Page 219 - Lindens Handbook of Batteries

ZINC-CARBON BATTERIES—LECLANCHÉ AND ZINC CHLORIDE CELL SYSTEMS 9.13

TABLE 9.3 Electrolyte Formulations*
Constituent Weight %
Electrolyte I
NH Cl 26.0
4
ZnCl 2 8.8
H O 65.2
2
Zinc-corrosion inhibitor 0.25–1.0
Electrolyte II
ZnCl 2 15–40
H O 60–85
2
Zinc-corrosion inhibitor 0.02–1.0
*Electrolyte I based on Kozawa and Powers. 7
Electrolyte II based on Cahoon. 5

9.5.6 Corrosion Inhibitor
The classical zinc corrosion inhibitor has been mercuric or mercurous chloride, which forms an
amalgam with the zinc. Cadmium and lead, which reside in the zinc alloy, also provide zinc anode
corrosion protection. Other materials like potassium chromate or dichromate, used successfully
in the past, form oxide films on the zinc and protect via passivation. Surface-active organic com-
pounds, which coat the zinc, usually from solution, improve the wetting characteristic of the surface
unifying the potential. Inhibitors are usually introduced into the cell via the electrolyte or as part
of the coating on the paper separator. Zinc cans could be pretreated; however, this is ordinarily not
practical.
Environmental concerns have generally eliminated the use of mercury and cadmium in these bat-
teries. These restrictions are posing problems for battery manufacturers in the areas of sealing, shelf
storage reliability, and leakage. This is critical for zinc chloride cells in that the lower pH electrolyte
can result in the formation of excessive hydrogen gas due to zinc dissolution. Certain classes of
materials considered for use to supplant mercury include gallium, indium, lead, tin, and bismuth,
either alloyed into the zinc or added to the electrolyte from their soluble salts. Other organic materi-
als, like glycols and silicates, offer protection alternatives. Additional restrictions on lead use, which
are already stringent, may also be imposed in the future.

9.5.7 Carbon Rod
The carbon rod used in round cells is inserted into the bobbin and performs the functions of current
collector. It also performs as a seal vent in systems without a positive venting seal. It is typically
made of compressed carbon, graphite, and binder, formed by extrusion, and cured by baking. It has,
by design, a very low electrical resistance. In Leclanché and zinc chloride cells with asphalt seals,
it provides a vent path for hydrogen and carbon dioxide gases that might build up in and above
the cathode during heavy discharge or elevated temperature storage. Raw carbon rods are initially
porous, but are treated with enough oils or waxes to prevent water loss (very harmful to cell shelf
life) and electrolyte leakage. A specific level of porosity is maintained to allow passage of the
evolved gases. Ideally, the treated carbon should pass internally evolved gases, but not pass oxygen
into the cell, which could add to zinc corrosion during storage. Typically this method of venting
gases is variable and less reliable then the use of venting seals. 4,6