Q: Can I anodize titanium with my 12v Power Supply?

June 19, 2011

Christopher asked:

I want to do a large piece of titanium (28″ x 3″ x .5″), around how many amps would I need to push through it, or how long would it take?

I have a power supply that is 12V @ ~19A, could I use this to color my titanium if I just leave it on for a long period of time?

If I color my titanium and dont like it, can I do it again and will I get around the same results? I’m scared that I will try to color it blue and get a terrible result and be stuck with it.

Okay, three questions, but the most critical one is in the title. Titanium colors are voltage controlled. A twelve volt power supply (or battery charger) would work for electroplating or aluminum anodizing, but not for titanium. More precisely, you can get the fingerprint-prone bronzes and deep purple at or under 12 volts. But not any of the other colors.

Because the final color is voltage limited, the current is less critical, in theory.  In practice I find that to reach well saturated colors beyond about 50 volts you need a supply that can support an initial surge of at least 0.1 amps/sq.in. This can be done with lower rated supplies by charging a large capacitor in parallel with the electrodes. Your total piece is 184.5 sq.in, so 19 amps should be enough.

Burrs or sharp edges can have a negative effect on your final color.

If you don’t like your color, you can subsequently anodize to higher voltages, but not lower. The best color results appear on a clean and freshly etched surface. If you overshoot a color, or get a hazy or gray result, the only recourse is to grind, polish, or etch the color off and start over.

Q: What role does pH play in electrolyte solution when anodizing titanium?

April 20, 2011

This question from Jack is a good one. I hadn’t really considered it before, and finding information on it online is either tricky or expensive. In short, I don’t know.

I have used electrolytes with a wide range of pH (acidity and alkalinity) but had not been looking for the differences. Some of my favorites are phosphoric acid (pH = 1.7), ammonium phosphate (4.2) , and tri-sodium phosphate (12). Quite different pH’s, but all slam that phosphate ion against the titanium anode and drop off an oxygen atom. Borates work fairly well, too. I’ve read that alkali sulfates can be used. But personal experience says, stay away from nitrates and chlorides.

According to some guidelines/requirements for anodizing titanium medical implants, a strong alkaline should be used. I suspect that this is to guarantee that nothing living can be in the solution.

According to one for-fee article from 1985  (Studies on anodizing of aluminium in alkaline electrolyte using alternating current) found “Electrolyte pH was found to affect the growth of anodic films considerably.” But I didn’t buy it to see how. This article is not quite to the point because it a) was about aluminum, b) they used alternating current, and c) it focused on only part of the pH spectrum.

If anyone has played with pH in anodizing titanium, please let me know if you notice any anodizing differences by pH.

Q: Is the Color Layer Hypoallergenic?

April 17, 2011

Marlene asked:

I’m desperately trying to find a solution to my recently-developed metal allergy as I LOVE my earrings. Titanium seems like a great option to try, but I am wondering, what is the colored coating composed of? Is there anything in that I could be allergic to?

As I explain on pages such as A short article on the physics of Anodized Titanium Color, the color layer is pure titanium dioxide. Just oxygen bonded to the surface of the metal creating a material that has been used for artificial “Titania” diamonds.

Thus this layer actually is the hypoallergenic coating that makes titanium safe. The “silver” color has a thin layer of the oxide on it, whether I want it or not. Titanium spontaneously grabs oxygen from air or water to protect itself. When anodizing or heat coloring, the higher the voltage, the thicker the protective layer becomes. But there is no practical difference for hardness or sensitivity, as the thickest layer (green) is about 0.00000003 inches thick.

I use a simple phosphate detergent as my electrolyte, then soak the ear hooks in clear water. In the 30 years since I started doing this I have had only two customers who were too sensitive even for titanium wires. I suspect it was a non-chemical tactile or contact sensitivity; the rubbing or pressure itself was causing the reaction.

Some people claim that niobium is even better for sensitive ears than titanium. They are chemically similar, and involve the same method of coloring. It’s been discussed before here: Hypoallergenic: Titanium versus Niobium

I suggest trying both, as a spare pair of wires is cheaper than another shipping charge.  Shop my Ear Hooks.

Can I Color Titanium in an Oven?

March 22, 2011

Kathleen asked:

Is it possible to color titanium in an oven (to control the temperature)? If so, what temperature does the oven have to have?

Assuming a kitchen oven, the answer is, No.

If you have a laboratory oven, a kiln, or some such, then the answer is, “Probably”.

Titanium colors by heat are controlled by temperature much like anodized color is controlled by voltage. The temperature at which you should start seeing the lowest tan/bronze is about 640°F. This is easy to reach with a direct flame, but not in a household oven.

I have not found a color/temperature scale, but would love to publish one. If anyone with a lab oven wants to play with this, please share your results.

Black or Gray Titanium

March 4, 2011

John Asks:

I’m trying to get a black or dark gray finish on the face of a titanium driver head. What voltage achieves that color?

Sorry, John. Anodizing produces a particular spectrum of colors limited by the first two octaves of optical interference. I explain it here.

Black and gray are shades, not colors. One cannot make titanium black by anodizing.

So, how is black titanium made? Everyone who does it is keeping the actual process a tight secret. But my  educated guess is that it is produced by implanting nitrogen into the titanium using an industrial vacuum effusion furnace. This produces a relatively thick layer of titanium nitride in a similar chemical manner that titanium dioxide is made by anodizing. But nitrogen implanting cannot be done in an oxygen rich environment, like air or water. Air is 21% oxygen by volume, and water is 33% oxygen by atoms, or 88% by weight.

Anode vs. Cathode Terminology

January 25, 2011

A reader named Scott suggested that I may have made an error:

“On your Anodizing page,  point #3 & #4 are backwards. The anode is negative and the cathode is positive. The work goes on the negative side (the anode) and we are ‘anode-izing’

“Just the first few words of each line are backwards.”

His contention is that the “Anode” should be the negative side. I guess that he is familiar with batteries or sacrificial anodes, where the polarity is opposite that of the electrolytic process that I use.

Rather than just calling him “wrong”, I thought that I would explain it here, in case it comes up again:

The anode is the side of an ion exchange that supplies positive ions.

In the case of an electromotive source (like a galvanic cell, “battery”) you would be correct. The immersed source of positive ions into the solution (anode) produces the negative voltage by pumping electrons around the circuit to balance the positive ions lost to the solution. So in a battery, the anode is the negative side.

But in an electrolytic cell, like an anodizing or plating bath, the anode is where the positive external voltage pumps positive ions into the solution. So the anode is the positive side.

For my purposes, I need to bond oxygen to titanium. Oxygen is a negative ion (2-), pulled toward the positive electrode by the external power source.  The anode simply absorbs electrons from the solution and oxygen is split from the water to keep the accounts balanced. Titanium loves oxygen, so sucks it up as long as there is current. Hydrogen (+) bubbles off at the cathode (negative electrode).

Here’s the Wikipedia article on Anodes, if you want to corroborate what I’m saying and follow to even more authoritative sources.

Programmable Voltage Supply?

December 19, 2010

John asked:

Is there any value in a programmable(manual or C/C++) power supply for anodizing?

Say Vout=20+(110*N/(255)); // N=0,1,2,3,4,…255

Giving {20,20.4, 20.8,21.2…129.6, 130}

I can also make this power limited to approximately 13W(0.10A at 130VDC)

I am not selling anything! I am just wondering if this is a worth while adventure.

As a fellow electrical and programming geek, I see the appeal of the project. But practically speaking in terms of anodizing titanium, no. The color is determined by the final voltage, and the faster you get there, the better.

Also, I use down to 8 volts on occasion. And the lower voltages are more color sensitive than the higher, so it should either be 16 bit linear, or have exponential or quadratic output, as in

vOut = (((N/64)^2 + N) *120/255) + 5 // N={0…255}

But if you were to rig an x-y table to such a supply, one could then “print” in anodized colors. However, there is a limited palette. And also one would have trouble with certain adjacent colors, and have to adjust the lateral speed to be proportional to voltage, and maybe fluid flow through the dielectric cathode, and several other engineering considerations.

As such, it becomes fun and useful. But a lot more work. Then you would be able to share it on HackADay.com or Makezine.tv or some such.

In order to make such a project marketable, one would have to write the CADD end to prevent unfulfillable designs. Artists have to have limits imposed.