Current density anodic

The temperature in the ceU is 40°C. Most electrolyte ceUs are equipped with 24 anodes spaced approximately 10 cm apart, center to center 25 cathode starting sheets are used, one at each end and others evenly spaced between the anodes. Current density is typicaUy 15 mA/cm of cathode area ceU voltage ranges from 0.30 to 0.70 V, and a current efficiency of 90—95% is usuaUy realized. 

The electrolysis is conducted at 90—95°C and an anode current density of about 50 120 A/m when using lead alloy anodes and lead cathodes. Using graphite electrodes, the current density is from 70 100 A/m using titanium anodes and graphite cathodes, the current density is 50 80 A/m (82). 

Piebaked anodes aie produced by molding petroleum coke and coal tar pitch binder into blocks typically 70 cm x 125 cm x 50 cm, and baking to 1000—1200°C. Petroleum coke is used because of its low impurity (ash) content. The more noble impurities, such as iron and siUcon, deposit in the aluminum whereas less noble ones such as calcium and magnesium, accumulate as fluorides in the bath. Coal-based coke could be used, but extensive and expensive prepurification would be required. Steel stubs seated in the anode using cast iron support the anodes (via anode rods) in the electrolyte and conduct electric current into the anodes (Fig. 3). Electrical resistivity of prebaked anodes ranges from 5-6 Hm anode current density ranges from 0.65 to 1.3 A/crn. ... 

Higher currents polarize anodes current density is critical. 

If using bron2e anodes, Sn should be less than 10% and anode current density <1 A/dm. ... 

Spontaneous Passivation The anodic nose of the first curve describes the primary passive potential Epp and critical anodic current density (the transition from active to passive corrosion), if the initial active/passive transition is 10 lA/cm or less, the alloy will spontaneously passivate in the presence of oxygen or any strong oxidizing agent. 

Composition Density Anode current density (A m ) Anode consumption... 

Impressed current anodes of the previously described substrate materials always have a much higher consumption rate, even at moderately low anode current densities. If long life at high anode current densities is to be achieved, one must resort to anodes whose surfaces consist of anodically stable noble metals, mostly platinum, more seldom iridium or metal oxide films (see Table 7-3). 

Substrate metal Density (g cm-3) Coating Density (g cm-3) Coating thickness ( m) Anode current density (A m ) max. avg. Allowable maximum driving voltage (maxA ) Loss (mg A a )... 

In such cases basket anodes are frequently used. These have a relatively large surface and work at a low driving voltage due to their special construction. A cylinder of platinized titanium-expanded metal serves as the basket to which a titanium rod is welded. This serves as the current lead and carrier, and ends in a plastic foot that contains the cable lead and at the same time serves as the mounting plate. The expanded metal anode exhibits a very uniform anode current density distribution, even at large dimensions, in contrast to the plate anode. The reason is the many comers and edges of the metal that make the point effect only evident at the outer edges of the anode. 

The submitters used a cathode of nickel foil (140 x 71 x 0.5 mm.) rolled into a cylinder 3.5 cm. in diameter surrounded by three curved platinum anodes each having the dimensions 70 x 30 x 1 mm. (total surface area 130 cm. ) with a distance of 0.5-1 cm. between the cathode and the anodes. The submitters electrolyzed for 6 hours at a current maintained at 3.25 amp. This corresponds to a total of 19.5 amp.-hours and an anodic current density of 0.025 amp./cm.Under these conditions the submitters report yields of 81-84%. 

Hatch, G. B., Maximum Self-generated Anodic Current Density as an Inhibitor Pitting Index , III. State Water Surv., Circ. No. 91, 24 (1966) C.A., 66, 8l8l4f Herbsleb, G., Pitting Corrosion on Metals with Elearon-conductive Passive Layers , tVerksl. Korros., 17, 649 (1966) C.A., 66, 5337m ... 

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