Anode contamination metal

This section reviews additional anodic processes in water cleanup, leaching pure metals from ores, cleaning contaminated metallic surfaces, and anode behavior in harsh electrolytic environments. 

Cells for metal-powder production may utilize soluble replaceable anodes, as in eleclrorcfining, and the anode be installed inside a porous container to prevent anode sludge contaminating the catholyte or the powder deposit In the case of cells utilizing very insoluble anodes, the metal concentration in the cell may be maintained by solution fiow. 

An attempt has been made to use the inert anodes in large cells, but apparently it was unsuccessful, probably due to cracking and corrosion of the anodes, contaminating the aluminium metal. Likewise the use of wettable cathodes in commercial cells has so far been unsuccessM due to material problems [4, 7]. 

In electrogalvanizing, copper foil, and other oxygen-evolving appHcations, the greatest environmental contribution has been the elimination of lead-contaminated waste streams through replacement of the lead anode. In addition, the dimensionally stable characteristic of the metal anode iatroduces greater consistency and simplification of the process, thus creating a measure of predictabiUty, and a resultant iacreased level of safety. 

As a final step, anodized parts must be sealed to ensure corrosion resistance of the anodic coating. Sealing involves plugging the anodic pores completely so contaminants caimot reach the base metal. A variety of sealing methods are used by anodizers (see Sealants). 

Figure 19-1 shows the experimental setup with the position of the steel test pieces and the anodes. The anodes were oxide-coated titanium wires and polymer cable anodes (see Sections 7.2.3 and 7.2.4). The mixed-metal experimental details are given in Table 19-1. The experiments were carried out galvanostatically with reference electrodes equipped to measure the potential once a day. Thus, contamination of the concrete by the electrolytes of the reference electrodes was excluded. The potentials of the protected steel test pieces are shown in Table 19-1. The potentials of the anodes were between U(2u-cuso4 = -1-15 and -1.35 V. 

In the first reconstruction [27] of road slabs contaminated with CL, silicon iron anodes were embedded in a layer of coke breeze as shown in Fig. 19-4a or the current connection was achieved with noble metal wires in a conducting mineral bedding material. Slots were ground into the concrete surface for this purpose at spacings of about 0.3 m (see Fig. 19-4b). This system is not suitable for vertical structures. 

There are obviously situations which demand considerable over-design of a cathodic protection system, in particular where regular and efficient maintenance of anodes is not practical, or where temporary failure of the system could cause costly damage to plant or product. Furthermore, contamination of potable waters by chromium-containing or lead-based alloy anodes must lead to the choice of the more expensive, but more inert, precious metal-coated anodes. The choice of material is then not unusual in being one of economics coupled with practicability. 

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