Letter 41077

Color change of titanium racks used for anodizing [South Korea] 

June 12, 2006

Dear sirs,

I work for aluminum surface treatment.
I have some questions about titanium racks.

We are using titanium racks for chromic acid anodizing and phosphoric acid anodizing of aluminum alloys.

When we use new titanium racks for anodizing, we experienced that the original color of titanium racks changed to blue or purple after they were used for first anodizing. And the blue or purple color disappeared after next anodizig. I heard that the color change of titanium racks shows too much current carring. But in my experience, there was no problem on anodized parts. So I performed simple test for this. I completly sanded short titanium rack and we used it for anodizing. When we use the titanium rack, we did not rack any aluminum parts on it. After first anodizing, we found the color change on immersed area of titanium rack. There was no color change on exposed area above the solution level. And then we used this empty titanium rack for anodizing again. After second anodizing, the blue color disappeard.
After this test, I thought that color change cannot occur on empty titanium rack if it shows too much current carring. So I conclueded that color chagne of titaninum rack is not related with too much current carring of rack.

Is my conclusion right?
What is the bule or purple color? Why does the color chagne occur?
Why does the boue or purple color disappear after second anodizing?

Thanks in advance.

Insub Hwang
Chemical process for aluminum - Kyungnam, South Korea

It sounds like you are anodizing the titanium - do you happen to be right arond 18-22 volts? The color change is the result of the anodize on the rack - it isn't actually colored, rather the coating is very uniform and it bends the light like a prism to result in the color you see. The degree of bend (and therefore color) is dependent upon the thickness (which in turn is dependent on the voltage) and is why you see a range of colors. I suspect that it goes away the second time because it is no longer uniform enough to bend light uniformally and the conditions necessary to make the color appear are interupted.

The only problem with it is that you might build up enough coating the you start to loose conductivity at the connection points of your rack - it is not a problem at all with the parts that are not actually touching your aluminum parts. Once the racks stop conducting enough electricity, you'll run into various problems - fluctuating current in the tank, extra burning, extra current required; problems similar to not having enough contact points.

To prevent that, it is necessary to remove the anodized titanium from the contact points - either by physically sanding it off or by chemically disolving it (letting the racks hang in your anodizing bath overnight withough the electricity on is the simplest way, but can pollute your tank overnight, the fastest is to use a titanium etch of nitric and hydrofluoric - but it is nasty and dangerous and has the potential to dissolve too much of your rack if you don't watch it carefully). How you choose to do it and how often you need to do it can be determined by how you run your process - either way you are removing a minute amount of titanium from your rack when you do it, so try to avoid cleaning them more often than you need to. Some companies clean their racks every shift, some every few months - base it on your needs and experiment a bit and you'll be fine.

If you want more info, do a search through the archives for anodized titanium racks and you should find more info - I know that there are a few letters in there.

Good luck!

Jim Gorsich Accurate Anodizing Inc. Compton, CA, USA

I also witnessed the titanium color change on some racks of ours when they were new.. I attributed it to the temperature change and the introduction to the various chemicals in the anodizing process tanks while it was used to plate some parts..

Ryan Cook Team Metal Finishing , Inc. Toccoa, Georgia

Your observation of color change in solution and not outside the solution is correct. Titanium has not been observed to change color unless it is immersed in the right electrolyte. Color change may occur at 1000s of degree in the air. I doubt that you have that temperature, but that maybe what someone was trying to explain. They may have thought your Titanium had very high volt/amps that made it glowing hot and hence a change occurred.

What is the blue or purple color?
The Blue color is a thin film of Titanium oxide , just like Aluminum oxide.

Why does the color change occur?
The Titanium rack gets anodized in the electrolyte just as the Aluminum.

Why does the blue or purple color disappear after second anodizing?
The Electrolyte dissolves the Titanium oxide leaving a layer of other metals in the alloy of the rack.

Note that the thin Titanium anodize layer has negligible resistance. I will not recommend Nitric/HF stripping, especially when your electrolyte strips the colored oxide film the second time around.

Good luck!

Kas Amadi
- Dayton, OH, USA

The purple layer is oxidized titanium. For anodizers using titanium racks, the oxide that builds up from time to time on titanium and has to be removed to avoid burning of parts.

It can be removed by a wire brush or acid etching.

John Pun
- San Diego, CA

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