Impressed current equipment control

Tanks that are enamelled or that have other high-resistance coatings with volumes below 4 m do not require automatic exhausts if they are equipped with galvanic anodes in which the surface area does not exceed 5 dm per square meter of surface to be protected, or with controlled impressed current anodes with a low current output of 150 mA. 

The impressed current protection method is used mainly for the internal protection of large objects and particularly where high initial current densities have to be achieved (e.g., in activated charcoal filter tanks and in uncoated steel tanks). There are basically two types of equipment those with potential control, and those with current control. 

As an example. Fig. 20-7 shows potential and protection currents of two parallel-connected 750-liter tanks as a function of service life. The protection equipment consists of a potential-controlled protection current rectifier, a 0.4-m long impressed current anode built into the manhole cover, and an Ag-AgCl electrode built into the same manhole [10,11]. A second reference electrode serves to control the tank potential this is attached separately to the opposite wall of the tank. During the whole of the control period, cathodic protection is ensured on the basis of the potential measurement. The sharp decrease in protection current in the first few months is due to the formation of calcareous deposits. 

Disadvantages no control to ensure that protection is maintained, limited anode life (say 15 5 years), no published standards other than impressed current criteria which require installation of monitoring equipment and circuitry. Anodes intrusive either thermal sprayed zinc, anodes in repairs or cored holes. 

The complexity of offshore production platforms and associated pipelines necessitates specialized seawater corrosion control methods. Coatings are used to protect steel components in the atmospheric zone with CRAs used for special items. The splash zone represents a special case, with either special coatings and/or CRAs being used. Control of corrosion of submerged steel equipment is by cathodic protection via either sacrificial anodes or impressed current. Standards covering some of these control measures are listed below ... 

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