Impressed current systems cathodic protection system

Impressed-current-type cathodic protection systems have a number of advantages ... 

In impressed current systems cathodic protection is applied by means of an external power current source (Fig. 11.7). In contrast to the sacrificial anode systems, the anode consumption rate is usually much lower. Unless a consumable scrap anode is used, a negligible anode consumption rate is actually a key requirement for long system hfe. Impressed current systems typically are favored under high-current requirements and/or high-resistance electrolytes. The following advantages can be cited for impressed current systems ... 

The system was straightforward. One of the popular impressed current pipeline cathodic protection anodes of that time was made of a corrosion resistant silicon iron, surrounded by a carbon cokebreeze backfill. A well was dug near the pipeline, the anode put in surrounded by the backfill and the system connected to a DC power supply, with the negative terminal connected to the pipeline to make a cathodic protection system. Richard Stratfull look pancake silicon iron anodes, fixed them on a bridge deck and applied a carbon cokebreeze asphalt overlay (Stratfull, 1974). The systems installed in 1973 and 1974 were reviewed in 1989 and were still working (Broomfield and Tinnea, 1992),... 

Cathodic Protection Systems. Metal anodes using either platinum [7440-06 ] metal or precious metal oxide coatings on titanium, niobium [7440-03-17, or tantalum [7440-25-7] substrates are extensively used for impressed current cathodic protection systems. A prime appHcation is the use of platinum-coated titanium anodes for protection of the hulls of marine vessels. The controUed feature of these systems has created an attractive alternative... 

The use of an impressed-current system or sacrificial anodes will both provide satisfactory cathodic protection, but each has advantages and disadvantages with respect to the other (Table 10.24). 

A typical soil resistivity survey is shown in Fig. 10.22. Soil resistivities will normally indicate whether a cathodic-protection system is advisable in principle and whether impressed current or sacrificial anode schemes in particular are preferable. It may, as a result of the survey, be considered desirable to apply protection to the whole line or to limit protection to certain areas of low soil resistivity or hot spots . 

The major stray-current corrosion problems now result in cathodic protection systems. Current from an impressed-current cathodic protection system will pass through the metal of a neighboring pipeline at some distance before it returns to the protected surface. Increased anodic corrosion is frequently localized on the pipe at the zone where the current leaves the pipe back to the protected steel tank. 

Corrosion damage is a major factor in ship maintenance and availability. Coatings combined with the impressed current cathodic protection (ICCP) system are the most common means for shipboard corrosion control. They interact with each other to protect the shipboard. Coatings provide primary corrosion protection by isolating the hull metal from seawater, while ICCP systems protect the hull by applying an external source of current to the ship where the paint is damaged or degraded. 

In this technique, the electrical current is delivered to the structure to be protected from a direct current (dc) power source through an auxiliary electrode. The structure acts as a cathode and the auxiliary electrode becomes the anode in the cell [26, 27]. Figure 8 shows an impressed current system used to protect a pipeline. Both the buried anode(s) and the pipeline are coimected to an electrical rectifier, which supphes direct current to the buried... 

Fig. 15.6 Schematic of cathodic protection system using impressed current.

In this case of a cathodic protection system, the weight and number of anodes to be used is calculated from the cycle life of the system. Prolonged life of the anode is essential to decrease maintenance cost of the impressed current cathodic protection system. Dissipation rate of the anode is a measure of the life of the anode. For high silicon, chromium bearing cast iron, the dissipation rate is 0.5 kg/A year. The weight of the anode is given by... 

During the design stages of a cathodic-protection system, the designer must make an informed economic decision on the suitability of either a galvanic or impressed-current scheme. In some instances, the use of both systems may be adequate however, absolute care must be taken by proper choice of insulators at relevant junctions to avoid interactions between them. 

An alloy of 2% Ag-Pb is used as a corrosion-resistant anode in impressed current systems for cathodic protection of structures in seawater (see Section 13.6) [4]. Alloying with 6-12% Sb increases strength [only at temperatures <120 °C (<250°F)] of the otherwise weak metal, but corrosion resistance of the alloy in some media is below that of pure lead. 

This Is another vital part of an impressed current system. The T/R must be rugged and reliable with minimal maintenance requirements. It should be easy to maintain with good instruction manuals, circuit diagrams for maintenance and easy access to fuses and other replaceable components. Compared with pipeline or marine cathodic protection applications (steel piles, etc.) the power demand is modest. Steel in concrete needs less than 20 mA m - to provide protection, usually at less than 10 V, The power for a 100 W light bulb will protect 5000 m, This means that a single-phase, air cooled T/R vill usually protect even the largest structure and power consumption is rarely an economic concern. 

Platinised titanium anodes (titanium carrying a thin surface film of platinum, of the order of 0-0025 mm thick) have proved successful in cathodic-protection systems employing impressed-current techniques, as electrodes for electrodialysis of brackish water, and in many applications where established anode materials suffer significant corrosion. Platinum-coated titanium anodes can operate without breakdown at very high current densities, of the order of 5 0(X)A/m, in sea water, as although the very thin platinum coating may be porous the underlying titanium exposed at the pores will become anodically passivated... 

Regulation 542-0 l-07(i) means that the connections to earth must be effective and the earth loop impedance low enough for the protective devices to operate within the prescribed time in the event of an earth fault. Regulations 542-01-08 and 542-02-03 deal with electrolysis and corrosion. The electrolysis in Regulation 542-01-08 is probably that which occurs in d.c. supply systems where earth leakage can cause corrosion from electrolysis at the positions where the current enters or leaves metallic parts. This problem occurs in d.c. traction systems and certain types of electrochemical plants, and is often associated with cathodic protection systems which impress d.c. currents in the ground. 

Cathodic Protection Systems—Generally, an engineering study is required to locate and size bonding when cathodic protection systems are employed to protect a facility against corrosion. For example, the option of deenergizing an impressed current system does not immediately remove the... 

When an impressed current cathodic protection (ICCP) system is in full operation there is a high possibility for oxygen to be produced at the anode, and in nearly all cells, hydrogen is formed at the cathode. If chloride ions are present, chlorine gas may be formed at the anode. This generation of gas, either oxygen or chlorine, at the anode is not nearly as likely to occur in a natural corroding cell as it is when an ICCP system is used, particularly when inert anodes are used. 

In the galvanic or impressed current system, the metallic structure is made the cathode (negative) by connecting it to galvanic anodes, which are more negative than the metallic structure to be protected. In this system, the current is generated by the corrosion of active metals, such as magnesium, zinc and also aluminum, which are... 

Impressed current system A cathodic protection system which receives the required current for protection from a transformer-rectifier. 

Finally, in an impressed current system (used in cathodic protection systems), the magnitude of the current density is specified. 

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. 

In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus falsify the potential measurement. In such cases the anodes should be separated. 

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. 

On the other hand, the costs for an average cathodic protection station for 6 A come to = 40,000 DM according to Table 22-2. For very small installations as, for example, the external cathodic protection of a tank, the costs of an impressed current system where a current supply is already available without cost, with lower current output, can be reduced to about 4000 DM. With larger tanks and greater soil resistivity, the following considerations point to the increased suitability of an impressed current system. 

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