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 aluconsult.com from now on.
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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:
- Intro to Hard Anodizing, also called Hard Coat
- The history of Hard Anodizing
- How to create a Hard Coat
- Properties of a Hard Anodized surface
- How to specify a Hard Anodized layer
- What defects and other problems arising from a Hard Coated Surface
- Some Key takeaways and Q&A
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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)?
Answer:
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
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!
Spangle on etch aluminum EN AW 6063
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.
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.
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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.