Anodizing World: 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 V_{high temp} is lower than your V_{low temp}

Then Ohms law gives you V = R x I

V_high
temp = R_{high temp} x I

V_low
temp = R_{low temp} x I

I = same for the two temperatures

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

High temp:

R_{high temp} = R_{elec, high temp} + R_{thickness,
high temp} + …..

Low temp:

R_{low temp} = R_{elec, low temp} + R_{thickness,
high temp} + …..

Lower temperature gives lower conductivity, so R_{elec,
low temp} ˃ R_{elec, high temp} which is the reason for a higher V_{low 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 R_{thickness, 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 V_{0, low temp} than V_{0, 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 R_{thickness,
low temp} will be higher than R_{thickness, high temp.}

This is the reason for a continues increase in V_{low 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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