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.

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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.


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.


Is TSP/90 as Good as TSP for Anodizing?

December 13, 2010
TSP90
TSP 

James asked, “Will the TSP/90 Phosphate Free products work as well as the standard TSP brands?”

An excellent question. My first impulse is, “I doubt it.” But I am not sure. The folks at ReactiveMetals.com might have some insight (that I would share here if passed along).

TSP/90 is made with Sodium metasilicate and pentahydrate. So it is an alkali electrolyte with plenty of oxygen carriers in it. So far, so good. But as a cleanser it appears to suffer from leaving behind a film; a bad sign.

If you are concerned about the potential harm of artificial phosphates in the environment, anodizing is not a significant supply. I have been using the same 8 oz. box of actual TSP for the last dozen years. That’s equivalent to a few weeks of laundry. The same batch of electrolyte can keep on going for months, by adding distilled water (the part that is used up) and an occasional pinch of TSP crystals (to keep up the concentration from the drops removed by pulling out pieces). Occasionally, I filter out the dust and bring it to a boil to make sure it stays sterile.

If you want to be even more environmentally correct, use ammonium phosphate (lawn fertilizer) and then dispose of your old electrolyte by spraying it on your lawn. I used a box of this through the 1980’s and 1990’s.

One of these days, I’ll probably expound why the lingering phosphate meme of the 1970’s was somewhat misguided in the first place.


Q: Can I Stamp and Rivet with Titanium?

December 3, 2010

Kelley asks:

I make stamped jewelry and I have never worked with titanium before. Can titanium be stamped with a usual tool hardened steel stamp set? A client is wanting inspirational words stamped on a premade titanium bracelet.
Another question, would my usual method using a riveting hammer and a bench block do with titanium wire to create a rivet?

The short and qualified answer is, “Yes”.

Odds are that you are working with one of the less hard alloys or grades. Titanium behaves much like stainless steel, in that it takes more force to work it than do silver and other “normal” jewelry materials. Hit harder, maybe with a bigger hammer or mallet than you use to mark silver. I use normal steel stamps, myself  (as in this picture).

Where titanium gets its tough reputation from is that it is harder than steel under pressure. Stamps and especially cutting tools wear out faster because titanium, unlike steel, doesn’t get softer at a mere few thousand psi. Silver, gold, etc are softer to start with. I used a 20 oz. hammer to strike this stamp.

I have also used titanium wire to make rivets, using an ordinary little chasing hammer to set them. Nice, soft Grade #1 wire. Here is a List of Grades and their relative hardnesses.