Interference coloring of aluminum by pulse anodizing

A. That is true .

Yes, companies are selling a piece of equipment which enables you to obtain a lot of different colors by anodising . I do not know if they save power though.

We have been using this technology with success for more than a year now.

A. One company sells such a process, but as far as I know it only produces blue-gray finishes, and the cost of investment goes around US $100,000. It is a two step process, first you sulfuric acid anodize and then you color treat the part using a very hi-tech computer controlled rectifier. In the second step two processes occur, pore enlargement and metal deposition.

Another process which is described in US patents numbers (you can read a resume of them in www.uspto.gov) 4,152,222 and 4,066,816 involves a three step process, sulfuric acid anodize - phosphoric acid anodize to enlarge pores and color treatment (Sn or Ni usually), this process uses an alternating current power supply and you can get at least eight colors to my knowledge.

This process is also described in what happens to be the bible of anodizers, The Surface Treatment & Finishing of Aluminum and its Alloys by S. Wernick, R. Pinner, P.G. Sheasby, fifth edition, vol I, pages 641-649. Here both processes are described in a more general manner, but valuable information can be obtained.

Saludos desde MEXICO,

A. During the last 15 years I have been working with pulse anodizing. I am not selling any rectifier but have been so lucky as consultant to be involved in various projects about pulse anodizing.

These projects have shown that you will reduce the energy consumption when you use square wave formed pulse anodizing. This is purely a DC process where you ask your rectifier to change value during the process. This means that you can use your old rectifier.

Regarding coloring, interference coloring gives you a whole spectra of colors including the common ones.

Sunny regards

A. The pulse rectifiers allow you to run at a higher amperage because the pause between the pulses allow the piece to cool so that it doesn't burn. The density of the coating is increased. The coating is harder. Anodizing to the desired thickness is faster.

The disadvantage is that you have to purchase a new rectifier to get the advantages. As far as the US trend and additives I don't know. You may want to contact Dynapower in Vermont for a high quality pulse rectifier.

Regards

A. Both Dynapower and Rapid offer pulse units, and both have presented papers at various AESF shows. There is an excellent paper in the May 92 issue of Metal Finishing on pulse rectification. Pulse rectification has been around for a good 20 years or more, but hasn't taken over because we are a mature industry, and there is a reluctance to discard a perfectly good DC rectifier. There is also the element of exploring the unknown. This may initially be a disadvantage, as there is an educational process in learning how to use the technology to its maximum efficiency, and at the same time maintain production. Any hardcoater knows it is not too difficult to ruin parts in a hardcoat tank, and one needs to make sure one knows what is going to happen when the rectifier is turned on.

I agree with Mr. Jones that the key to higher efficiency is that the part is receiving pulses of very high amperage without the problems of increased part temperature. Not only does this prevent burning, but this results in a denser coating and less Al dissolution from the coating. I believe that at the same time the barrier layer retains a higher conductivity than a straight DC rectifier would provide with very high current densities, contributing to faster film build and ability to grow thicker coatings if desired. There are several other advantages as a result of this concept, including the choice of anodizing for shorter cycle times or, putting more parts on a rack. In some cases we used to hardcoat more parts at lower current densities and still got 2 mils in about 40 minutes.

Two other advantages: we did away with work bar agitation, and for many parts we were able to use titanium finger racks (caution: make sure the splines can carry the required current, and use the old rule of 8 amps per contact point). The higher voltages did make the titanium racks brittle over time, reducing titanium rack life, but we saved a great deal of racking time.

Hardcoat additives work by dissipating heat faster than a straight sulfuric acid solution, reducing the rate of aluminum dissolution from the film, again resulting in denser coatings and faster film build. The advantages are similar to what pulse rectification offers, but as you can probably tell, I liked the pulse rectifier. (I hope my boss doesn't read this; we sell additives but not rectifiers.)

Phil Johnson
- Madison Heights, Michigan