Anodizing
Make use of mechanical finishing techniques when necessary. Etching alone isn’t enough to correct pre-existing surface defects like scratches and dents.
If these marks are evident on a part’s surface, you might as well use mechanical polishing techniques like grinding, buffing, and sandblasting.
Be mindful of your packaging. Before sending out your parts for anodizing, make sure that you have packed them to be dent-proof and well-cushioned. Additionally, avoid sticking tape directly to the surface to be anodized.
Ensure that your parts are properly cleaned and degreased. Look out for uncleaned swarf seated on blind holes, fingerprints on the surface (avoid handling parts with bare hands!), and residual oils from the fabrication processes.
Remove steel inserts like rivets and pins on parts to be anodized as these can corrode differently throughout the different treatment processes.
How to Anodize Aluminum Parts: Step by Step
The working principle behind anodized aluminum parts involves inducing the oxidation of aluminum by submerging it into an electrically conductive acid electrolyte solution where oxygen ions are released. This method uses anodic films to get aesthetic effects.
To further understand the process of anodizing aluminum parts, here are 4 major steps involved:
Step 1: Pre-treating.
The pretreatment process involves achieving a visible finish and a clean work part.
Cleaning: Pretreatment cleaning is important to remove residual grease, oils, and other impurities from a previous fabrication process like extrusion or CNC aluminum machining to avoid impurities and inconsistencies in the finished part.
Etching: The surface finish of a part before anodizing is relevant because it will dictate the quality of your final result.
Tweaking your desired surface finish can be done through etching, where minor imperfections on the surface are corrected while being prepared for anodizing.
Step 2: Anodic Oxide
After the surface is prepared, the parts will go through the anodizing step, submerged into a bath of sulfuric acid electrolyte solution (which may vary depending on the type of anodizing process chosen).
The electrolyte solution has many positive and negative ions and is an electrically conductive solution.
The positive ions can attract the negative plates and the negative ions to the positive anode. An electric current causes it in the circuit.
The negative ion attracts the aluminum parts, which is the positive anode. The aluminum parts will serve as an anode.
At the same time, a cathode is installed within the tank to allow an active passing of electrical current through the system and induce the release of oxygen ions from the electrolytic solution.
This process will create aluminum oxide in the substrate, which is also called a barrier layer. But it is rougher than the aluminum surface.
Step 3: Coloring
The anodic coating formed on the surface layer is porous by the structure, which allows the addition of color into the part.
Various ways of adding colors to an anodized part involve submerging it into dyes or dissolved metal salts. This process achieves finishes like black anodized aluminum, gold, nickel, and stainless.
Step 4: Sealing
To close the porous surface created in anodizing and provide a uniform surface, the parts are subjected to a final step that will submerge them into a nickel acetate solution.
Sealing ensures long-lasting color and prevents further corrosion for the anodized part.
How To Anodize Aluminum At Home?
Below are the basic step-by-step instructions to follow for the at-home hobbyist to anodizing aluminum parts yourself. Please keep in mind that it does take time to master the process, and your materials may require some tweaks.
- Use the Scotch-Brite pad to clean the surface and remove any machining marks.
- Put on your safety equipment, including gloves.
- Use the degreaser to clean the part well, then rinse with the distilled water.
- Etch the part by dipping in a lye bath for 3-5 minutes. Use roughly 4 tablespoons of lye to 1 gallon of water.
- Remove from the lye bath and rinse with distilled water.
- Check for cleanliness by pouring water on the piece. If clean, the water should runoff. If the water beads on the surface, the part is not clean enough for anodizing.
- Rack the part by securing it to the aluminum (or titanium) wire. Ensure the connection is good, and don’t forget there will be an unanodized mark where the wire is in contact with the part.
- Create the bath by adding sulfuric acid to the distilled water in your anodizing container at a ratio of 1 part acid to 3 parts water. Note that anodizing is most successful when the bath temperature is 70 degrees Fahrenheit. Anything above 75 F or below 65 F won’t yield good results.
- Now everything goes in the tank. Add the cathode, making sure it won’t touch the part(s). Add the agitator. Suspend your parts in the tank, making sure they aren’t touching anything. Add the heater and thermometer. Add the ping pong balls to cover the surface.
- Make sure the temperature is 70 degrees before proceeding.
- Set up your power supply and connect the positive terminal to the wire attached to the part(s). The negative side is attached to the cathode. Now things can get touchy.
- The amperage is set based on the total surface area being anodized. For a good, hard surface, use 0.03 amps per square inch, and for a softer surface that will take dye better, use 0.02 amps per square inch.
- Start at 16 volts. You can find all sorts of online calculators, but a home setup starting at 16 volts should get you pretty close. Keep an eye on the tank temperature as the process goes along. The temperature can increase when the process is underway.
- While the anodizing process is going on, heat your dyes. For most colors, 140° F works best for most colors, but some work better at cooler temperatures. You’ll have to experiment with your brands and colors.
- Prepare one tank of distilled water and another with your acid neutralizer.
- Turn off the power and remove your parts from the anodizing tank. Dip first in the distilled water for 10-15 seconds, then rinse in the acid neutralizer tank for 5 minutes. Rinse a second time in the distilled water and dunk the parts in the dye. They should immediately begin to take the color, but be sure to leave the parts in the dye for 10-15 minutes, depending on how deep you want the color to be.
- After finishing with the dye, you want to boil the parts for 15 minutes. Boiling hardens and seals the dye.
Types of Aluminum Anodizing Processes
There are three types of aluminum anodizing processes. They are described in more detail below:
Type I – Chromic Acid Anodizing.
This process uses chromic acid as the electrolyte and produces the thinnest coating of all the methods, 2.5 μ (0.0001 in).
Despite the reduced thickness, this process produces comparable corrosion resistance to the other two processes.
The coating produced tends to be darker and does not accept color as well due to its reduced thickness and reduced porosity.
Type II – Sulfuric Acid Anodizing.
This process uses dilute sulfuric acid as the electrolyte. It is the most commonly used technique. The coating thickness ranges from 5.1 to 30.5 μ (.0002-.0012 in).
A common industry specification Xometry and many other manufacturers follow is MIL-A-8625/MIL-PRF-8625 Type II, Class 1 (non-dyed) or Class 2 (dyed).
This coating is harder than the one produced by chromic acid anodizing.
Parts anodized using the Type II process can easily be colored with various dyes, however, it should be noted that colors typically cannot be matched to specific Pantone or RAL colors due to variability in the process.
Sulfuric acid is a relatively low-cost electrolyte when compared to chromic acid.
Type III – Hardcoat Anodizing.
This process makes use of sulfuric acid as the electrolyte but is used to produce much thicker coatings than Type II MIL-A-8625/MIL-PRF-8625 anodizing — typically 12.7 to 50.8 μ (0.0005 to 0.002 in) due to a higher voltage, longer bath immersion time and lower bath temperature.
This coating is harder than tool steel and is used where high levels of wear resistance are required.
Although unpigmented, a clear hardcoat anodized finish will darken the part surface due to the higher thickness of the coating compared to standard anodizing.
Depending on various factors, the natural color can vary from light to darker gray or brown.
Which Type Of Anodizing Is The Best For You?
If your aluminum part is destined for a high-wear environment or will be exposed to harsh conditions, you’ll likely want to opt for Type III or hard anodizing.
This process creates a much thicker, denser oxide layer that will withstand significant abuse and protect the aluminum underneath from corrosion or wear.
This makes it an excellent choice for aerospace, military, or industrial machinery components.
On the other hand, if your part needs to maintain tight dimensional tolerances or will undergo further processing like adhesive bonding or painting, Type I chromic acid anodizing may be the best choice.
This process results in a thinner oxide layer, minimizing dimensional changes, and provides an excellent surface for further coatings.
Type II sulfuric acid anodizing is a good all-rounder, balancing increased surface hardness and aesthetic flexibility.
Compared to Type I, its thicker, more porous layer allows color dyes to be absorbed effectively.
Therefore, if your project calls for vibrant, customizable colors and a balance of durability, Type II could be the ideal fit.
Remember, your project’s specific demands and requirements should dictate your decision.
Anodizing Colors For Aluminum
One of the unique and appealing aspects of aluminum anodizing is the wide variety of color options available.
Anodizing opens up the aluminum’s pores, which allows it to absorb color dyes effectively. The result is a vibrant, durable color that won’t chip or peel like a paint can.
The range of colors is practically limitless, and the precise shade can be controlled through the type of dye used, the concentration, and the length of time the part is immersed in the dye. Here are some of the most common options:
Clear/ Silver:
No dye is used in this process, and the resulting aluminum retains its natural metallic silver color. This is a popular choice for products where a sleek, modern aesthetic is desired.
Black:
Black is one of the most frequently seen colors for anodized aluminum alloys, largely due to its versatility. Black anodized aluminum offers a sophisticated and sleek look that suits many applications.
Gold, Blue, Red, Green and More:
With the correct dyes, virtually any color can be achieved. Vibrant blues, greens, reds, and gold are all possible, allowing you to customize your aluminum parts to suit your specific aesthetic preferences.
It’s important to note that while color anodizing adds to aluminum’s aesthetic appeal, it does not significantly impact the improved corrosion and wear resistance the anodize process offers.
Whether clear, black, or neon green, your anodized aluminum will be robust and durable.
Advantages Of Anodized Aluminum
The unique anodized finish is the only one in the metals industry that satisfies each of the factors that must be considered when selecting a high performance aluminum finish:
#1. Durability.
Most anodized products have an extremely long life span and offer significant economic advantages through maintenance and operating savings.
Anodizing is a reacted finish that is integrated with the underlying aluminum for total bonding and unmatched adhesion.
#2. Color Stability.
Most anodized products have an extremely long life span and offer significant economic advantages through maintenance and operating savings.
Anodizing is a reacted finish that is integrated with the underlying aluminum for total bonding and unmatched adhesion.
#3. Ease of Maintenance.
Scars and wear from fabrication, handling, installation, frequent surface dirt cleaning and usage are virtually non-existent.
Rinsing or mild soap and water cleaning usually will restore an anodized surface to its original appearance. Mild abrasive cleaners can be used for more difficult deposits.
#4. Aesthetics.
Anodizing offers a large increasing number of gloss and color alternatives and minimizes or eliminates color variations. Unlike other finishes, anodizing allows the aluminum to maintain its metallic appearance.
#5. Cost.
A lower initial finishing cost combines with lower maintenance costs for greater long-term value.
#6. Health and Safety.
Anodizing is a safe process that is not harmful to human health. An anodized finish is chemically stable, will not decompose; is non-toxic; and is heat-resistant to the melting point of aluminum (1,221 degrees F.)
Since the anodizing process is a reinforcement of a naturally occurring oxide process, it is non-hazardous and produces no harmful or dangerous by-products.
What Are The Limitations Of Aluminum Anodizing?
While anodizing offers many advantages, like any process, it does come with certain limitations. Here are some to keep in mind:
#1. Limited to Aluminum and Aluminum Alloys.
The anodizing process as described, is specifically designed for aluminum and its alloys. Other metals require different processes to achieve a similar result.
You might need to explore other surface treatment methods if your project involves different materials.
#2. Variations in Aluminum Alloy Can Affect Color.
Different alloys can react differently to the anodizing process and produce slightly different colors when dyed. Even within the same alloy, inconsistencies can cause variations in shade.
Therefore, achieving a perfectly uniform color across all parts can sometimes be challenging, especially between batches.
#3. Dimensional Changes.
The anodizing process adds a layer of oxide to the aluminum, which can alter the part’s dimensions. This is typically a negligible increase, but it could be a consideration for applications requiring exact dimensions.
#4. Hard Anodizing Can Affect Surface Finish.
Type III or hard anodizing can cause a slight change to the surface texture of the aluminum, making it slightly more rough. While this isn’t a problem for many applications, if a perfectly smooth finish is required, this could be a limitation.
#5. Anodized Oxide Layer Can Be Damaged.
The anodized oxide layer offers excellent hardness and durability but isn’t invincible. Intense impacts or abrasions could potentially compromise this layer, subsequently exposing the underlying aluminum to potential corrosion.
Consequently, it’s critical to exercise caution while handling and installing anodized parts to maintain the corrosion resistance attribute of the anodized surface.
This way, the integrity of the protective layer remains intact, ensuring long-lasting performance and durability.
#6. Not Suitable for Large Parts.
The size of the part that can be anodized is limited by the size of the anodizing bath. This means that very large parts may not be suitable for anodizing or require specialized equipment.
Applications For Anodizing Aluminum
Anodized aluminum finds usage in a broad spectrum of applications due to its superior properties. Here are some of the most common applications, along with specific examples:
#1. Aerospace and Automotive Industry.
Anodized aluminum’s superior strength and lightweight characteristics make it perfect for the aerospace and automotive industries.
For example, anodized aluminum is often used in constructing aircraft frames and automotive parts, such as engine components, due to its resistance to corrosion and high temperatures.
#2. Electronics.
Anodized aluminum is widely used in electronics due to its electrical insulation properties. For example, it manufactures MP3 players, cell phones, laptops, and other electronic devices.
Apple uses anodized aluminum in its MacBook Pro and iPhone models for durability and a high-quality finish.
#3. Architectural Structures and Building Materials.
The resistance to weathering and corrosion makes anodized aluminum an excellent choice for architectural applications. Examples include window frames, roofs, and façade cladding on buildings.
The Golden Gate Bridge, for instance, uses anodized aluminum in its construction due to its resistance to the corrosive effects of salty sea air.
#4. Medical Devices.
In the medical field, anodized aluminum creates various devices like surgical instruments, optical equipment, and orthopedic implants. Its corrosion-resistant and non-toxic properties make it safe for contact with the human body.
#5. Sporting Goods.
Sporting goods such as baseball bats, golf carts, and bicycle components often use anodized aluminum. This is because it’s lightweight, corrosion-resistant, and can be colored for aesthetics.
What’s the difference between anodizing and electroplating?
If you’re familiar with electroplating, you may recognize that anodizing is similar in many ways. Comparing the two processes definitely helps to understand anodizing as a unique and important step in fabricating CNC aluminum parts.
Similar to anodizing, electroplating involves placing the part in an electrolyte bath and applying a charge to it. The key difference is that with electroplating, the part gets negatively charged, becoming the cathode instead of the anode.
The anode is applied to a piece of the desired plating material, such as gold. As with anodizing, this circuit allows positively charged ions to flow to the cathode (the part). These ions coat the part with a thin, even layer.