Today, Anodized Aluminum enjoys use in many applications from simple kitchen appliances to parts for aircraft and trains. The wide range of use it enjoys is made possible by its excellent properties like corrosion and wear resistance.
In this guide, we’ve compiled everything you need to know about anodizing aluminum including how it’s made and how it is used. So, let’s begin with a simple question:
What Is Anodizing Aluminum?
Anodizing is an electrochemical finishing process used to create a protective oxide coat over the base metal’s surface. This coat gives the metal protection against corrosion, wear, and other chemical attacks. It also enhances the visual aesthetics of the metal, giving it a high-quality look.
So, how does it work?
Aluminum and its alloys undergo an oxidization process when exposed to oxygen. The pure metal reacts with oxygen to form an oxide layer on the metal to protect it from further oxidation. This process is known as passivization. Anodizing accelerates and amplifies this natural process to deepen the protective oxide layer in the metal and increase its protective properties.
What Are The Benefits?
Aluminum anodizing offers many benefits. Like we mentioned before, pure aluminum is already a pretty remarkable material but with anodizing, it gains many additional properties that transform it into top-class material. Let’s look at some of these properties:
Increased wear and corrosion resistance.
The metal retains its color for a long time.
Improved and visually appealing surface finish.
Electrical and thermal insulation.
Increased durability and service life of the part.
The metal becomes easier to clean.
What is the Manufacturing Process?
Over the years, there have been many different methods developed for anodizing Aluminum. But, the chemical manufacturing process behind anodizing remains the same. Let’s take you through it.
First, the Aluminum is cleaned with an alkaline detergent to remove any foreign contaminants and oil. Cleaning is important to ensure a smooth and even coat.
Next, the top layer of the aluminum is removed by chemical etching. The etching is the process of removing the native oxide film with an acid. It is to make sure the surface to be anodized is free of imperfections. The residue left behind by the etching can be removed by dipping it in a solution of nitric acid
Anodizing is done by submerging the aluminum into an electrolytic cell and passing a direct current through it. The aluminum forms the anode(positive terminal) of the cell. The current initializes an electrochemical reaction at the surface of the aluminum. During this reaction, aluminum atoms are extracted from the metal, giving it a pitted, porous structure.
As the reaction goes on, the electrolytic solution releases oxygen at the anode. This oxygen combines with the free aluminum atoms at the surface to form the aluminum oxide layer. The thickness of this oxide layer depends on the voltage applied, temperature, and the solution in the electrolytic cell.
3. Post-Anodizing Treatments
Different types of treatments are done on the Aluminum after anodizing to give it some desired properties. One of these treatments is coloring. Anodized aluminum can be dyed into a variety of colors. The open-pore structure of the surface after anodizing makes it easier for the metal to retain the dye.
After coloring, the aluminum is sealed to prevent the dye from leaching out of the pores. Sealing is done in many ways, but the most popular methods used are boiling in de-ionized water or infusing the aluminum’s pores with a sealant like Nickel Acetate or Teflon.
What are Different Types Of Aluminum Anodizing?
Aluminum anodizing processes are classified based on the type of acidic electrolyte used and the thickness of the oxide layer created. The thickness of the oxide layer is important as it determines the protective properties of the oxide layer.
Based on these criteria, there are three commonly used processes. Let’s take a look at them:
1. Type I (Chromic Acid)
It involves using Chromic acid to create a thin oxide layer on the metal. The oxide layer crated is ductile, flexible, and does not easily break tolerances. The thickness of the oxide layer is between (0.5μm to 18μm). Although the oxide layer created is thin, it still has increased corrosion and wear resistance. However, the resulting surface isn’t easily colored.
Type I is one of the oldest anodizing methods. It has been discontinued and replaced by the new Boric-Sulphuric methods in some countries due to environmental concerns. However, it is used in some cases where tight tolerances are needed.
2. Type II (Sulphuric Acid)
This process is the most popularly used anodizing method in the industry. It uses Sulphuric acid to create a moderately thick oxide layer(1.8μm to 25μm). The increased thickness means it has better wear and corrosion resistance. It also means it can easily retain dyes and lubricants on its surfaces.
3. Type III(Hard)
Type III anodizing is also done with sulphuric acid. It is called “Hard” anodizing because the oxide layer created is far thicker than those previously seen (13μm to 150μm). Anodized aluminum created using this method has the best wear and corrosion-resistant properties. It can also be colored and dyed, but this is discouraged.
The type III method isn’t suitable for projects where tight tolerances are needed. It’s best suited for creating parts for applications where extreme wear is expected.
What are The Common Applications?
Anodized Aluminum can be found in almost every technological sector. The impressive structural properties of Aluminum, coupled with the given to it by anodizing have made it a favorite. Let’s take a look at some of the ways it is used in various sectors.
It is a popular material in the construction industry. It is the second most used material after steel. Its lightweight structure and beautiful surface finish make it ideal for external finishes and decorative facades for buildings and other architectural structures.
It is also used extensively in the auto and aerospace industry. The superior strength to weight characteristics of aluminum makes it a better alternative to steel. So, airplane makers, carmakers, and even spacecraft manufacturers have turned to anodized aluminum to create lasting lightweight components.
Anodized aluminum is one of the hottest materials used in consumer electronics right now. Its premium look and durability have made it a material of choice for creating premium electronics. Companies like Dell and Apple use this finishing to create some of their more premium offerings.
What Colors Can You Anodize Aluminum?
Aluminum can be dyed into many different colors. Many bright decorative colors like green, purple, red, black, etc, can be gotten with organic acids while darker colors like bronze can be gotten with metallic salts.
The receptiveness of the aluminum surface to color is dependent on the thickness of the oxide layer. Chromic acid anodizing has the least receptiveness, type 2 can receive all sorts of colors and sealing while type 3 can also be colored easily. But coloring and sealing type III aluminum is advised against because it loses about 20% of its corrosion resistance when it is sealed.
What Is Aluminum Anodizing Thickness?
Aluminum anodizing thickness is the thickness of the oxide layer. Like we mentioned before, the thickness of the oxide layer varies with the type of anodizing process used. Chromic anodizing produces the least thickness while hard anodizing produces the highest thickness.
The increase in dimension of the part after anodizing is equal to half of the anodizing thickness. During the anodizing process, half of the oxide layer grows out on the surface; the other half grows into the surface.
The depth of the anodizing layer thickness directly influences the wear resistance and corrosion resistance properties of the oxide layer. Deeper anodizing thickness often produces surfaces with better wear properties.
However, the oxide layer becomes more brittle and prone to thermal cracking with increasing thickness. Also, abrasion resistance decreases with increasing thickness.
What Are Design Tips For Anodizing Aluminum Parts?
When anodizing aluminum, there are some tips and practices that need to be followed to get the best results out of the process.
1. Work In Small Batches.
Working in small batches is advised especially when coloring the Aluminum. This is done to ensure color uniformity among the parts. The shades of colors can differ between processes, so it’s best to work in batches.
2. Monitor Tolerances
Dimensional changes occur during anodizing. For some parts that require tight tolerances, this can affect their fit. It’s best to make allowances for the dimensional changes in the original design or use either type I or type II anodizing to minimize the dimensional change.
3. Forming And Shaping
All metalworking processes like forming drilling, shaping et. Should be done before anodizing. After anodizing, the metal, the coating formed on the surface is very hard and resistant to elastic deformation. Cracking can occur if the metal is deformed in any way, and cracks in the surface can compromise the integrity of the coating.
4. Remove All Steel And Iron Parts.
Steel or Iron parts like rivets, screws, etc should be removed before anodizing. Anodizing is only suitable for aluminum and materials like Iron, steel can be destroyed during the process.
Can We Anodize An Entire Part?
It is nearly impossible to anodize an entire part. During the anodizing process, the metal has to be dipped into various baths and tanks. The metal needs to be held or supported in some way while it’s in the tank. These support points are often shielded from the anodizing process.
Also, to supply power to the anode, electrodes needed to be placed on the metal. The larger the part, the larger the surface area of electrodes needed. These electrodes often shield the surface from the anodizing solution.
To solve this problem, special grips can be designed into the part specifically for holding the part during anodizing. After the anodizing process is complete, these grips can be cut or machined off.
Does Anodizing Wear Off?
No, it does not. Anodizing, unlike all other finishing methods, electrochemically bonds the Aluminum oxide to the base metal. This means that the anodizing will not peel, chip, or flake as it is part of the metal.
However, in colored aluminum, some telltale signs of wear, like the fading of the colors, can be observed after some time. This sort of wear and tear cannot be avoided, but it can be postponed by using proper anodizing and sealing processes and periodic cleaning and maintenance of the part.
How Long Anodized Aluminum Can Keep?
Anodized aluminum has a very long lifespan. If the anodizing is done right, the part can end up lasting for several decades. But, there are still some factors that can make the lifespan of some parts shorter than others.
The lifespan of the part depends on the service conditions and the environment it operates in. For example, the lifespan of an anodized part operating in a Heavy Industrial environment might not be up to 20 years while the lifespan of an anodized part operating in construction may be as much as 50 years.
How To Strip Anodizing From Aluminum?
The anodizing coat might need to be stripped off for several reasons. Maybe, it has already become dirty, faded, and needs to be re-anodized or the part needs a new polished aluminum look. Anyway, there are several methods for stripping aluminum. Let’s look at some of them.
The first method is through chemical etching. This involves submerging the anodized part in a caustic solution like (NaOH) or (KOH). This solution dissolves the anodizing and also removes a layer of aluminum under the coating giving the metal a pitted or matte surface.
Acids can also be used to strip anodizing from aluminum. This method dissolves the anodizing coat away using a combination of phosphoric acid and chromic acid. Unlike the previous method, it does not affect the surface of the metal.
What Is The Difference Between Anodizing And Powder Coating Aluminum?
Powder coating is a finishing process that uses a dry free-flowing powder to coat the metal’s surface. The metal to be coated is grounded while the powder is negatively charged to make it stick to the surface. The coating is then cured with heat.
Anodizing differs from powder coating in one fundamental respect; the former is an electrochemical process while the latter is an electrostatic process. In anodizing, the coating is grown from the metal’s surface while in powder coating, the decorative finish is applied to the part.
Can Other Materials Be Anodized Other Than Aluminum?
Yes, there are procedures for anodizing materials other than aluminum. Non-ferrous metals like titanium, magnesium, and tantalum can also be anodized using different procedures.
How Much Does Anodizing Aluminum Cost?
Anodizing Aluminum is cheap when compared to other finishing methods. It is also very environmentally safe producing little non-recyclable waste. The total cost of anodizing varies from vendor to vendor, but it is usually dependent on these factors;
The type of anodizing process used.
The dimensions of the parts to be anodized.
The post-anodizing treatments (coloring, sealing) are required.
The thickness of the oxide layer.
To get a suitable price quote, you can apply to your local anodizing provider to find out the going rates for anodizing.
What To Look For In An Anodizing Provider?
Anodizing providers are metalworking professionals that can provide any service from fabrication to finishing. Selecting the right vendor for your job is very important because the quality of the final product depends on it.
So, how do you select the right professional? Let’s take a look at some factors to consider when selecting the vendor.
1. Industry Experience
There are many anodizing vendors out there, and each vendor might cater to a specific industry. For example, an aerospace metalworker might not know the standards involved in anodizing aluminum for construction purposes. So, it’s best to go with a vendor with proven experience in the appropriate field.
Finding the vendor with the right certifications from the appropriate regulatory agencies is very important. Certifications like the QC pass percentage and the ISO standards are very important factors to check out when selecting a vendor.
It’s best to go with a vendor that has a very reliable supply chain to get the highest quality materials on time. The type of materials used can have a great impact on the final product. So, it’s best to go with vendors that only use the best materials from reliable suppliers.
When selecting an anodizing provider, word of mouth can be an invaluable tool. There are several forums and review sites online that you can browse to get an accurate picture of the services provided by vendors. With comments and testimonials from past works, choosing a vendor is made significantly easier.
Now that you know more about anodizing aluminum, you can judge if this process works well for your project.