The most common process when hard anodizing uses a high current density and a low temperature of the electrolyte. This will allow the balance between the formation and dissolution of the oxide layer to become difficult to maintain. This unbalance can lead to the two problems, Burning and Powdering.
Burning is described as an uneven growth and destruction of the oxide layer. The interface between the barrier layer and the aluminum is not smooth on a microscopic scale. Metallic aluminum extend as many small projections into the barrier layer.
These metallic needles, intermetallic phases, and/or other "impurities" transport the current easier than the aluminum oxide and give rise to a short circuit through the barrier layer in these pores. When the current density concentrates in these pores the temperature will rise at the bottom of the pores. When the voltage U is maintained constant, we have by Ohm’s Law:
Burning is described as an uneven growth and destruction of the oxide layer. The interface between the barrier layer and the aluminum is not smooth on a microscopic scale. Metallic aluminum extend as many small projections into the barrier layer.
These metallic needles, intermetallic phases, and/or other "impurities" transport the current easier than the aluminum oxide and give rise to a short circuit through the barrier layer in these pores. When the current density concentrates in these pores the temperature will rise at the bottom of the pores. When the voltage U is maintained constant, we have by Ohm’s Law:
U = R * I
In pores with impurities a drastic increase in current will be found, when the resistance R is zero. The temperature will rise because of the effect P = R * I*I. In pores without these impurities there will be a high resistance and only a small current will flow here.
In pores with low resistivity formation of oxide will increase dramatically. A thick oxide layer will be formed here and the temperature will increases due to Joule’s heat, which will lead to an increase in the rate of chemical dissolution. Hence the oxide layer will be non-uniform and in some places the electrolyte even attacks the underlying aluminum.
To avoid burning the current density should be low. Hereby the chemical dissolution reaction will be able to dissolve irregularities in the barrier layer and a uniform oxide layer will be created during the anodizing process.
However, lower current density means lower formation rate of the oxide, and hereby a longer anodizing time to obtain a certain thickness of the oxide layer.
Powdering appears as a consequence of this prolonged treatment time. The acidic electrolyte will dissolve the aluminum oxide. Since the chemical dissolution is independent of the electrical field the attack on the oxide will happen everywhere on the surface, contrary to burning which is limited to certain areas.
This powdering effect is due to a combination of long anodizing time and a high concentration of the electrolyte. It can be prevented if a high current density can be applied and the concentration and temperature can be kept low.
So therefore to prevent burning it is advisable to- Decrease the current density
- Increase the temperature of the electrolyte
- Increase the concentration of the electrolyte
and to prevent powdering it is advisable to
- Increase the current density
- Decrease the temperature of the electrolyte
- Decrease the concentration of the electrolyte
If these criteria are compared, it is seen that in order to prevent burning, powdering will be favoured and vice verse. By pulse anodizing both of these two phenomena can be reduced, see an earlier post, and at the same time it will be possible to form a thick and dense oxide layer.
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