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.

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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.


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.


Why add a capacitor to the Anodizer?

August 6, 2008

I received the following question:

I have a variac and full wave rectifier but no cap.
What is the reason behind adding a capasitor to the anodizer? I know it will reduce electrical ripple but what will it mean to the anodize process or final results?

In principle, the smoother, ripple-reduced output allows more even anodizing starting at the initial surge. Whether this is truly useful, I don’t really know. My experience is almost exclusively with a smoothed DC supply. But I have a switch on my main anodizer to disconnect the capacitor for those occasions when I feel like it.

RMS vs Peak Voltage The voltage will read wrong with ripple. The anodized color depends on the peak voltage. But a rippled current shows on a meter as the rms voltage, that is somewhat lower. So the color is less predictable, and the time spent at that voltage is more critical to watch.

Also, once you reach your final voltage (or at least asymptotically close enough), the smooth DC current is stopped. But a rippling supply still produces a trickle of  current as the piece you are anodizing acts as a capacitor. If you wait long enough, you can see the color continues to rise at a fixed ripply voltage.

This latter point is more important if you mask and do a succession of lower voltages for multiple colors. With ripple, the higher voltage colors will creep as you anodize the lower voltage areas.

Another note is that AC is more dangerous than DC. Edison (General Electric) made sure that the first electric chair used the AC current promoted by his rival Tesla (Westinghouse), to popularize that point. (source) But I doubt it makes much difference in any practical sense of anodizer safety.


How can I get consistent colors?

March 26, 2008

This frustrated email arrived today:

I live in Belgium (Europe) and have been making titanium jewellery for 4 or 5 years now. Sometimes I colour the pieces. I have a machine from Wieland, a German company. All this time I did not have any problems because the pieces I made had always the same size. Now lately we are doing titanium leaves in various sizes; Grade 2 thickness 0.5 mm. The problem with coloring these pieces is that I do not seem to have any control over the colours due to the variable sizes of the pieces. Is there a way to make a formula that gives me control (more or less) by taking the weight of the piece? Because of the irregular shape it is impossible to know the amount of surface I am working with.

At the moment I am really taking what comes out of the machine. I did make several colour charts using various shapes but with pieces 6 to7 times as big as my trials I have no control .

I would be very grateful if you have some advise on this, or maybe even a solution.

Thanks in advance.

Peter

My reply: I can’t know exactly what the problem is, for I encounter the same difficulties.
The color you end up with is a function of the electrical current density, the total time, the surface finish, and the grade/alloy (which also affects finish and current flow).

The weight is not as good an indicator as the surface area. If all the pieces are always the same thickness, then the two are functionally interchangeable. But the risk is that if you calibrate on weight, and then start working with other thicknesses, the calibration will not stand.

I passed the question on to Bill Seeley.


How can I make some of those Fancy Titanium Colors?

March 21, 2008

Another question from a visitor to my regular site:

I’m starting to anodize on my own. How do they create that color “oil slick” in the picture or the “rainbow”?


The many simple colors are all based on voltage, as described in my anodizing page.

The stripes are made by masking off areas with something waterproof, like automotive striping tape. Then anodize to a high voltage color. Then remove the tape, and anodize to a lower color. The high voltage color blocks the lower voltage colors. Voila, stripes!

The rainbow can be made in several ways. The fastest is to turn the voltage up and the contacts off, immerse the piece,  then turn the contacts on and draw the piece out of the electrolyte. The color is now dependent on the immersion time rather than the voltage setting.

The oil slick is trickier. This is probably done by sponge or brush anodizing (clip the positive lead to the piece, and the negative to something absorbent soaked in electrolyte. Then very carefully apply the high voltage wet thing to the charged piece. Rubber gloves and goggles are required. If metal touches metal, then you are practicing welding. Bright sparks, damaged pieces, and possibly damaged electronics.