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topic 0985A

Looking for information on anodizing


Q. I'm looking for information on anodizing. I've heard of processes that involve voltage controlled growth of an oxide (more voltage, more oxide, different color) any one know what I'm talking about?

The process involves using 10% trisodium phosphate [linked by editor to product info at Amazon] to 90% water solution and a variable DC power supply(0-150 volts)

So far my experimentation has led to a neat way to put pinholes in aluminum foil!


Peter Geraci


A. Peter:

It sounds like you are describing two-step anodizing. The first step is a conventional sulphuric anodize, and the second is an anodize bath containing a lithium salt. The lithium deposits into the pores of the coating. the longer you anodize in the second bath, the deeper the color becomes. Unfortunately, all my reference books are at work...

Gerald Janssen
- coil anodizing


A. Peter,

The process you described sounds like titanium anodizing. It is true that the color is controlled by changing voltage for anodizing Ti in phosphate solution.

If you want to anodize aluminum alloys with different colors, however, it is a different story. In this case, you may refer to the information in Gerald's email.


Ling Hao
- Grand Rapids, Michigan

December 6, 2012

A. Hi Peter. Please search our site for "color anodizing of aluminum". As Ling says, the colors of aluminum anodizing are from dyes absorbed into the pore structure, not from thin diffraction films as in titanium anodizing. The usual process solution is 10-12% sulphuric acid by volume, and the usual voltage is about 12 volts, with a current density of perhaps 12 ASF. After anodizing to a thickness of .0002" to about .0008", depending on how dark and saturated the color must be, you dip it in dye, then close off the pores by swelling the aluminum oxide in steam, hot water, or other sealants.

Best of luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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