Impressed current system conductivity

If CP is the chosen rehabilitation methodology then the correct choice of anode is vital. For applications where the life is less then 20 years and suitable anodes are available, galvanic cathodic protection. For longer lives, impressed current systems are more likely to be suitable assuming that power is available and maintenance will be conducted. When it comes to individual anode choice then the Table in HA BA 83/02 summarizes the merits and limitations discussed in Section 7.3.1 to 7.3.7. 

If cathodic protection is the chosen rehabilitation methodology then the correct choice of anode is vital. For non-marine applications the usual choice will be an impressed current system. If it is a wearing surface then coatings are usually excluded. This leads to the use of the conductive concrete anode or one of the titanium configurations in an overlay or titanium ribbons in slots. 

Another approach for reducing corrosion is to employ mechanisms that can modify the electrochemical processes that consume materials. Cathodic protection, either through the use of sacrificial anodes or an impressed current system, can convert a material that normally will corrode quite readily into a material that resists corrosion. This approach, which is the topic of Chap. 13, works very well for protecting fixed assets in contact with potentially corrosive environments such as soils, seawater, or any other electrolytically conducting medium. 

The low cost, light weight, and exceUent electrical conductivity of graphite anodes have made this impressed current protection system valuable for cathodic protection of pipelines, storage vessels, process equipment, and also for weU casings both on- and offshore. 

Figure 20-9 shows the negative effect of uninsulated heating elements on corrosion protection. In a 250-liter tank, an electric tube heating element with a 0.05-m surface area was screwed into the upper third without electrical separation, and in the lower third a tinned copper tube heat exchanger with a 0.61 -m surface area was built in. The Cu heat exchanger was short-circuited for measurements, as required. For cathodic protection, a potential-controlled protection system with impressed current anodes was installed between the two heating elements. The measurements were carried out with two different samples of water with different conductivities. 

By virtue of the high breakdown potential of the oxide film (approximately 155 V in sea water and 280 V in low conductivity water of pH = 7) tantalum has found use as a substrate for platinum in impressed-current cathodic-protection anodes, which can be used at high impressed voltages (50 V) and high current densities. However, because of its lower cost, niobium is preferred for systems that have to operate at high voltages... 

Figure 9.14 Cathodic protection with an impressed current, (a) Leadjsilver anode for the protection of a ship s hull. Note that the anode is insulated from the hull and surrounded by a non-conducting shield to improve the potential distribution over the hull The anodes will be placed at intervals around the hull (b) Anode cans for the protection of an underground pipeline (some distance away). The anode cans contain a Fe/Si rod surrounded by coke breeze (to increase anode area), (c) Monitor and controlled power supply for a ship s corrosion protection system. Photographs supplied by Corrintec UK Ltd.

To illustrate randomized block designs, consider the following example an experiment is conducted to test the effectiveness of two humectants (wetting agents) on the operation of impressed current cathodic protection systems on reinforced concrete bridges. There are 120 reinforced... 

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