Anodic protection system design

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. 

Corrosion monitoring must be coupled with diagnostic work and remedial action. In some cases of corrosion, remedial action may be obvious or easily deduced. In other cases, diagnostic work precedes a decision or remedial action. The options for remedying corrosion problems in a process plant are (i) install a CP system (ii) install an anodic protection system (iii) change equipment design (iv) improve feed stock purity (v) alter process variables (vi) change the alloy/material (vii) institute inhibitor additions (viii) institute planned maintenance. 

The following are the requirements for designing anodic protection systems ... 

J. Jenkins, Cathodic protection system design 3. Sacrificial anode system design principles for underground structures. Report No. NFESC-TDS-2022-SHR, Order No. AD-A301 912/2GAR, 1995, Naval Facilities Engineering Service Center, Port Hueneme, CA, USA. 

Chapter 4 presents the fundamentals of passivity the film and adsorption theories of passivity criterion for passivation methods for spontaneous passivation factors affecting passivation, such as the effect of solution velocity and acid concentration alloy evaluation anodic protection systems and design requirements. A fuU discussion on stainless steel composition and crystalline structure, oxidizer concentration, and alloy evaluation is included. The chapter also considers anodic protection to establish a basis for anodic... 

As already mentioned, no general principles concerning the design of anodic protection systems have been prepared up to now. Such problems as the number and location of cathodes and reference electrodes are solved experimentally. However, on the basis of information obtained for many industrial objects with anodic protection, some principles may be established concerning the method of polarization of the structure (Foroulis, 1980). 

The dc power supplies used in anodic protection systems have similar design and requirements as the rectifiers for cathodic protection, with one exception. Because of the nature of the active-passive behavior of the vessel, the currents required to maintain the potential of the vessel wall in the passive range can become very small. Some designs of dc power supplies must be specially modified to reduce the minimum amount of current put out of the power supply ... 

Designing an anodic protection system requires knowledge of the basic electrochemical behavior of the system and of the geometry of the... 

The required number, n, of anodes can be calculated using Eq. (17-2) from the current requirement, together with the maximum current output 1 of the anodes. The arrangement of the anodes is dealt with in Section 17.3.2.2. Galvanic protection systems are usually designed to give protection for 2-4 years. After this period, a maximum of up to 80% of the anodes should be consumed. 

By contrast a cathodic protection system based on sacrificial anodes is designed from the outset to achieve the required protection potential. If this is not achieved in practice there is no control function that can be exercised to improve the situation. Some remodelling of the system will be required. Moreover, the currents from each current source (the sacrificial anodes) is modest so that field gradients in the environment are not significant. It is at once clear that potential measurements are less significant in this case and instant-off measurements are neither necessary nor possible. 

Before a satisfactory cathodic protection system using sacrificial anodes can be designed, the following information has to be available or decided upon ... 

System Life Cathodic protection systems may be designed with a life of between 1 and 40 years. The greater the time of protection, the greater the mass of anode material that is required. 

Obviously, the total weight of the anode material must equal or be greater than the total weight, IF, calculated above. Similarly each anode must be of sufficient size to supply current for the design life of the cathodic protection system. The anodes must also deliver sufficient current to meet the requirements of the structure at the beginning and end of the system life. That is, if current demand increases (as a result of coating breakdown, for example) the output from the anodes should meet the current demands of the structure. 

The latter part of this chapter has dealt with the design considerations for a sacrificial anode cathodic protection system. It has outlined the important parameters and how each contributes to the overall design. This is only an introduction and guide to the basic principles cathodic protection design using sacrificial anodes and should be viewed as such. In practice the design of these systems can be complex and can require experienced personnel. 

There are obviously situations which demand considerable over-design of a cathodic protection system, in particular where regular and efficient maintenance of anodes is not practical, or where temporary failure of the system could cause costly damage to plant or product. Furthermore, contamination of potable waters by chromium-containing or lead-based alloy anodes must lead to the choice of the more expensive, but more inert, precious metal-coated anodes. The choice of material is then not unusual in being one of economics coupled with practicability. 

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. 

Installing direct current (DC) electrical-based remediation systems in urban areas also requires containment of stray voltage and current. DC systems can cause corrosion of buried gas and water lines or wreak havoc on cathodic protection systems. A good design can minimize the impacts, but sometimes, extra sacrificial anodes need to be installed to contain the electric field, adding to the cost of installation. 

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