Thoughts on plating baths.

There has been a lot of words wrote about different types of plating baths and additives that are incorporated in baths for to achieve the 'best'/'superior'/'highest quality' coating of Lead Dioxide. Theses words themselves are hard to define exactly. When plating a substrate that is not inert to the chlorate/perchorate cell you need a non-porous coating of Lead Dioxide with NO flaws or 'pin holes' (invisible or visible). If there are any flaws the Chlorate/Perchlorate electrolyte will get to the substrate and cause the destruction of same.
In general you must have a coating of Lead Dioxide that will not shred or crack and fall off of substrate when it goes into service. It must adhere well to the substrate (inert or otherwise).
The most popular bath for the electro deposition of Lead Dioxide is the bath containing Lead Nitrate. It will plate under a wide range of temperatures, pH's, Lead ion concentration, and current densities when compared to other baths. The Lead Nitrate concentration should not be let fall below 200g/l. Use a bath large enough so that when you are finished plating the Lead Nitrate concentration will not have fallen below 200g/liter. The bigger the bath the better.
The Lead Nitrate bath mainly plates Beta Lead Dioxide (the type we want on the outside of the anode). The amount of Beta or Alpha that is plated can be controlled by current density on the anode according to one source I have communicated with. High current density (70mA per square cm) giving the Alpha, 40mA per square cm giving the Beta. Alpha gives better adhesion to Ti substrates ( and possible others excluding graphite), see US 4822459 for a discussion of adhesion. Some articles I have read have used a high current density at the start of plating to create a large amount of PbO2 nucleation sights on the substrate to improve adhesion. Perhaps these's sights were Alpha PbO2. The different crystal structure of Alpha makes it more adherent to Ti and doped Tin Oxide.

There is also the question of additives for plating baths. The main additive that is added to baths are surface active agents (surfactants). When plating Lead Dioxide, bubbles of oxygen appear on the anode that is being plated. Theses bubbles stick to the anode and cause a 'pin hole' to form where the bubble is sitting. This is a disaster if you are using a non inert substrate as this hole will allow Chlorate/Perchlorate electrolyte to corrode the substrate when the anode goes onto service and cause failure.
The literature suggests many different surfactants that should be added to the plating bath both ionic and cationic. See surfactants for more info regarding surfactants.
The surfactant will not allow a bubble to stick to the anode and therefor you get an improved coating of Lead Dioxide. There is also a mention of the surfactant Cetyl Trimethylammonium Bromide alleviating stress in the electro deposited Lead Dioxide in JES Sept. 1976 p1294 (see elsewhere on this page). Also CTAB decreases stress as stated in J. Applied Electrochemistry 12 (1982) 171-183, though they say that high temperature and low CD do a better job that the CTAB.
An alternative strategy for the elimination of bubbles is to spin (cylinder) or rock to and fro (flat plat) the substrate which keeps bubbles from sticking to the anode. The movement has also the added advantage of keeping the bath stirred.
Adding ceramic particles to the bath has been mentioned in patents 4,026,786 and 4,159,231. The bath is stirred and the particles brush against the plating anode and keep bubbles swept off it. This method has no complications like breakdown products etc.

There are other additives that are mentioned in the literature including
Nitric acid!, limited to 4 to 6 grams per liter, US 4038170, US 3,463,707, from 2 to 20 grams per liter.
Nickle Nitrate,as a grain refiner
Gelatin, as a deposit leveller
Fluoride, to improve plating current efficiency, reduce bubbles on anode and for to dope the Lead Dioxide. Na Fluoride has an approx. saturation value of 4.5 x 10 ^-2 M, at which point Lead Fluoride precipitates. (JOURNAL OF APPLIED ELECTROCHEMISTRY 10 (1980) 511-525). F is also considered in US 3,463,707 for removal of Iron.
Iron, Cobalt, Nickle and F to increase catalytic effect, (J.Serb.Chem.Soc. 66(11–12)835–845(2001))
Flourine resin, (Environ. Sci. Technol. 2005, 39, 363-370)
Hydrogen Peroxide, to oxidise Nitrites, that are formed at the Cathode, back to Nitrates (US 2994649).
NaClO3, ???
Sodium/Lead Acetate to relieve stress and as a pH buffer (Electrochimica Acta. 1971. Vol. 16, pp. 1301 to 1310).
Ta & Nb Oxide powder to reduce stress (US 5545306 & 4822459).
Teflon emulsion to reduce stress.
Bismuth Nitrate to make the Anode much more resistant to wear (Hypochloride production), US 4038170 & 4101390.
Bismuth has also been touted as having increased cathylitic effect for the formation of Perchlorate.
CTAB to decrease stress (JAE 12 (1982) 171-183) though they say that high temperature and low CD do a better job that the CTAB
and more.....
Surfactants will improve the coating of the Lead Dioxide but it is not essential to get an excellent coating of Lead Dioxide when using an inert substrate or inert fibre mesh reinforcement as theses materials have resistance to the Chlorate/Perchlorate electrolyte.
The addition of surfactant has the added complication of the break down products produced by the surfactant in the plating bath which will eventually stop the bath from plating successfully. Some have suggested resting the neutralised bath for 24 hours for to allow the breakdown products to recombine. After a lot of plating the tank has to be washed using amyl alcohol, which is a lot of extra work. See US Patent No. 2,945,791.

There are (roughly speaking) two types of Lead Nitrate plating baths, high Nitric acid baths, and low Nitric acid baths. The bath recommended in this page is of the low variety. It initially contains a few grams of Nitric acid per liter when plating is commenced. The Nitric acid concentration will increase as plating progresses. The high Nitric acid plating baths (See US Patent No. 4,026,786) contain about 100g/l actual acid when plating commences. The patent claims that there are advantages regarding using this type of bath. Stirring the Lead Nitrate electrolyte is ESSENTIAL when using this type of bath because Lead Nitrate is inclined to precipitate out on the plating anode which causes adherence problems between the Lead Dioxide and the substrate. The high Nitric acid patents appear to be for making Lead Dioxide as a product (powdered) and also as a metal electrowinning anode only.

The function of Copper Nitrate in the plating bath is to stop Lead metal from plating onto the cathodes and wasting Lead ions.

If I understand this process correctly, the Copper
is added to the solution in order to plate out
first. After this initial Copper plating at the
cathode, some Lead will start to deposit, but due
to the fact that there is already a layer of
Copper on the cathode, the Lead and Copper will
form a shorted galvanic couple, and the Lead,
being placed above Copper in the electrochemical
series, will be corroded or re dissolved. This
couple can supposedly also cause Hydrogen to be
evolved at the cathode, instead of any metal
plating out. I have always used a cathode with a
larger area than the anode being plated in order
to (hopefully) allow the Lead deposited there to
be re dissolved as fast as it forms. I do not know
of any magic area ratio, but I suppose you could
expect some unusual things to happen if you used a
very small cathode.

Another parameter that is mentioned regarding plating baths is 'throwing power'. The addition of Nitric acid is said to improve the throwing power (an advantage).
Throwing power refers to the ability of the solution to plate into nooks and crannies and around corners that are not in a direct line of sight of the cathode. To put this another way, it is the ability of the electrolyte to even-out the current distribution on all areas of the anode given a certain tank and electrode set up. There is a good explanation of throwing power in "Treatise on Electrochemistry" , G. Kortum, (Elsevier publishing Co.).
It is related to the conductivity of the solution and Nitric acid increases this. The current on the anode is more evenly distributed as a result of the greater conductivity. The current distribution is also a function of cell geometry, spacing and arrangement of the anode and cathode. The cathode should surround the anode being plated and there should not be any sharp corners (this increases the current density and increases plating on that area) or indents (less plating) on the anode.

Another problem that may accur is lowering of plating efficiency. The problem is caused by Nitrites building up in the tank (see US 2994649).

 My theory is that since Nitrite contains less Oxygen than Nitrate, it can be
formed at the cathode since reduction takes place there. The equation could be
something like:


NO3-  +  2e  +  2H+ ===>   NO2-  +  H2O


Now at the anode the oxidation of Nitrite  to Nitrate would be competing with
the oxidation of Pb2+ to PbO2. The  equation would be something like:


NO2-  +  H2O  -  2e  ->  NO3-  +  2H+


Since the potential required for the above process is far lower than that
required for the formation of PbO2, current efficiency will drop.
US PATENT 4,064,035 discusses an alkaline bath using NaOH and Lead Hydroxide. This patent is trying to eliminate stress in the deposited Lead Dioxide layer by alternatively depositing Beta and they Alpha Lead Dioxide which, seems to eliminate stress. It should be noted that Alpha Lead Dioxide is not good at standing up to a (Per)Chlorate electrolyte and the outside layer of the anode must be Beta. It should also be noted that if the substrate is Graphite, it is not possible to plate on a Alpha layer to start with and in that case you must start with a Beta layer and end with a Beta layer.

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