Using slow Square Pulse Anodizing for Hard Coat

An earlier post on this blog gave an introduction to Hard Anodizing. In short, what we like is to form a thick, hard and wear resistant aluminum oxide during the hard anodizing process.

This weeks posts will be about Pulse Anodizing.

The two main reasons for using pulses during the hard anodizing process claims that it is possible to increase wear resistance and corrosion resistance of the formed oxide layer. Another advantage shown is an improvement in thickness uniformity and reduction in total time for the process. In addition, the low temperature used for making hard coatings can be raised and the maximum thickness can be increased.

In later posts about pulse anodizing I will explain and verify this reduction in total process time, for the moment you can visit Finishing Market to buy the only book about pulse anodizing.

Pulsing between two values of direct current instead of using the same value during the whole process gives several possibilities for individual process conditions

This can be utilized in the best pulse anodizing process, in my opinion, namely the one using square wave-formed, low frequency pulses. This process was presented by Yokoyama, K., Konno, H., Takahashi, H. and Nagayama M. in the magazine Plating and Surface Finishing in July, 1982. The article “Advantages of Pulse Anodizing” shows the first results regarding this work.

The main purpose was to create a thick and dense aluminum oxide without having troubles with burning and powdering. By periodically applying a high and low voltage the recovery phenomenon can be utilized to avoid these two effects.

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. During anodizing, the anodic current tends to concentrate on these projections giving.

Powdering appears as a consequence of a 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.

By this decrease in voltage the following are expected to occur: 1) Roughness at the metal/barrier layer interface is reduced and 2) the accumulation of heat is dispersed.

The four parameters of importance is E1 (i1), E2 (i2), t1 and t2. These should be determined for each individual production. The parameters are highly dependent on the solution, temperature, alloy and the desired properties of the film.

A way to find the parameters is to run some test loads with the specific aluminum part. These first runs should be voltage controlled and the value of the high voltage E1 should be:
  • 30 - 40 V for AlSi alloys with the silicon content of 5 - 7 %

  • 20 - 25 V for AlSi alloys with a low content of silicon and AlZn alloys
Be careful with aluminum alloys containing copper and high silicon alloys. Here a different strategy should be applied dependent on the amount of copper and which other alloying elements are found in the aluminum.

E2 should be as low as possible and at the same time have the possibility to draw some current. 15 V would be a good place to start.

Using slow square wave formed pulses t1 is often longer than t2 with duration of pulses varying from 10 - 180 seconds, or even longer. To find the best values of these pulse periods there need to be a system behind the rectifier which can monitor the resultant current during the whole process. In this way a couple of test runs will show how long each period should be.

The diagram below shows a test run on a low silicon aluminum alloy.

The period t1 should be as long as possible because this is the part of the process which forms a lot of oxide.

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