Impressed current composite anodes

A higher content of AI2O3 and SiOj is critical for the composition of the protective films in the tubing, assuming the water contains silicates or silicic acid. The protective films have a maximum thickness of 1.5 mm and cannot grow further. The corrosion process can be stopped even in copper pipe networks with type I pitting [21] by providing a reaction tank with impressed current aluminum anodes. 

Table 7-2 Composition and properties of solid impressed current anodes (without coke backfill)...

It has now gained acceptance as an impressed current anode for cathodic protection and has been in use for this purpose since 1971. The anode consists of a thin film of valve and precious metal oxides baked onto a titanium substrate and when first developed was given the proprietary name dimensionally stable anode , sometimes shortened to DSA. Developments on the composition of the oxide film have taken place since Beer s patent, and this type of anode is now marketed under a number of different trade names. 

As an example, reinforced concrete slabs are prepared for an experiment to compare effectiveness of arc-sprayed zinc and titanium anodes. All the slabs will be put under impressed current cathodic protection, and exposed to controlled applications of chloride to the concrete to promote corrosion of the reinforcing bar. Slabs are assigned randomly to be coated with either arc-sprayed zinc or titanium. In this case, randomization is a desirable precaution to control small inconsistencies due to mixing of the concrete, the chemical composition of the concrete, or the distribution of aggregate in the concrete. 

Backfill The special soil placed around the anodes to provide uniform resistivity. The material used as backfill in an impressed current system is generally coke breeze whereas in the galvanic system the backfill is composed of a mixture of gypsum, bentonite and clay, the composition being dependent on soil resistivity. Bond An electrical connection between two metallic structures. 

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