Finishing Talk Southern Metal Finishing Newsletter
Account Manager - Metal Finishing Account Manager
News Room - Surface Finishing Announcements
Home Metal Finishing Back To Home

The Basics Of Pulse Anodizing

January - 2010
Previous Page

Most of the anodizing job shops use direct current (DC) for Type II anodizing. This article will talk about slow pulses using low frequency pulse, which gives pulses in seconds to minutes as accounted for in the early eighties by Yokoyama, Konno and Takahashi1.

Before going into depth with the square wave formed pulse anodizing a short introduction of anodizing and the formation of the oxide layer is necessary.
Anodizing takes place when a certain current is applied to aluminum as the anode in an electrolyte, the oxidation will start. This current will pass through the electrolyte and the aluminum oxide, Al2O3, will form at the aluminum surface. When the electrolyte consists of an acid, e.g. sulfuric acid, dissolution of the aluminum oxide will take place. Hereby a porous oxide layer is formed. It is important to realize that the growth takes place at the bottom of the pores. Therefore the outer part of the oxide layer has been formed first and the acid during anodizing may have noticeably attacked it.
The two main reactions involved when forming porous aluminum oxide is a formation and a dissolution, which should be kept at a steady state to form an optimal porous oxide film.

2Al + 3H2O  Al2O3 + 6H+ + 6e
Al2O3 + 6H+  2Al3+ + 3H2O

The last one is the chemical dissolution of the formed oxide layer. It would be very difficult to explain how the chemical dissolution should be able to double for the same anodizing conditions when the current density is twice as high to keep a steady state for formation and dissolution. This indicates that another dissolution process must take place beside reaction (2).

This process is called the field-assisted dissolution and is a result of concentration of the field across the barrier layer, and thus of the current, probably thermally enhanced through local Joule’s heating. According to Thompson et al.2 this dissolution mechanism is due to a weakening of the Al – O bonds in the oxide lattice causing a dissolution at the film/electrolyte interface.

These two dissolution mechanisms take place with very different rates. The field-assisted dissolution takes place with rates up to 300 nm oxide pr. minute whereas the chemical dissolution is much slower with rates up to 0.1 nm oxide pr. minute2.

With this in mind, which happens both in conventional DC anodizing and pulse anodizing, let us take a look at the current - time curve obtained during the first 100 seconds of the anodizing, see figure 1. Here the process is performed with constant voltage during the anodizing process.

Period a in figure 1 shows the formation of the first microns of oxide. In the beginning the current is high due to the fact that the current only passes through the metallic aluminum. Then the current starts to decrease because of the formation of a thin non-porous oxide layer. This oxide layer has a higher resistance than the metallic aluminum. The increase in thickness and therefore an increasing resistance result in a further decrease in the current in period b.

The tendency of the curve to turn upwards in period b is due to small imperfections (roughness) in the compact oxide layer. These small imperfections are formed by the concentration of the current in areas with thinner oxide than on the rest of the surface. These areas with the small imperfection are the subgrain boundaries found on aluminum. They stated that these areas are the places where the initial formation of cells starts. The natural oxide film on either side of these subgrain boundaries is not as compact or uniform as on the rest of the surface. Therefore these areas offer less resistance to  current.

Considering the formation of a single cell of oxide. At a single point the dissolution reaction is started, hence the oxide thickness is reduced and the current will start to flow to repair the damage. This will increase the temperature of the electrolyte and the solution will be more reactive and thus increase the rate of dissolution. This mechanism will perpetuate a pore once formed. Therefore some pores will perpetuate and others will never get started.

Hence the current will concentrate on these small imperfections. This will increase the electrolyte temperature in these areas. Therefore the dissolution will increase and the oxide layer will become even thinner. The current will increase as seen in period c in figure 1. Now the formation of the porous oxide layer has started. In period d the current will reach a constant level where the rates of dissolution and formation of the oxide layer reach a steady state level.

The idea of using square pulse anodizing is to have a higher average current density for the total process and hereby reducing the process time. When pulsating between two values of current density, a high period and a low period, lets the aluminum surface time to recover during the low current density period1,2,3. The time for these periods should be in the range of 10 – 240 seconds to let the two different dissolution mechanisms happen that take place during anodizing.

Both dissolution reactions take place in conventional DC and slow square pulse anodizing, though it is only by the square pulse anodizing the use of both is beneficial. During conventional DC anodizing the chemical dissolution will only be utilized to attack the surface of the formed oxide if the process time is too long, giving a soft outer layer. In slow square pulse anodizing both reactions are used to their best.

It is best explained by the recovery effect. When a high voltage E1 is applied, the responding current will reach a steady level I1 as seen in figure 2. During this period t1, the resistance R1 (thickness of the interface between the aluminum and the formed oxide layer) will reach a level corresponding to the forming voltage E1. When the voltage is suddenly lowered to E2 the current density will decrease drastically to a very small value as seen in period t2. This low value of the current, some times in the range of Amps, corresponds to the very high resistance R1. The electrical  field across this interface in this period is very low. Hence the formation of oxide is almost zero and the field-assisted dissolution is 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, depending on many factors such as alloying elements, concentration of the electrolyte, temperature of the electrolyte and the value of E, the thickness of the interfacial layer has become thinner hereby increasing the electrical field across it. Now the field-assisted dissolution and formation will take over increasing the total dissolution rate as seen by the steep increase in current to a value of I2, due to less resistance in the reduced thickness of the oxide layer.

So by using square wave slow pulses it is possible to form an oxide film with a higher average current density, and by this a shorter process time. A shorter process time gives a higher productivity with more loads through the anodizing tank4.

This article was submitted by Anne Deacon Juhl, of Aluconsult.  See it online for references, pics and contact information