There are several explainations of this recovery effect. The one I like is found in Wernick, Pinner and Sheasby´s book The Surface Treatment And Finishing of Aluminum And Its Alloys
, 5. Ed., Finshing Publications LTD., Teddington, Middlesex, England, 1987.Explanation by Murphy.
If an anodizing voltage E1 is quickly reduced to a lower value E2, the current falls to a very low value and may take a considerable period of time, amounting to minutes, to attain the steady state condition characteristic of the second voltage: but if the voltage reduction is carried out slowly the recovery is much quicker.
This recovery effect is affected by:
- The values of E1 and E2 as well as the differences E1-E2.
- The rate of change of E from E1 to E2.
- The concentration of the electrolyte in which the anodic coating at E1 was formed.
- The temperature of the electrolyte in which the recovery process occurred.
- The treatment of the anodic coating between the time E1 was switched off and E2 applied. Drying of the film between formation and recovery approximately doubled the recovery time.
Murphy postulated that readjustment of the barrier layer to the new voltage E2 is field assisted. Therefore the recovery effect should be dependent on field assisted migration of protons out of the film and/or neutralization of protons by field assisted migration of anions into the film.
Several years later, after Murphy´s explanation, Takahashi, Nagayama, Akahori and Kitahara presented their explanation of the recovery phenomenon. According to these authors the main features of the recovery effect can be explained by the drawing and explanation below.
When the high voltage E1 is applied, the current will reach a steady level i1, stage 1. In this period the barrier layer will reach a thickness d1 corresponding to the forming voltage E1. The structure of the cells will also be controlled by E1.
When the voltage is suddenly lowered to E2 the current density will decrease drastically to a very small value as seen in stage 2. This small current density with values in the range of mA, corresponds to the very high resistance in the barrier layer d1.
The electrical field across the barrier layer in this period is very low. Hence the formation of oxide is almost zero and the field assisted dissolution also very slow. The main reaction in this period will be the chemical dissolution of oxide. This period is called the recovery period.
After a certain time, dependent on many factors such as alloying elements, concentration of the electrolyte, temperature and the value of (E1 - E2), the thickness of the barrier layer has become thinner hereby increasing the electrical field across the barrier layer.
Now the field-assisted dissolution and formation will take place increasing the total dissolution rate as seen by the steep increase in current density during stage 3, due to a less resistance in the reduced thickness of the oxide layer.
After a while the current density will reach a steady level corresponding to the value of E2, see stage 4. Now the barrier layer thickness has reached the value d2 (less than d1), that corresponds to the voltage E2. The oxide will also adopt another dimension with smaller cells corresponding to E2 and appearing beneath the oxide layer formed at E1.
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