Flipping the Switch on Anodizing

Most anodizing job shops use DC or pulse anodizing with pulses in milliseconds, and slow pulses mean low frequency as accounted for in the early 1980s by researchers Yokoyama, Konno and Takahashi.

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

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

Both dissolution reactions take place in conventional DC and slow square-pulse anodizing, though it is only by the square-pulse anodizing that the use of both are 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.

The Recovery Effect

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 1. 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, sometimes in the range of A, 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.

Results from the Danish Project

In this project it was found that by using slow square-formed pulses, the process time could be decreased up to 50 percent, and, at the same time, the total energy consumption in the anodizing tank was decreased up to 30 percent.

The aluminum alloy used in the experimental loads was a 6063 alloy. The chart shown in Figure 2 gives an idea of the tremendous reduction in time forming the same thickness of the oxide layer. The process parameters were conventional DC anodizing at 18 A/ft2, and three different values of current densities used for the slow square-pulse anodizing.

The first one was run by 22.5 A/ft2 in the high-current period and 10 A/ft2 in the low period. The second one used 30 A/ft2 in the high-current period and 10 A/ft2 in the low, and the last one using 40 A/ft2 and 10 A/ft2.

The process time was calculated to give a thickness of the oxide layer of 0.8 mils, and the pulse time was 120 seconds for the high current and 30 seconds for the low current.

The voltage drop was logged every second during the anodizing process in different places on the load. The energy consumption is calculated from the logged voltage drops and the current running through the system.

The energy consumption was then calculated in MWh/m3, which gives an idea of the amount of energy used per micron-formed oxide layer. (See Figure 2.)

The conventional DC anodizing process uses 87 MWh/m3 compared to the slow square-pulse anodizing process with 40 A/ft2 in 120 seconds and 10 A/ft2 in 30 seconds with a total process time of 18 minutes.

ROI Increasing Revenue

The invented company is 20 years old and has a mix of parts coming in for anodizing. The shop works in two shifts a day. Figure 3 shows a production line for the anodizing process.

The volumes of the tanks are 800 gal, and the red tanks are the heated tanks. There are three anodizing tanks which each has a DC rectifier of 25V/4000A. The sealing process is hot sealing.

To make it easier to calculate, the company runs only loads of 0.8 mil sulfuric acid anodizing type II, class 1 coatings, which take 40 minutes each (1 load per rectifier per hour). This gives a production of 24 loads per shift. The total area on each load is 20 m2.

The average value of each load is set to $100, and the average number of workdays in a year is 260 days. With 48 loads a day, the total revenue for a year is $1,248.

Changing the total production to slow square-pulse anodizing will increase the total revenue per year to $2,782. The increase in revenue is due to the reduction in the anodizing time (18 minutes vs. 40 minutes) for the 40 A/ft2 and 10 A/ft2 current density process, giving a total of 107 loads a day.

Investment

Most of the anodizing job shops run their rectifiers to the maximum limit, so to be able to use pulses three new rectifiers should be purchased to get the total increase in revenue. At the same time, new customers or increasing orders from current customers should be obtained to be able to fill more than double the number of loads.

Most anodizers will probably buy one new pulse rectifier to begin with, giving roughly a 33 percent increase in total loads. So instead of 107 loads per day it will only be 67 loads, compared with 48 loads for the conventional DC anodizing line. The total revenue per year is then $1,742, giving an increase of $494 per year with one new rectifier.

The price of rectifiers varies a lot depending on their abilities, the way they are built and what supplier is chosen. The company would need to buy one new rectifier with a 30 V/8,000 A specification.

Depending on the cooling capacity of the existing cooling system and the agitation system already installed in the tank that is changed to pulse anodizing, there could be a need for investment in these two systems as well. Again, depending on the condition of the contact points, busbar and rack design, there could be a need for upgrading of these parts, too.

Two different scenarios of investments for the company are set up. 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 and 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 upgrading of the rest of the equipment mentioned above. The upgrade is estimated at $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 U.S. 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 required 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 on the current, so this scenario will have an even faster ROI.

Energy considerations

Energy is not a part of the above calculation. As measured in the Danish project, there is not only a decrease in time but also a 30 percent decrease in the energy consumption during the slow square-pulse anodizing.

According to the Energy Information Administration, energy costs approximately 5 cents per kWh. The energy decreases by 27 MWh/m3 in going from conventional DC anodizing to slow square-pulse anodizing. This will give a small decrease in energy costs when replacing one DC rectifier, of $4,914 per year.

Another big source of energy consumption in an anodizing line is the heated tanks—the red italics tanks in Figure 3. When the anodizing tanks can handle double the number of loads, the same amount of parts can be processed in half the time.

So if the anodizer goes for one pulse rectifier without having more customer parts, there will still be an energy savings. Forty-eight conventional DC anodizing loads are processed in 16 hours (= 2 shifts). Replacing one of the DC rectifiers with a pulse rectifier would process the same amount of loads (48) in 12 hours instead of 16 hours. By this it would be possible to shut down the whole anodizing line approximately one day earlier a week, hereby reducing the energy used to keep the temperature up in the heated tanks.

As shown in the above calculations, it is clear that an increase in productivity would pay the investment in the new pulse technology. Though in a world where energy is an increasingly scarce resource, it is important to take the decrease in energy into consideration, also because anodizing is often seen as an energy-heavy process.

The calculation is for the pulse pattern 40 A/ft2 in 120 seconds and 10 A/ft2 in 30 seconds. The values are suggestions to start with, and some anodizing lines will need a lower current density in the high period. Every anodizing line will have its own optimum values, so to get the best results some test loads should be taken into account before running optimal.n

Anne Deacon Juhl is an anodizing consultant who has a website, AnodizingWorld.com. She can be reached at 619-261-8288 or at info@aluconsult.dk. Please visit pfonline.com to see full references for this article.

References
1. Yokoyama, K., Konno, H., Takahashi, H. and Nagayama M., Anodic oxidation of aluminum utilizing current recovery effect , AES, 2nd. International Symposium on Pulse Plating, Rosemont, Ill., USA, Oct. 6-7, 1981.
2. Juhl, A. Deacon, Pulse Anodizing of Extruded and Cast Aluminium Alloys , Ph.D. thesis, Inst. of Manufacturing Engineering, The Technical University of Denmark, July, 1999.
3. Thompson, G.E., Xu, Y., Skeldon, P., Shimizu, K., Han, S.H. and Wood, G.C., Anodic Oxidation of Aluminum , Philosophical Magazine B, Vol. 55, No. 6, 1987, pp. 651 667.
4. Juhl, A. Deacon, Burfelt, K. and Weldingh, P., A new approach to pulse anodizing Decreasing
energy consumption Increasing productivity , presented at the 15th Annual International Anodizing
Conference & Exposition, October 2 5, 2006, Toronto, Canada.
5. American Plating Power, www.usplating.com
6. http://www.eia.doe.gov/neic/brochure/electricity/electricity.html