Anodizing Aluminum: What it is and Why I love it

I have been working in the anodizing industry for more than 25 years and I am still as passionate about it as on my first day! 

Therefore, I decided to re-launch my blog about anodizing. You can still access my old blog here, Anodizing World, but I will publish new blog posts at AluConsult new website from now on. 

To get noticed about a new blog post and learn more about anodizing, make sure you sign up at AnodizingSchool to receive new blog posts!

I will continue to use this space for awhile but will eventually redirect you to  AnodizingSchool.

What is aluminum anodizing?

Anodizing is an electrochemical process to thicken the naturally formed oxide layer of aluminum. Through this process, the oxide layer grows from nanometres to microns in thickness. As a result, your aluminum product not only gains a consistent appearance but also higher corrosion resistance. Improving the corrosion resistance will also make your aluminum more versatile for other applications. 

Another reason why I love aluminum anodizing is that anodizing is the most environmentally friendly surface treatment for aluminum. You will learn more about the sustainability aspect of anodizing later in this blog post. But in short, anodizing allows us to recycle the whole aluminum product. So if we want to think green, anodizing is our solution.

If you are working with aluminum and you want to learn the basics of anodizing applications, the benefits, and the process steps of anodizing, this article is for you! 

What do you use aluminum anodizing for?

It is probably easier to answer the question “what can anodized aluminum NOT be used for?” Anodized aluminum is an all-rounder and can be used for airplanes, solar panels, windows, curtain walls, laboratory equipment, and much more. However, there are some limits. Usually, anodizing is not used for electrical connection, products exposed to high or low pH or parts that will be bent.

After all, aluminum products need surface protection that ensures a life-long usage. Anodizing does not compromise the low density and high strength of aluminum but enhances the surface properties, such as corrosion resistance, scratch and wear resistance, chemical, heat, and electrical resistance.

But to give you an even simpler overview, the properties of anodized aluminum can be divided into three main categories: 

PROTECTIVE PROPERTIES, meaning it is designed to protect the aluminum application from external damage. You can see in the example instrumentation pods for the FOCUS 3 made of anodized aluminum from MacArtney. Thanks to the anodized surface they are more resistant to potential impact with the seabed.

Aluminum Anodizing:  instrumentation pods for the FOCUS 3
Instrumentation pods for the FOCUS 3

TECHNICAL PROPERTIES, involving or concerned with applied and industrial use. The picture below shows the hard-anodized FGR 250 front fork from Ohlins. Due to the anodizing treatment, the surface has lower friction than if the surface was bare aluminum.

You can learn more about hard anodizing here.

aluminum anodizing: hard anodized FGR 250 front fork
Hard-anodized FGR 250 front fork

DECORATIVE PROPERTIES, using anodizing for the aesthetic factor are probably one of the most important surface properties. Often the decorative appearance is connected with some necessity for protection too. For example, if your smartphone has a colored aluminum backside, it is most likely anodized.

How does the aluminum anodizing process work?

The anodizing process consists of a list of process steps and can be divided into three phases:

Phase 1: Pre-Treatment

To prepare the aluminum surface for the anodizing step you need to follow the steps of degreasing, etching, and desmutting. 

Phase 2: Anodizing

In the electrolyte in the anodizing process tank, the passing electrical current builds the anodic film on the aluminum surface and thereby thickens the natural oxide layer.

Phase 3: Post-Treatment

Finally, the aluminum can be colored and will afterward be sealed to close the pores in the anodic film.

Of course, in each phase, you have to follow various steps, which I explain in-depth in my Anodizing Masterclass.

In the Anodizing Masterclass, the first and only online anodizing course, I also explain why rinsing is one of the most neglected steps in the anodizing process. As will be explained in a later blog post, rinsing should be done repeatedly throughout the whole anodizing process to ensure a clean surface and by this a flawless anodizing result for your aluminum product.

9 benefits of aluminum anodizing

We have already touched on some benefits of anodized aluminum such as corrosion resistance and the increased sustainability. But there are many, many more benefits to anodizing your aluminum!

  1. The thickened oxide layer is almost as hard as diamond protecting a relatively soft aluminum material.
  2. The properties of the anodized surface can be customized to unique and specific applications.
  3. Anodizing decreases the life-cycle cost of aluminum by a longer lifetime, lower finishing costs, higher durability, and lower maintenance costs.
  4. The anodic coating is a conversion of the aluminum to aluminum oxide, creating a chemical bond to the metal, and therefore cannot – in theory – chip or peel.
  5. The anodized layer is “glasslike” or transparent. This way the surface looks exactly as before the anodizing (if you don’t apply any coloring).
  6. The anodic film is porous and is therefore perfectly suitable for long-lasting coloring.
  7. Anodized aluminum has a metallic appearance – unlike a painted surface which creates a more plastic-like appearance.
  8. Most of the colors included in the porous layer have a high UV and weathering resistance – when processed properly
  9. Anodizing is the most environmentally friendly surface finishing process. If we use sulfuric acid anodizing and colors without chromium included, you can call the anodizing process the greenest and most protective coating of aluminum.

What are the different anodizing types

Type I: Chromic Acid Anodizing

The first anodizing process that was invented is the chromic acid anodizing (CAA) also known as the Bengough-Stuart process. This type of anodized aluminum is mostly used in the aerospace industry because of its ductility.

Type II: Sulfuric Acid Anodizing

The most common type of anodizing is sulfuric acid anodizing (SAA). SAA is used in 70 % of all anodizing solutions. If you want to learn all about the type II anodizing process, have a look at the free preview of my Anodizing Masterclass.  

Type III: Hard Anodizing

Hard anodizing gives you an even more corrosion-resistant surface than type II anodizing with a hardness of high-strength steel. If you would like to learn more about hard anodizing, I invite you to join my free webinars. Sign up for my newsletter to get informed.

Clear Anodizing

Clear anodizing is not a type of anodizing itself. Instead, it refers to type II anodizing but specifies that no color has been used to change the appearance of the surface. As anodizing already gives your aluminum product a metallic appearance you can use clear anodizing as a design feature.

Anodizing Aluminum: Example of a clear anodized geometric figure
Clear anodized geometric figure

Black Anodizing

Black anodizing also refers to type II anodizing but with the surface colored in black. 

Black Anodizing: Black aluminum surface
Black anodized aluminum

Can anodized aluminum be colored?

The aluminum oxide layer formed in the anodizing process is a porous aluminum oxide that can be colored. However, you have to be careful when coloring your anodized aluminum. Depending on the chosen aluminum alloy you might have a coloration of the aluminum oxide layer. This can change the appearance of your final color layer by a few nuances.

When talking about the color specifications with your aluminum supplier (if you are the end-user) or with your customer (if you are the anodizer) the common procedure is to just use two colored coupons to show the acceptable color range. However, this procedure is unreliable as people simply have different abilities to judge a color variation, which can lead to unpleasant and unproductive discussions.

Now, let me share some learning from my 25 years in the anodizing industry: when talking about color specifications, always agree on a number defining the desired color. Color difference can be evaluated by a DE which is a number consisting of the darkness/lightness, how blue/yellow, or how green/red your color is. The left side of the sample below is more yellowish with a b+-value = 1.47. In comparison, the right side has a b+-value = 1.038.

A later blog post will explain this color measuring process in depth. But if you can’t wait to learn about it, have a look at module #8 in Anodizing Masterclass where you will be given the full explanation.

Aluminum Anodizing: examples of colored anodized aluminum samples
Black Anodizing Example

Sustainability: Why aluminum anodizing is the best surface finish

The reason why anodizing is one of the most environmentally friendly surface treatment processes for aluminum is that coatings such as paint as opposed to anodizing can dramatically reduce the ability to recycle the aluminum. Paints, plastics, and plating rely on problematic materials such as organic solvents that can compromise the recyclability of aluminum whereas anodized aluminum is “recycle-neutral”. 

The reason why anodizing does not need to use organic solvents lies in the anodizing process itself. The anodic coating is generated from the base metal in a water-based process and, thus, has essentially the same constituents as the aluminum itself: the anodized layer consists of ultra-thin, nontoxic aluminum oxide.

Do you want to join the movement and make the aluminum industry more sustainable? Then have a look at the AnodizingSchool, where I share free materials for a growing knowledge community.

Getting the perfect anodized product

Anodizing is no black magic, but you need to understand the process to get consistent and high-quality anodizing products that will never fail your customers. If you want to become a master in anodizing, I invite you to have a look at AnodizingSchool, where I share all my knowledge about anodizing.

If you need immediate help with your anodizing line, you can also hire me as a consultant. Contact me here.


Join the Free Webinar about Hard Anodizing

Dear Anodizing World readers

Happy New Year! 

As you know, AnodizingSchool was launched in 2020 as a new learning community for anodizers and end-users of aluminum anodized products.

The aim is to create a one-stop online space, where you can get access to articles and webinars for free, but also buy specialized e-learning courses.

We currently offer the Anodizing Masterclass, and in 2021 we will publish new courses about Hard Anodizing, Pulse Anodizing and Anodizing for End-users.

The first event will be a FREE webinar about Hard Anodizing!

The webinar will be a guide to Hard Anodizing, also called Hard Coat including the following topics:

  1. Intro to Hard Anodizing, also called Hard Coat
  2. The history of Hard Anodizing
  3. How to create a Hard Coat
  4. Properties of a Hard Anodized surface
  5. How to specify a Hard Anodized layer
  6. What defects and other problems arising from a Hard Coated Surface
  7. Some Key takeaways and Q&A

Reserve your spot by sending an email to

If you find this article useful and you would like to know more please contact me

Get 3 amazing benefits!

Introduction to Anodizing 
has evolved and is changing its name to 

Anodizing Masterclass 

All 12 modules of the masterclass is now available! 
Learn the Better than Best practice of aluminum anodizing and bring your anodizing process to the next level!

Take a look at Anodizing School to read more about our aim to create an new online learning community for anodizers and end-users of aluminum anodized products. 

Interested? Get 3 amazing benefits if you sign up now

First, get a FREE 1-2-1 call to take the temperature of your anodizing process and identify the full potential of your line.

Then get access to FREE lessons from the Anodizing Masterclass!

And lastly, get the Christmas discount 25% of your purchase


Follow the link below to read more about the full course

Use the code CHRISTMAS to get a 25 % discount when purchasing the online course Anodizing Masterclass. (Offer expires on 24th of December.)


Influence of temperature on Voltage response when Anodizing

A question from a customer:

We have investigated the influence of the electrolyte temperature in our anodizing tank - Anodizing at a low and a warm temperature, current controlled and in our standard anodizing electrolyte.

The voltage at the higher temperature runs with a lower value than the voltage with the lower temperature and at the same time the voltage stays constant for the duration of the anodizing process. 

At the lower temperature the voltage keeps rising until we turn of the power. 

Do you have an explanation of why the voltage at the higher temperature stays constant, although the layer thickness is identical with both temperatures (with identical current settings and anodising time)?


Higher temperature in the anodizing electrolyte leads to higher conductivity, which means lower voltage for same current and time.

So your Vhigh temp is lower than your Vlow temp


Then Ohms law gives you             V = R x I

Vhigh temp = Rhigh temp x I

 Vlow temp = Rlow temp x I


I = same for the two temperatures


The total resistance of the electrical circuit consists of several resistances:


High temp:


Rhigh temp = Relec, high temp + Rthickness, high temp + …..


Low temp:


Rlow temp = Relec, low temp + Rthickness, high temp + …..


Lower temperature gives lower conductivity, so Relec, low temp ˃ Relec, high temp which is the reason for a higher Vlow temp

Higher temperature of the electrolyte will lead to faster chemical attack of the formed aluminum oxide changing the structure of the formed oxide and the resistance Rthickness, high temp

This will lead to an equilibrium thickness, where formation rate of oxide = dissolution rate of oxide leading to a constant voltage.

Lower temperature of the electrolyte will lead to a higher V0, low temp than V0, high temp for same current I.

The coating weight of the oxide layer is higher when formed at lower temperature - more compact and by this Rthickness, low temp will be higher than Rthickness, high temp. 

This is the reason for a continues increase in Vlow temp during the process time but eventually you should see a steady voltage here too.

If you are curious and want to know more about temperature, voltage and other parameters when anodizing in sulfuric acid you should sign up for the first and only online anodizing course!

Introduction to Anodizing - Click here!

If you find this article useful and you would like to know more please contact me 

Spangle on etch aluminum EN AW 6063

The spangle effect on the aluminum EN AW 6063 alloy has been known for many years.

There are still a lot of unanswered questions but more recently explanations have thrown light on some of the issues. 

The spangling effect on aluminum can be divided into three categories: grainy, galvanizing and sparkling.

The type where the grains are visible by the naked eye after etching is called grainy. Galvanizing is used when only few of the grains are shiny, and the sparkling is when the whole surface is shiny.

The first picture shows a EN AW 6063 aluminum alloy directly from extrusion - not a very beautiful appearance and with a lot of die lines - looking closer you can maybe see some of the grainy structure already. 

The next picture shows the surface after etching and anodizing. Here a very grainy structure is seen leaving to a very unattractive surface - especially if it should fit together with areas which are not having this surface appearance.

If you as a designer want to create something special - this could be a very interesting surface appearance to work with.

Well, back to the subject - why do we see this spangle effect on anodized aluminum.

There are two reason to look for - the first one is the content of zinc (Zn) in the etching tank and the second is the content of Zn in the aluminum alloy.

The grainy appearance is caused by the chemical composition, whereas the two more shiny appearance are due to the content of Zn in the solution and grain orientation.

The orientation of the grains has an influence on how much they are etched in the alkaline solution. Zinc is more noble than aluminum so in the alkaline solution there will be a selective dissolution of aluminum. This will lead to a higher concentration of Zn in the grains with a certain orientation creating small galvanic elements from grain to grain.

If at the same time there is zinc in the alkaline solution, even as little as 5 ppm can cause spangle under the right conditions.

Addition of a small amount of sodium sulphide can be used to precipitate the zinc decreasing the spangling effect of the aluminum alloy.

The EN AW 6063 itself should not contain more than 0.03% zinc.

If you want to know a lot more about anodizing and defects

Please sign up for more information regarding the first and only online anodizing course - Introduction to Anodizing presented by the Anodizing School.

If you find this article useful and you would like to know more, please contact me 

How to remove a heavy, mixed oxide film on aluminum

Working on the materials for the first and only online course about anodizing - the Introduction to Anodizing and thought I would like to share some information about how the natural formed oxide layer can be difficult to remove in our etching tank.

Pre-enroll here for more information about the online anodizing course

Aluminum reacts immediately in contact with air forming a very thin oxide film - normally not more than 1 - 3 nm in thickness.

If you want to read more about this you will find an earlier blog post about 
The natural formed oxide layer by clicking the link

When aluminum alloys are heat treated this natural formed oxide layer thickness up to 10 times of the thickness of the air formed. The oxide layers formed at high temperature are more difficult to remove in the etching solution than the air formed ones.

The reason for this is the alloying elements - these alloying elements create an oxide with uneven thickness, as you can see here in the drawing.

The major alloying element incorporated within the film is magnesium. Magnesium is the primary alloying element in the 5000 series - aluminum magnesium alloy and is the major alloying element together with silicon in the 6000 series aluminum magnesium silicon alloys.

The mobility of magnesium atoms at temperatures above 340𝇈C are much higher than for aluminum atoms.

Magnesium atoms will diffuse from the bulk of the alloy through the surface and oxidize up to 5 - 10 times faster than the aluminum atoms.

Leaving 5000 and 6000 series aluminum alloy with a heavier and mixed oxide - containing aluminum oxide - the alumina and magnesium oxide - the magnesia.

Because the magnesium oxide is significantly more resistant to alkaline etching than aluminum oxide a patchy appearance can be found after etching.

The patchy appearance is due to dissolution of the aluminum oxide creating a surface with areas with a film of magnesium oxide and by this localized attack can roughens the surface.

A deoxidizing before etching can be used to remove this film, or some of the new types of acid etch - another topic which is covered in an earlier blog post - Acid etch - the hot topic.

If you find this article useful and you would like to know more please, contact me

Open for Early Registrations: The "Introduction to Anodizing" online course

Anodizing is a very complex process, undertaken by highly specialized and experienced personnel. Sometimes however, problems occur out of nowhere and technical personnel working with anodizing may not be able to solve the problem. Perhaps they are working from experience only, or they have never learned the actual reasons behind the many processes involved.

Often you stand as an anodizer and really don´t know what happens and why. Sometimes you have solved the problem, but then it comes back months later, or sometimes already after a week. Most of the times you work under pressure to solve the problem because anodizing is always the last step in supply chain.

The five reasons for why it often goes wrong when anodizing aluminum!

1. Lack of understanding the anodizing process steps besides experience

2. Lack of chemical and electrochemical knowledge
3. Lack of understading of the aluminum material
4. Failure in understading the importance of narrowing fluctuations in all process steps
5. Forget to acknowledge the rinse tanks as process tanks

Do you want to be better than "Best Practice"?

Then pre-enroll for early registration today - SIGN UP

This anodizing course "Introduction to Anodizing" will take you through the background and step-by-step processes, and help you deal with some of the challenges that come up quite often. This course is therefore important in allowing you to produce products of consistent quality for your customers.

The Benefits by taken this online "Introduction to Anodizing" course!

1. Repeated and consistent high quality of your anodizing
2. Delivery security to your customers
3. Optimized production - more quality parts through your anodizing line
4. Happier customers and employees

If you can´t wait to get started then pre-enroll for early registrations today  - SIGN UP

If you find this article useful and you would like to know more please contact me