Impressed current cathodic

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... 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

Fig. 8-1 Construction of an impressed current cathodic protection station.

According to Ref. 32, the functioning of impressed current cathodic protection stations should be monitored every 2 months, and the stray current protection station every 1 month. If protection installations are provided with measuring instruments for current and potential, this supervision can be carried out by operating staff, so that the readings are recorded and sent to the technical department for... 

In addition, the reactions occurring at the impressed current cathode should be heeded. As an example. Fig. 21-7 shows the electrochemical behavior of a stainless steel in flowing 98% H2SO4 at various temperatures. The passivating current density and the protection current requirement increase with increased temperature, while the passive range narrows. Preliminary assessments for a potential-controlled installation can be deduced from such curves. 

Materials that are corrosion resistant to the expected cathodic polarization qualify as impressed current cathodes. Austenitic CrNi steels are used with strong acids. The oleum (i.e., fuming sulfuric acid) and concentrated sulfuric acid tanks used in sulfonating alkanes and in the neutralization of sulfonic acids are anodi-cally protected using platinized brass as cathodes [15]. Lead cathodes are used to protect titanium heat exchangers in rayon spinning baths [16]. 

In spite of the possibility of cathodic corrosion discussed in relation to Eq. (2-56), practical experience has shown that carbon steel is a suitable material for impressed current cathodes. Stress corrosion of the cathode material does not have to be considered because of the strong cathodic polarization as shown in Fig. 2-18. 

The use of high-silicon irons as anodes for impressed-current cathodic-protection systems is described in Sections 10.3 and 10.4. 

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

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... 

In an impressed-current cathodic protection system the power source has a substantial capacity to deliver current and it is possible to change the state of polarisation of the structure by altering that current. Thus effective control of the system depends on credible potential measurements. Since the current output from any given anode is substantial, the possibility of an IR error which may reach many hundreds of millivolts in any potential measurements made is high. Thus the instant-off technique (or some other means of avoiding IR error) is essential to effective system management. 

An impressed-current cathodic protection system circuit comprises an anode, the power source, the structure and the environment in which it... 

Soil resistivity surveys are often impractical in built-up areas, but in such areas impressed-current cathodic protection is usually avoided on account of the danger of interaction. Under such conditions adequate protection can be achieved by installing magnesium anodes in the pipe trench should the soil resistivity measurements made when the trench is opened indicate that this is necessary. 

Dietl, B. and Gleason, J. D., Compatibility of Impressed Current Cathodic Protection with Paint Systems , Mat. Prot. and Perf., 10 No. 11, 44, Nov. (1971)... 

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

Impressed Current cathodic current supplied by a d.c. source to a structure in order to lower the potential to the protective potential for cathodic protection. 

Applied research recently reported in the literature has been conducted mostly on marine iron wrecks, aiming at evaluating the efficiency of chloride removal from the artifacts from a marine site under cathodic polarization [301] as well as the use of impressed current-cathodic protection to conserve parts of the first ironclad warship [302]. 

This method of corrosion protection consists of (i) the sacrificial anode method and (ii) impressed current cathodic protection. The other related impressed current protection method is anodic protection. 

Figure 1.70 The principle of impressed current cathodic protection using a potentiostat. (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

A. Kumar, J.B. Bushman, J.H. Fitzgerald, A.E. Brown and T.M. Kelly, Impressed Current Cathodic Protection Systems Utilizing Ceramic Anodes, US Army Res. Labs, Champaign, Inc., 1990. 

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. 

A schematic of an impressed current cathodic system is depicted in Figure 7.26. By an impressed current the potential of the steel is shifted to greater than —850 mV, thus making the steel bar cathodic and prevent the corrosion. 

The best way to overcome the limitations of cathodic protection related to changes in the environment is to monitor the potential and adjust the cathodic currents accordingly. This cannot be readily done with sacrificial anodes, and the method of impressed-current cathodic protection is sometimes preferred, in spite of its higher cost. 

Impressed-current cathodic protection entails the use of an external power source in combination with a stable anode. The potential of the specimen being protected is forced to negative values with respect to its open-circuit corrosion potential, and its rate of anodic dissolution is consequently reduced. The result of impressing a cathodic current on the structure is shown in Fig. 21M for the parameters used to draw this figure we obtain i = 48.8 pA/cm and E = - 0.554 V, NHE. Applying a cathodic current density of 72... 

Fig. 21M Evans diagram for iron at pH = 6, showing the principle of impressed-current cathodic protection. The two horizontal lines show tw>o levels of cathodic protection. The impressed current is the difference between the cathodic and the anodic currents shown.

Coatings combined with impressed current cathodic protection (ICCP) are the most common means for the protection of shipboard. They interact with each other to protect shipboard. Coatings provide primary corrosion protection by isolating the hull metal from the seawater, while ICCP provides secondary corrosion protection in those areas where the paint is damaged or degraded. The protective effect is directly related to the ICCP configurations an incorrectly designed ICCP system would not only influence the protective effect but also... 

DeGiorgi V.G, Thomas E.D, Lucas K.E and Kee A. A combined Design Methodology for Impressed Current Cathodic Protection Systems. Boundary Element Technology XI, Computational Mechanics Pub. 1996 335-345... 

R. W. Ditchfield, J. N. McGrath and D. J. Tighe-Ford. Theoretical validation of the physical scale modelling of the electrical potential characteristics of marine impressed current cathodic protection. J. Applied Electrochemistry. 1995.25 54-60... 

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