Practical Large Anodes

It is not possible to fabricate a practical anode such as that shown in Figs. 20 and 21. One approach is to drill and tap a hole in the carbon piece and screw a piece of threaded copper into the hole. This works fairly well for a hole 10 cm deep, perhaps 15 cm deep if one is careful, but not very much beyond that. There are two major problems (1) It is difficult to get a good 

A second approach is to carefully machine the rod and hole to obtain a good press fit. The copper is soft and difficult to machine, but it can be done. The carbon is hard and difficult to machine, but it also can be done. It is a lot of work and it is difficult to push a piece of copper into a closely fitting hole 110 cm deep in a piece of carbon. 

However, even when this is successful there remains a serious problem. Dense carbon is very porous. Even if the surface is not wetted by the electrolyte, below about a 10-cm depth there is enough hydrostatic pressure to push electrolyte into the pores and into the copper/carbon interface. When the electrolyte reaches it, the copper corrodes. Since the corrosion products occupy more volume than the copper, the carbon is put under tensile stress and fails by cracking. 

Araldite PY306 resin, HY917 hardener, and DY070 catalyst, all from Ciba-Geigy.) The epoxy was mixed and cured according to the manufacturer s directions. 

The central metal (copper) conductor was carefully put in place, with copper wool packed around it to hold it in place and to conduct current from the central conductor to the copper plate and then to the carbon piece and out to the electrolyte. The 6-mm spacing (108 channels or grooves) appears to be adequate, but anodes with 160 channels or grooves run at slightly lower voltages. 

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Figure 22 shows a practical large anode. The anode started as a rough cylinder of YBD carbon about 20 cm in diameter and 120 cm long. A central cavity 10 cm in diameter and 110 cm deep was machined in as shown. The pores in the carbon were filled with a commercial epoxy material using standard techniques. (The material was a mixture of... 

The divided, flow-through, concentric cylindrical geometry is preferred by some manufacturers, The inner electrode may be a hollow cathode rod, surrounded by a cylindrical cation-exchange membrane, and an outer, perforated tubular anode (Fig. 7,20(a) This arrangement provides a relatively large anode area while minimizing the area of the membrane and the size of the cathode. In practice, a number of cell modules are incorporated into a common anolyte tank (Fig. 7.20(b)), with air-sparging to provide improved anolyte flow. 

Where the use of zinc anodes is practicable, the low driving potential is a great advantage since the resistance of the steel to be cathodically protected is the controlling resistance, and the current output of the anode varies with the requirements of the cathode. Thus it can be said that zinc anodes are largely self-governing. 

The production of large quantities of alloy material in anode form, and possessing the desired mechanical properties, must obviously be practicable and economic. Thus secondary processing such as heat treatment is undesirable. 

Performance testing is long term (months to years). Once a potentially attractive formulation has been determined it is used to produce detailed data on its performance and behaviour as an anode material under the anticipated exposure conditions. For this reason the test should mirror as closely as possible the expected operating conditions, or where practicable be conducted in the field. Large specimens (tens or hundreds of kilograms) may be used for these tests. 

Cathodic protection is therefore normally practicable only in large bore pipes carrying salt water. Under special circumstances, however, it has been found necessary to use cathodic protection in fresh water and in these instances anodes have been run longitudinally down the entire length of the pipe, but the cost of such schemes is usually prohibitive. 

Advantages 1. More impurities can be tolerated in the copper anode since the electrode distances are relatively large. 2. The fabrication of anodes and the operation of the electrolytic cell is relatively simple. 3. More suited for refining copper of varied impurity contents. Advantages 1. Energy losses are comparatively less because of small interelectrode distances and contacts are practically eliminated. 2. The refining cycle is shorter due to higher number of electrodes and the anodic residue is relatively small. 

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