Anodic protection system design applications

Although the first industrial application of anodic protection was as recent as 1954, it is now widely used, particularly in the USA and USSR. This has been made possible by the recent development of equipment capable of the control of precise potentials at high current outputs. It has been applied to protect mild-steel vessels containing sulphuric acid as large as 49 m in diameter and 15 m high, and commercial equipment is available for use with tanks of capacities from 38 000 to 7 600000 litre . A properly designed anodic-protection system has been shown to be both effective and economically viable, but care must be taken to avoid power failure or the formation of local active-passive cells which lead to the breakdown of passivity and intense corrosion. 

Because these variables have a very pronounced effect on the current density required to produce and also maintain passivity, it is necessary to know the exact operating conditions of the electrolyte before designing a system of anodic protection. In the paper and pulp industry a current of 4(KX) A was required for 3 min to passivate the steel surfaces after passivation with thiosulphates etc. in the black liquor the current was reduced to 2 7(X) A for 12 min and then only 600 A was necessary for the remainder of the process . From an economic aspect, it is normal, in the first instance, to consider anodically protecting a cheap metal or alloy, such as mild steel. If this is not satisfactory, the alloying of mild steel with a small percentage of a more passive metal, such as chromium, molybdenum or nickel, may decrease both the critical and passivation current densities to a sufficiently low value. It is fortunate that the effect of these alloying additions can be determined by laboratory experiments before application on an industrial scale is undertaken. 

See the NACE Papers Oliver W. Siebert, Correlation of Laboratory Electrochemical Investigations with Field Applications of Anodic Protection, Materials Performance, vol. 20, no. 2, pp. 38-43, February 1981 Anodic Protection, Materials Performance, vol. 28, no. 11, p. 28, November 1989, adapted by NACE from Corrosion Basics— An Introduction. (Houston, Tex. NACE, 1984, pp. 105-107) J. Ian Munro and Winston W. Shim, Anodic Protection— Its Operation and Appheations, vol. 41, no. 5, pp. 22-24, May 2001 and a two-part series, J. Ian Munro, Anodic Protection of White and Green Kraft Liquor Tankage, Part I, Electrochemistry of Kraft Liquors, and Part 11, Anodic Protection Design and System Operation, Materials Performance, vol. 42, no. 2, pp. 22-26, February 2002, and vol. 42, no. 3, pp. 24-28, March 2002. 

Cathodic protection has many applications, e.g. in refineries, power stations, gas, water, and oil utilities on marine structures, e.g. jetties, piers, locks, offshore platforms, pipelines, ships hulls, etc. and on land structures, e.g. buried pipeline, storage tanks, cables, etc. For each use, the cathodic protection system requires careful design, either impressed current, sacrificial anodes, or a combination of both may be chosen. There may also be other protection systems, e.g. paint, the nature of which will affect the design parameters and must be taken into consideration. 

The oil and gas industries have probably been responsible for the greatest seawater applications of sacrificial anodes in seawater applications. New technologies had to be developed to support the exploitation of deep sea resources which had expanded at a great pace since the mid 1970s. Corrosion protection of the expensive and intricate structures had to be based on CP systems, for which the available scientific data were sparse. Designs were often based more on inspired guesswork than on the application of science, particularly for impressed current systems. Designers preferred to use copious quantities of inexpensive zinc anodes in the belief that overprotection was safer than the risk of underprotection [8]. 

The designing of cathodic protection systems is rather complex, however, it is based on simple electrochemical principles described earlier in Chapter 2. Corrosion current flows between the local action anodes and cathodes due to the existence of a potential difference between the two (Fig. 5.1). As shown in Fig. 5.2, electrons released in an anodic reaction are consumed in the cathodic reaction. If we supply additional electrons to a metallic structure, more electrons would be available for a cathodic reaction which would cause the rate of cathodic reaction to increase and that of anodic reaction to decrease, which would eventually minimize or eliminate corrosion. This is basically the objective of cathodic protection. The additional electrons are supplied by direct electric current. On application of direct current, the potential of the cathode shifts to the potential of the anodic area. If sufficient direct current is applied, the potential difference between the anode and cathode is eliminated and corrosion would eventually cease to occur. 

The second system is based on the application of impressed current that is forced through anodes to the protected structure such as the tank by a current source of sufficient potential. Properly designed CP systems that are well maintained and operate at the correct current density are a proven method of protecting tanks from the corrosive effects of contact with corrosive soils. In addition to protection of underground tanks, CP is also useful for aboveground double-bottom tanks and for internal corrosion protection. 

For the application of cathodic protection to structures to be protected, the initial considerations are best made at the early design and preconstruction phase of the structure. For underground structures, it may be necessary to visit the proposed site, or for pipelines the proposed route, to obtain additional information on low-resistivity areas, availability of electric power, and the existence of stray dc current or other possible interactions. Other considerations will include fundamental design decisions to select the type of system and the most suitable type of anode appropriate to that system. In addition, it will be a requirement to determine the size and number of the power sources or sacrificial anodes and their distribution on the structure. Other factors that must be considered to ensure that cathodic protection is applied in the most economic and reliable manner are given as follows. 

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