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
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.
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