Aluminium 2000 congress in Bologna, Italy

Last week the 7th Aluminium Two Thousand International Congress took place in Bologna, Italy. Around 350 people attended from 25 different countries.

With the motto "Let´s build the future of the aluminum world together" the days were fully packed with experts from all around the world. The three day program with parallel sessions included analysis of the aluminum industry, interesting new developments within all different aspects of the aluminum industry; foundry, casting, extrusion, anodizing and painting, automation, architecture, transport industry, environmental protection and recycling, measuring, testing and quality techniques.

A lot of the presentations were about how to save money, which is probably understandable because this is a topic we all can relate to and want to hear more about.

So I was very honored to be asked to present a paper on the cost savings when changing from conventional DC anodizing to Pulse anodizing.

The paper and presentation showed that the ROI is less than a year for an Anodizing line to switch from conventional DC to Pulse anodizing - all because of the increase in productivity.

I am proud to say that the paper, among three others, was awarded "Most interesting presentation", at the congress.

Purchase my paper presented at the 7th Aluminium Two Thousand Congress

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Research project about Pulse Anodizing in Spain

Cemitec, the Research and Development Center in Navarra, Spain has a Chemical Engineer working in a research project related to pulse anodizing of aluminium.

It is always wonderful to see other people working with something you find important. The work I did in my PhD project was a comparison of various pulse methods and also verifying the use of square wave formed pulses when hard anodizing different alloys.

The project in Spain will also look at the influence of pulse anodizing in the final color of aluminium pieces.

Hopefully Cemitec will present a paper at one of the anodizing events this year. The two anodizing events in 2010 are the IHAA symposium in Las Vegas and the AAC conference in Montreal.

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Hard anodizing of high silicon containing aluminum alloys by pulse anodizing

Sometimes pulse anodizing seems difficult to figure out, specially for the aluminum alloys which are difficult to anodize.

So a question about Hard Anodizing an aluminum alloy with 10% silicon aluminum Alloy to 75µm thick layer with a pulse rectifier, suggesting the current density per square feet will get the following answer.

To get 75 µm you can use different kinds of electrolytes; a suggestion would be an electrolyte with 15wt% sulfuric acid at 54 - 60F or one created from sulfuric acid and an organic acid at a lower or same temperature.

With the sulfuric acid electrolyte established you should then proceed to use 40 A/ft² for the high current density period and 10 A/ft² for the low current density period. Then depending on the geometry you should try different pulse periods. My suggestion is to start with 60 seconds in the high current density period and 20 seconds in the low period.

It is important to remember that a pretreatment with fluoride will improve your result.

Also remember that lower silicon content will give you a possibility to use a higher current density in the high current density period.

Use higher temperature than conventional hard anodizing when processing high silicon alloys, you could try using a high current density period of 20 - 100 A/ft², and pulse periods t1=t2=30 seconds.

You should always consider starting your test runs with voltage controlled anodizing, high current density period 20 - 40V and low 15 - 25 V.

If you find this article useful, Anne Deacon Juhl is available for consulting, please send an inquiry to blog@aluconsult.com

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Surface treatment for Aerospace applications in Paris

I returned back to Copenhagen Tuesday afternoon from a great conference in Paris held by the French Cluster, ASTech and A3ST (Association de Traitement Thermique et de Traitement de Surface).

Lots of talk about Chrome VI free processes on the market, advantages and disadvantages.

My presentation for the 150 attendees was about the ROI of pulse anodizing, and why there are no excuses for not changing your conventional DC anodizing to Square wave pulse anodizing.

Being able to pay the investment of a new rectifier back in less than a year by increasing the productivity in the anodizing line, should be of any managers interest.

Discussions about anodizing as a pretreatment before paint and comparison with other Chrome VI free pretreatments, more about this later.

Paris as wonderful as ever :o)

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How to implement Pulse Anodizing

Many people have talked about pulse anodizing for many years but it seems like a lot are still confuse and don´t see the benefits of using pulses for anodizing, both in hard anodizing and conventional anodizing.

The only whole book you'll find about pulse anodizing is my phd thesis, "Pulse Anodising of Extruded and Cast aluminium alloys" and then The Surface Treatment And Finishing of Aluminum And Its Alloys, by Wernick, Pinner & Sheasby does talk about pulse anodizing in several of its chapters.

There are several reasons for why so many anodizers have resistance against introducing pulse anodizing.

• The investment in buying a new rectifier
• Some have tried pulse anodizing already and it doesn´t seem to work
• You already form oxide fast with DC
• Education of employees

You will probably have to buy a new rectifier, a bigger one. This is true in most cases if you are already running full load on the rack, so the rectifier capacity is used fully. What we need is to be able to increase the current density (A/ft2).

So if it normally takes the company 40 minutes to run a 0.8 mil load sulfuric acid anodizing type II, class 1 coatings, by using pulse anodizing we should be able to decrease this process time with 50%, forming 0.8 mil in 20 minutes.

To do this we need to double the average current density. To do this we have to utilize the recovery period, and this is one of the reasons why some have tried pulse anodizing without much luck. If the pulse periods are to fast, as in milliseconds, the recovery period will not have time to take place.

Perhaps you are already forming 0.8 mil in 20 minutes using additives and/or a lower temperature.

In Metal Finishing Magazine, Jul/Aug 2009, I had an article about the return of investment (ROI) when changing from conventional DC anodizing to pulse anodizing.

Two different scenarios of investments for the imaginary company were set up. The company is running 24 loads per shift. The total area on each load is 215 ft2.

The first scenario is a small investment with a new rectifier and a bigger cooling system. The other scenario is a big investment with a new rectifier, cooling equipment, contact, racks and a new agitation system. The last investment scenario will probably only be interesting if the anodizing process is fully switched from conventional DC anodizing to slow square pulse anodizing.

Scenario 1 has an estimated cost of $40.000 for the rectifier and $40.000 for the new cooling system that can be used for the other anodizing tanks too, giving a total cost of $80.000.

Scenario 2 has the same cost for the rectifier and then up grading of the rest of the equipment mentioned above. The upgrade is estimated to $150.000 giving a total cost of $190.000.

The ROI in both scenarios is less than a year, which must be considered to be a valid investment.

A lot of the anodizing shops in the US are already using 24 – 30 V rectifiers so it could be sufficient to add a process controller to those rectifiers to be able to pulse anodize instead of buying a new rectifier. To do this an inspection will be need at the anodizing job shop to see which investments are needed to upgrade the existing DC rectifier to pulse rectifier.

The price of a rectifier is mostly depending on the voltage, and not so much of the current, so this scenario will have an even faster ROI. A process controller could be the HS 100 offered by American Plating Power. This process controller offers ramping, pulse as well as a surface mode. The operator only needs to enter the surface area on the parts that need to be anodized.

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Invited speaker by the French Heat and Surface treatment organization, A3ST

I have accepted to speak at the A3ST conference, which takes place in Paris from October, 26 - 27, 2009 and is dedicated

LIGHT ALLOYS SURFACE TREATMENT
The replacement of the hexavalent chromium in the surface treatment of the light alloys.

The reason is one of my published paper about Pulse Anodizing. The title of the presentation is “The basic of Pulse Anodizing and how to implement it in a conventional Type II anodizing line”, and will presented Monday 10/26/09 at 15.10.

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Burning and Powdering, two problems when Hard Anodizing

Two process related problems when hard anodizing are burning and powdering.

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:

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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The structure of the pulse anodized layer

This post will explain how the structural changes of the anodized oxide layer happens during the pulse anodizing process. There are four stages which happen during the pulse anodizing process. Check out my notes on the recovery phenomenon, this will definitely help you through this explanation.

In stage 1 the forming voltage has a value of E1. Anodizing at this value will give a defined oxide structure which is dependent on the anodizing parameters. Continuing anodizing with these parameters will give a uniform oxide layer with an array of hexagonal cells of a fixed size.

In stage 2 the forming voltage is abruptly decreased to a lower value E2. This will make the electrical field across the barrier layer, which still has the thickness d1, very small. This results in a zero formation rate of oxide. The dissolution will slowly increase during this period and the barrier layer will gradually diminish to an appropriate value corresponding to the lower anodizing voltage E2.

In stage 3, the thickness of the barrier layer has reached a level where the field strength has increased enough and suddenly the current increases steeply. The new cells formed are smaller and with a barrier layer thickness d2. Continue anodizing at these parameters will formed a uniform oxide layer which depends on the anodizing parameters.

In stage 4 the anodizing process continues at the current density i2 creating a uniform oxide layer with a characteristic array of hexagonal cells until the voltage is increased again, forming an oxide layer with the characteristic of the film formed in the first stage at the anodizing voltage E1.


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Using the recovery effect when anodizing

The post from yesterday only briefly mentioned the recovery phenomenon. Utilizing this effect could be used to avoid burning and powdering of the oxide layer, when hard anodizing and anodizing. These two defects are the main problems which can destroy the formed oxide layer entirely.

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:

1. The values of E1 and E2 as well as the differences E1-E2.
2. The rate of change of E from E1 to E2.
3. The concentration of the electrolyte in which the anodic coating at E1 was formed.
4. The temperature of the electrolyte in which the recovery process occurred.
5. 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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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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Why slow Pulse Anodizing is the only way to go !

You can now buy the Ph.D Thesis "Pulse Anodizing of Extruded and Cast Aluminum Alloys"

ABSTRACT

The main issue of this thesis is pulse anodising of aluminium, both extruded and cast aluminium alloys. The use of pulse anodising is of great interest to the industry whether the pulse time should be fast (high frequency pulses) or slow (low frequency pulses) to get the optimum use.

The pulse time has been investigated by choosing three different pulse anodising methods using the same extruded aluminium alloy 6063. They have each been evaluated through a number of different tests and all have been compared to conventional DC anodising.

It has not been possible to select a single method that has superior properties to the other concerning the properties of the oxide layer. Though a very important parameter for the industry, namely, the total treatment time has been shown to be almost twice as short when using the low frequency pulses compared to conventional DC anodising.

The most commonly used cast aluminium alloys are the ones with a content of silicon between 7 – 12 wt%. Investigations of the oxide layer formed on this alloy by hard anodising, both pulse and conventional hard anodising, shows a non uniform oxide layer without any corrosion or wear resistance. Five aluminium alloys were chosen in order to compare the oxide layer with each other by various tests. It was found that the oxide layer formed on the AlZn5 alloy shows the best properties according to the tests performed.

Send me an email, buy@aluconsult.com and I will send you the pdf-document.

Price 150,00 US dollars.
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