Tubular Anodes - Updated!
Entry posted by Swede
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There are two bothersome problems when you use a traditional strap of titanium to deliver current to either the anode or the cathode in a (per)chlorate cell. The first, and most troublesome, is the cutting and sealing of an appropriate slot in the lid of your cell for a strap that measures perhaps 1mm x 25mm. In thin plastic, it is not too much of an issue. In 1/2" thick PVC plastic, it is frankly a bitch, and regardless of your cell lid thickness, ultimately, the electrode strap will be floppy and loose, requiring some sort of external mechanical support, or perhaps a few grams of glue, which inevitably erodes from the chemical attack of the cell. A while back, I devised a method to flatten and weld a titanium tube to an electrode, and with a tubular shank secured to the MMO or Ti, it is a simple matter to route the tube through an appropriate plastic compression fitting made of PVDF.
In my September 4th, 2009 blog entry, I showed a method to create a basic tube anode or cathode shank, which solved the issue of mounting firmness and maintainability.
This solved some of the mounting issues, but there was still the very real problem of overheating, as a typical CP Ti tube is fairly thin-walled, and given the relatively poor conductivity of Ti, above about 35 amps, a 1/2" OD tube will overheat and cause problems with the plastic compression fittings. In addition, tube shank mark 1 required a lathe to get good results, as a brass or Ti button carrying female threads had to be pressed into the tube, and this requires some fairly precise machining, which is not something everyone can do.
Someone suggested (I forget who, forgive me) to fill the tube with lead, and drill and tap that material, and I thought it was a great idea. With the arrival of some very expensive Pt mesh, I decided to give it a shot, rather than spot-weld yet another Ti strap to this material. While I was at it, I figured I may as well start from the beginning, and show how to make a tube mounting that will appeal to the hobbyist bulk maker of these important oxidizers.
1/2" (12mm) OD Ti tube is a good starting point. Be sure that you can find a PVDF compression fitting designed for the diameter of the Ti tube you wish to use. In my case, 1/2" PVDF compression fittings are easily (and cheaply) available, an to mount such a shank in your cell requires nothing more than the appropriate tapered pipe tap, normally 3/8" to 1/2" for a 1/2" PVDF fitting - note that the dimension has nothing to do with the OD of the tubing. A 1/2" tube will easily fit through a 3/8" FTP hole.
To create a flattened portion requires several heating and flattening steps to avoid cracking. A vise with smooth jaws is a good way to start. If you have a thin parallel of steel or something similar at the bottom of the vise jaws, you can control the length of the flattened portion.
With a propane torch, the end of the tubing is heated to a dull red. Ti (like stainless steel) is a very poor conductor of heat, and the tube will glow red fairly quickly. Once the end of the tubing is dull red, insert the tubing in the vise, and quickly crank it closed about 1/2 of the diameter.
Remove the steel spacer or parallel, heat the tube to dull red again, and compress it all the way to flat:
The final step, if you can do it, is one final heating, and a massive compression of the flat portion in a hydraulic press between two hardened steel pieces, or any two smooth and flat metal pieces.
Cut the tubing to the desired shank length (longer for now is probably better) and prepare to fill the tubing with a tin, lead, or antimony alloy. For this first attempt, I decided to use pure tin. Rotometals has an excellent selection of hardened Pb alloys as well as tin. They offer tin as 1/4" (6.3mm) wire, and I decided to try an exceptionally simple setup to melt the tin into the tubing.
But I am getting ahead of myself. NOTHING is going to solder (which is what we are doing) well to Ti or Al, and the best we can hope for is a good mechanical and electrical contact through the length of the tubing. This can be much improved by grinding or filing out the oxide layer inside the tubing, and giving the molten alloy a good mechanical purchase. I did this with both a round hand file, and a flame-shaped carbide bit mounted in a dremel to score and clean up the inside of the tubing:
A portion of the tin wire was cut and inserted into the tubing, which was simply clamped in a bench vise.
Applying a propane torch to the bottom of the tubing, the tin melted within 10 seconds, and began to flow and fill up the hollow Ti tube.
I applied the torch evenly to be sure the tubing was filled with tin. The whole process took less than 3 minutes.
After the tin cooled, there was a depression in the center that was about 1/2" (12mm) deep, as the tin's volume went down due to cooling. One final application of heat at the top, and a little more raw tin, filled the tube completely. It was set aside to cool.
After cooling, all that remained was to do one of the following:
1) Lathe: Mount, face, drill and tap for the appropriate bolt, in my case, 1/4" X 20 316 stainless
or,
2) File and generally clean up the end of the tube. Spot the center, drill and tap the tin for your electrical connection bolt.
In all cases, it is best to create a permanently-mounted stud. Tin, or tin-lead alloys, are not particularly hard, and a stud will eliminate the need to screw a bolt in and out of the tin frequently, which would loosen the threaded connection.
Do NOT use loctite for a stud. Simply screw in an appropriate length of threaded rod, and overtighten the rod into the unthreaded portion of the drilled hole. You'll want nothing to interfere with the electrical flow from your power supply to the tin or tin-lead alloy! The electrons will travel nicely to the mesh or titanium plate electrode, and given the excellent conductivity of the tin or tin-lead alloy, the vast bulk of the current will flow through the core rather than the titanium skin, keeping conductivity high and above all, keeping your assembly cool.
It is labor intensive, no doubt, and for a bench-top experimental lab setup, it is tough to beat a simple strap, but for true oxidizer production, I can't think of anything that willl work (and seal) better than this particular method.
Happy electrolysis!
Swede
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