Lead Dioxide - PLATED!
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It has been a long road, but I now have two anodes plated... one of them looking fairly sad, while the other looks excellent. The weather has NOT cooperated, and in addition to the physical discomfort (cold!), there are process issues as well. Jugs of distilled water are ice cold, and the hot bath (60 to 70 degrees C) must really struggle to maintain heat. Evaporation from the bath is at a ferocious rate. Anyway, I am getting ahead of myself.
I've spent probably a month preparing for this moment, making equipment, gathering chemicals, and above all, educating myself. This process has been somewhat of a holy grail for home chemists. The electroplating itself of lead dioxide is well known, but the problems that are associated with it are many, and ultimately cause the lead dioxide anode to fail in use.
The primary failure mode is poor adhesion, meaning the electroplating simply does not stick well, to itself, or to the substrate. Pieces of lead dioxide simply flake away or fall off the anode. The second failure mode is chemical attack of the substrate rather than the lead dioxide, which by itself is quite inert in use. Assume for a moment that you have a nice titanium anode which you successfully plate with lead dioxide. It looks good. But in use, the potent liquor attacks the titanium beneath the lead dioxide; it oxidizes, and you lose continuity between the Ti and the lead dioxide. Eventually there is little or no conductivity between the two, and the anode fails.
Several patents address the adhesion problem with the use of surfactants, especially those of the polyoxyethylene variety. I found a good one in Triton X-100. Think of the surfactant as a very powerful detergent. The second failure mode (attack of the substrate) I hope to overcome by plating the very tough MMO mesh which has proven itself in extreme environments. By the way, I have plenty of this mesh available... (shameless plug) - check the Agora. It is GOOD stuff!
The setup is human and mechanical. The human, of course, is me. In my entire life, I have never worked with such noxious and toxic materials. I prepared accordingly:
Respirator, gloves, face shield, and a disposable painter's Tyvek suit that costed 7 bucks - worth it. Looks cool, too, or dorky, depending upon your love of science! I suppose the neighbors wondered a bit, though, when I crossed between the house and the shop!
The separate portions of the setup were gathered together on a portable table. I layed down a plastic drop cloth, and taped it to shelves behind the table. Several layers of newsprint paper (from "Staples") were placed on the plastic to soak up spills and droplets.
From left to right: a mag stirrer plus 4l beaker, a heater, a vacuum jar plus gauge (the vac pump is below the table), and a dual power supply. On top of the supply is a cheap powered HEPA filter that I was hoping would gather local airborne contaminants, and contain them. Finally, to the right, is the plating rig, seen before. It both vibrates AND rotates the anode. There was some doubt about the viability of rotation as a methodology, but I will flatly state that it works, and works well.
The grey box at the edge of the table is the heater controller, also seen in a previous blog.
The next batch of pictures will include pics of both anode 1 and anode 2 plating sessions. The difference between the two is primarily the bath size... for anode two, I went with the 6.5 liter bath. Anode 1 used a 2.5 liter, cheap plastic container that I was not happy with, but it had a good shape. There were important chemical differences as well, which I will describe as we proceed along.
The first step was to prep the MMO anode. I began by simply immersing it in a beaker full of acetone, and allowing it to soak for perhaps an hour, with occasional agitation. While soaking, I also prepared another beaker full of distilled water, and added to the beaker approximately 2 ml of Triton X-100, a powerful surfactant. The Triton has a viscosity about like that of heavy molasses, but disperses well and easily. This beaker went on the heater to heat up to 70 degrees.
Once heated, the Triton plus water mix became milky and a bit opaque.
While the anode was soaking, I filled the 4 liter beaker with 2.5 liters of distilled water, and on the magnetic stirrer, began to add the chemicals. The "recipe" I used was a carefully thought-out mixture from a number of patents. The MS Word doc I have created which contains this recipe and much other info can be found here: Lead Dioxide Plating for Dummies
Note the fancy custom labels. The two jars on the left were purchased in bulk from Duda Diesel, and the majority of my chemicals I bagged and stored in these HDPE jars. While not especially high quality, they are nice, and best of all, they stack. Very handy for tight storage locations. In the background is a 2.5 liter FEP bottle of concentrated nitric acid, the last chemical to add to the bath.
The plating bath (2.5 liter, anode #1) consisted of the following:
- Compound gm/l
- Lead Nitrate – Pb(NO3)2-- 375
- Nitric Acid – HNO3-- 10
- Bismuth Nitrate - Bi(NO3)3.5H2O-- 20
- Copper Nitrate - Cu(NO3)2.3H2O-- 14
- Nickel Nitrate Ni(NO3)2.6H2O-- 10
- Sodium Fluoride (NaF)-- 0.5
- Surfactant – Triton X-100-- 0.5
I started with the less toxic and more easily dissolved salts: The copper nitrate and nickel nitrate. There is some controversy on the use of nickel nitrate. Older patents make use of it as a grain-refiner, and anything that refines grain (finer, denser lead dioxide) is desirable to me. I decided to try it. Use caution: Nickel Nitrate is both toxic AND a suspected carcinogen. After the lead nitrate, it is probably the most noxious of the chemicals.
The Copper Nitrate is not required if using copper cathode(s), but in my case, I decided to use a CP (Commercially Pure) Titanium tube, so into the bath went 35 grams of Copper Nitrate:
The water in the 4 liter beaker had already been heated to about 80 degrees C on the hot plate to ease in the mixing of the chems. For the 6.5 liter bath, this process had to be done twice... the first 3 liters were loaded with all of the chems except the lead nitrate, and added to the polypropylene bath. The beaker was refilled with an additional 3.5 liters of distilled water, heated, and used to dissolve the huge amounts of lead nitrate the "big bath" required.
Back to the original bath, the 2.5 liter job... I decided to use Bismuth salts. From my research, in the patents, Bismuth was added to improve the efficiency of the anode in colder environments; not a player for perchlorate production, but there were some obscure references to it also being a grain-refiner. Into the bath it went. Being completely insoluble in a neutral solution, all it did was cloud the stirred electrolyte. One of the last chemicals to be added was the surfactant, 2.5 grams of Triton X-100. This too is controversial, because ultimately the acidic environment breaks down the polyoxyethylene chains; the bath life becomes limited as a result, unless the ruined surfactant is extracted with 1-propanol or similar. I decided to give this first run the very best chance for success, so it too went into the bath along with the Bi salts.
With the cloudy mixture being magnetically stirred, I began to weigh out the lead nitrate on a kilogram scale. The various patents have a range of 200 to 400 grams per liter of lead nitrate. Despite the 375 g/l in the "Dummy" doc, I used instead 300 g/l, so 750 grams of lead nitrate was used. The lead nitrate was from Chemsavers, one of my (and Tentacles) favorite resources. Chemsavers has since nearly DOUBLED the cost of lead nitrate, yielding not a few epiphets on my end. Fortunately, I had purchased 10 kg from them back when it was a bit cheaper. There are now less expensive sources of lead nitrate that Chemsavers. A bit of search will reveal them! Lead nitrate should cost approximately $12 to $15 per kilogram... much more than that, and you are getting ripped off.
Lead Nitrate is a useful chem beyond plating... it can be used to make nearly any other nitrate needed (such as Barium or Strontium) by simply adding the salt of interest to the lead nitrate solution. Since nearly every other lead salt is completely insoluble, the newly-formed lead salt precipitates and you can then harvest the aqueous nitrate of interest. Tentacles is the expert at this... I tend to be a lazy bastard and basically buy what I need. Anyway, I am rapidly drifting off topic.
The lead nitrate dissolves with ease if the water is hot enough. This is one case where a combined mag stirrer + heater is a good thing to have. Not being blessed with one of these, I simply heated the solution first, then moved it to the stirrer to dissolve the lead nitrate. The final addition was the nitric acid. In both cases, I added 5ml of stock 70% nitric per liter to the bath. This is definitely on the low end of various recommended acid concentration ranges. Since the process itself generates nitric acid, and excess nitric is fatal to a good plating job, I erred on the side of caution.
With the bath prepared, it was transferred from the large beaker to the 2.5 liter plastic container, and the immersion heater (encased in copper) was fixed into the bath and the controller turned on. Most resources state 60 to 80 degrees C. For both plating attempts, I set the controller for about 65 degrees, and the controller did a fantastic job throughout, maintaining +/- 2 degrees C.
While the bath warmed up and stabilized, and the anode continued to soak, I prepared the cathode. Resources state the cathode should be between 1/3 and 1/2 the are of the anode. These anodes measured aproximately 8 cm X 4cm, for a surface area (as cut) of 32 square cm. The mesh actually has 2.2X this area, so the surface are of the anode was determined to be 75 square cm, counting the shank portion immersed in the bath.
In my lathe, I took the titanium tubing (1/2" OD) and scored it every cm, so by counting the rings as they immerse, I was able to determine how much cathodic surface area is presented to the anode. Converting the 1/2" to metric, and doing a bit of math, I calculated roughly 4 cm squared for each cm of immersion.
The cathode carrier was machined from a strip of PVC plastic, and I set it up for three cathode diameters, 0.312", 0.500" and 0.750"
The grooves machined in the plastic carrier allows repositioning of the cathode on the plastic bath container, with stability.
Once the bath had stabilized at 70 degrees, I continued with the preparation of the anode. It had been soaking in hot water plus surfactant. I decided to apply vacuum to the beaker to pull any bubbles or voids on and in the MMO away from the surface of the anode.
The beaker + anode was set up in the bell jar, and the vacuum applied. The gauge slowly crept downward:
At this pressure, the water began to boil, so I allowed a bit of air to creep into the bell jar to cease the boiling, yet still continue to draw off any trapped air. It was left at this pressure for 1/2 hour.
To this point, both runs were identical with three major differences:
- The 2.5 liter bath had added Bismuth salts
- Additionally, it had added surfactant to the bath itself
- Finally, the volumes were greatly different, 2.5 liters vs. 6.5 liters.
The anode was attached to the rotating arm with 316SS hardware.
With everything completely ready to go, it was time, finally, to apply current. Like so much else associated with this process, suggested currents vary wildly. I took a number of them and essentially averaged the current. From "Plating for Dummies"... the list is phase of plating, Current in amps per square cm, and duration...
- Initial: 0.125 -- 15%
- Median: 0.050 -- 50%
- Final: 0.030 -- 35%
Note that NO TIME is specified, mainly because I was unable to truly determine how long a plating job should last! I figured I'd keep an eye on the anode in the initial phase, and when it exhibited 100% lead dioxide coverage, I'd move on to the next phase. At this moment, minor catastrophe #1 occurred. I didn't even bother to check the required current... for some stupid reason, I figured it would be below 3 amps, which is the maximum my smaller lab supply can do. At 75 square centimeters, 0.125 A per, I needed 9.37 amps! I had two supplies to choose from, a lightweight inverter supply, which is 240V, and an 80 pound beast, my old Sorensen. Since I had no 240V at the plating station, I dragged the Sorensen over, and ten minutes later I had my 9.37 amps on the anode. I am not sure if soaking (with no current) of the anode, during this period, was harmful. Normally, you do not want electrodes to soak unpowered in an electrochemical cell.
The rotation was turned on, setting the stage (again, I am referring to anode #1) for a more major catastrophe. I am not a completely stupid engineer... but for some reason, I decided a 0.375" copper round rod would rotate for days in a simple drilled hole in the aluminum anode support arm. Metal on metal at low RPM... normally not a problem. Immediately, I heard a bit of squalling from the system. That alone should have warned me! I added a couple of drops of oil, and soldiered on.
The anode support arm has both rotational and vibrational capabilities. The vibrator was nothing more than a Pittmann motor mounted to the boom arm, and a variable brass weight was attached to the output shaft. By varying the voltage to the vibrator, I could go from only a barely perceptible shaking, to a vibration so severe that the bath contents were being ejected from the container! THAT would not be good with this toxic bath. Here is a test with water from a previous day:
The purpose behind rotation was primarily to create an even coating; secondarily, it was hoped that the rotation would eject pinhead bubbles. The sole purpose behind the vibration was to eject nasty bubbles from the surface of the anode. Much thanks to Xenoid (from SMDB) for this ingenious idea.
I dialed the vibration down, set the rotation very slow, and waited... and watched. The anode darkened almost instantly, with the lead dioxide being flashed on. I ran anode #1 at the initial stage for approximately 1 hour. Then, I began to dial the current down to the median stage. In all stages of the project, the power supply was run in CC (Constant Current) mode.
Interestingly, I was able to easily control the nitric acid evolution (a very important function) using a pH meter and PbO, litharge. The pH of the initial bath in both cases was 0.5. Within an hour, due to evolved nitric acid, it had dropped to below 0.0, into the negative range. Most meters and probes have a difficult time dealing with such a low pH, but my pH system seemed to work fine. Patent (and other) sources suggested 10 grams of litharge per liter, per hour of operation, to control pH and replenish Pb++ ions. A bit of experimentation on the fly showed this number to be fairly accurate.
Approx 1/2 teaspoon of litharge every hour or so would bring the pH back up to 0.5, and that is where I tried to keep it. Lead Carbonate worked in much the same manner, with the bonus feature of a sizzle of CO2 gas as the carbonate hits the acidic bath, but litharge is cheaper and more convenient. Some sort of stirring or agitation is necessary during litharge additions. I used a PTFE rod as a stirrer. Future setups will definitely use some sort of motorized stirrer. I strongly believe that powerful agitation is helpful in this process.
About three hours into plating #1, the copper rotation shaft siezed with the aluminum boom arm. The gearmotor that powers the rotation pulled the belt right off the toothed timing pulley used on the Cu shaft. The only way to free it was to manipulate it with quite a bit of oil, some of which definitely drained down the shaft, polluting the bath.
With the entire rotational rig suspect, I pulled the plug at the 7 hour point; otherwise, I would have let it run overnight. Anode #1 is one ugly POS, and I doubt that it will be functional. The plating is rough and ugly. The lead dioxide that plated onto the pure CP Ti shank rubbed right off, probaby due to the oil dribbling down the shaft. LD on other parts of anode #1 seem flaky and fragile. To say I was unhappy was not an exaggeration.
I had one more free day... Waking up early, I tore down the boom arm and machined some PET plastic bushings for the rotating shaft. The 6.5 liter bath was prepared with the exceptions I mentioned... no Bismuth, and no surfactant in the bath. Two other major differences - I boiled anode #2 in a fairly heavy solution of surfactant, and from there, it went into the plating bath directly, no rinsing. The initial plating was onto a surfactant-slick MMO surface. The second difference was in the spacing of the anode to the cathode. I probably tripled it... being farther away, and as a simple tubular form, the cathode becomes more of a point source.
With good bearings in the boom arm, I was able to plate overnight. My goal was to close off the mesh. I didn't quite make it, but I am ecstatic with the quality of the plating. It is heavy, and so far, quite strong, shrugging off normal handling, whereas anode 1 is flakey, Since I have no basis for comparison, I am going to claim that yes indeed, it has that oft-mentioned "ceramic-like" surface.
Anode #1:
Anode #2: 
The odd blobules on the edge are even and strong. I don't think there is any way to make a mesh lead dioxide anode that doesn't have a slew of warts, and is generally ugly, but a strong, even grain is what is needed. I think that I may have nailed it with this one. Only testing will determine the truth of this.
Remembering that these are actually small test anodes, their specs:
MMO Mesh: 8.0 X 3.5 cm, 14.33 grams
Anode #1: Little change dimensionally; 27.17 grams PbO2 deposited
Anode #2: 9.0 X 4.7 cm, 182.78 grams PbO2 deposited
Assuming anode 2 works, I believe the following attributes were important to the success:
- Thorough prep, to include boiling in a polyoxyethylene surfactant
- A bath including lead, copper, and nickel nitrates, plus NaF
- Lead Nitrate concentration high, >350 g/l
- Introduction of anode strongly wetted with surfactant to the bath
- Lengthy hi-amperage session to deposit strong alpha PbO2
- Small Ti cathode, spacing 5" or greater
- Large, chemically stable bath
Next on the list is an analysis of the bath remnants, check of Pb++ ion concentration, filtration, and an attempt to continue plating with the "used" bath, assuming ion concentration is acceptable. This anode took on 182 grams of PbO2, or 0.76 moles of lead. The 6.5 liter bath started with 6.95 moles of Pb in solution, so the anode "took up" 10.9% of the lead. I did not track litharge addition. A guess would be 200 grams, or an addition of 0.90 moles of Pb, so the lead concentration of the bath should be fairly close to start. Concentration is easy to check... take a sample of the bath, precipitate the lead ions with NaCl or MgSO4, and weigh the dried precipitate. Calculate the molar amount in the sample, and extrapolate.
Historically, "used" baths don't plate as well as freshly-prepared baths, and I'm not sure why. It would be well worth investigating, given the cost of the chemicals, and the need to keep lead sequestered, safe, and not part of our local environment. You simply don't pour lead salts down the drain! If you experiment with lead, do so responsibly; be adult.
One of the goals was to plate enough lead dioxide to actually "close" the MMO mesh. In retrospect, I'm not sure this is necessary. It will require 275+ grams of PbO2, and a plating duration of at least 24 to 36 hours, on a small 8 X 4 cm anode.
An odd part of me prefers an aesthetically pleasing appearance for my anodes. I'd love to find a Ti rod, MMO coated, to plate with PbO2... this will definitely make a more attractive anode, and importantly, it will be strong.
This has been a lengthy and massive blog. Thank you for following along. I realize that this plating process as I have presented it is overly complicated, and certainly not for everyone. One of the future goals is a simple and relatively small "one-pot, one shot" plating process using a very basic bath with a SS wire "cage" cathode. I believe (again, always assuming that these anodes perform) that the anode prep was perhaps the most important aspect to the plating, along with, in the case of #2, a large, chemically stable bath.
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