How to Define the Hardness of Hard Coat Anodizing

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Anodizing Aluminum: What it is and Why I love it

We are moving our blog! Anodizing World will still be available, but new posts will be published on from now on.

To read the blog post "Anodizing Aluminum: What it is and Why I love it" please click 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

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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!

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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

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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.

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