Anode Shank Mounting
Entry posted by Swede
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This is going to be short and sweet.! 
Anyone who has created what might be called a true, "serious" (per)chlorate production cell tends to take time to make the cell durable, functional, gas tight, and above all, maintainable. 99% of the anodes available are electrically connected via a sheet metal strap, usually titanium. These are easy to create, weld, and connect to the power supply. But they suck at being maintainable and gas tight. It is easy to cut a thin slot in a similarly thin lid on an HDPE bucket, but if your lid has any thickness at all, cutting a 1/16" X 1" slot is quite a pain. And once the shank is installed, it needs to be sealed. I have tried hot glue, epoxy, PVC cement, teflon tape, any number of methods. They either fail mechanically, chemically, or both, and once glued in, they tend to be permanent.
There is a style of fitting available called a compression fitting, usually seen in plumbing, but it is also used extensively in industry, and is also available in two of the "big four" polymers that are suitable for a cell; PTFE or Kynar, with the latter being 15% the cost of the former.
Compression fittings form both a mechanical and a gas-tight seal around any round object such as tubing, yet they can be loosened so that the tubing can be removed for maintenance or replacement.
The idea, then, is to use a short section of titanium tube as the strap for the section of MMO or LD mesh. The tubing must make excellent electrical contact with the anode, and it must be able to accept the cable from the power supply on the outside of the cell. Additionally, the tubing must retain its cylindrical form at least on one end, so that it can be installed and removed from the fitting. All of these combine to make this more of a challenge than it appears to be.
The first step is to flatten one end of the tubing so that it can be welded to the anode mesh. My first thought was to slot the tubing, install the anode mesh, and flatten the junction for the weld. This was hopeless, with the Ti tubing cracking, and being a real mess.
The next attempt was to flatten the end of the tube in a hydraulic press. This showed more promise, but still cracked badly. By heating the tubing end dull red, then flattening, I found I was able to make a nice section for welding to the anode. This would best be done in a press, but a vise with smooth jaws might suffice.
In the picture above, the end of the tube is shown flattened, then sanded lightly. It would be best to abrade the inside of the tube to strip the oxides from the Ti for best welding.
The next step was to create two objects that will both seal the tube internally, and provide for an electrical attachment on the other end. This is best done with a lathe. I knw there are some imaginative guys out there, and no doubt there are other ways to do it, but since I had a lathe, I made use of it.
The ID of the tube was measured, and a plug of PET plastic was turned for a perfect fit internally. The function of this plug is to prevent any gasses from traveling UP the tubing, and corroding the electrical contact area.
I put a lot of thought into the electrical connection, but was unable to come up with a way to connect the cable other than what is shown here. The trick is that you cannot ruin the round shape of the tubing, so flattening and drilling is out. In the end, I turned the brass piece as shown. It's OD is 0.0015" wider than the ID of the tubing for a strong interference fit. If this fit is sloppy, the junction will corrode, and the fitting + tubing will not carry adequate current.
The business end of the brass piece was drilled and tapped 1/4 X 20 for the electrical cable lug.
The brass end is filed to a taper, and it barely starts into the tube under hand pressure, then stops.
With the parts made, the flattened portion was spot welded to the MMO mesh. It went OK, with more pressure than normal needed to create the weld. The length of the flat portion is a little shorter than I'd normally do. I prefer at least 1 square inch of contact in this area. But for this test, this will be fine.
The plastic plug was tapped into the tube with gentle tapping, and it went in as expected. The welded area was then clamped strongly in my press, and the brass turning was hammered on with a rubber mallet. The amount of force was perfect; not too tight, not too loose.
All that remained was to test the new shank using a 1/2 PVDF compression fitting. Sometimes, a 1/2" fitting might have an internal stop or some other protrusion which prevents the 1/2" tube from going entirely through the body. If this is the case, simply drill the body through with an appropriate drill. In my case, it fit through perfectly. On the top side, the two tapered collet pieces were added, then the outer nut was screwed on. As it begins to bottom, the collets squeeze the tube, and it locks in place perfectly!
To use this fitting, all I have to do is drill and tap my cell lid for the NPT thread that is on the compression fitting body. The anode is installed from below, the nut tightened down, and I've got a perfect mounting for the anode.
On the top of the shank is the 1/4" X 20 threaded hole for a stainless machine bolt which will secure the electrical cable (#2 stranded Cu) to the anode. Very strong, no wobble, this mounting will do everything I need it to. It will probably be a good idea to give the brass a light coat of lacquer on the non-contact portions to prevent the inevitable corrosion. Solid Ti bar could be used, but that would be expensive.
There is no doubt that this project took some specialized tools. I wanted to post it to maybe give some guys ideas. If anyone can think of a better way to attach the electrical cable to the round Ti tube, drop me a note. This is labor intensive, but for now, it is infinitely better than the strap shanks I had been using.
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