Cathodic protection system design

In the following discussion we do not intend to show the reader how to design a cathodic protection system from scratch. We will show some of the major processes that a qualified and experienced corrosion expert will go through in designing a system, showing how decisions are arrived at concerning the components and the system performance. Design should always be undertaken by a suitably qualified and experienced expert. 

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 the structure is a wearing surface then coatings are usually excluded. This usually leads to the use of one of the titanium configurations in an overlay or titanium ribbons in slots. 

The main restrictions on the overlay type systems are because of the cementitious overlay  

Galvanic anodes are extensively used in continuously wetted environments. They need less maintenance than the impressed current systems. One problem with the zinc system is the environmental impact of the during spraying. If enclosure is required during spraying, the costs can be very high. 

---

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. 

Cathodic protection can be useful, although its ability to protect tube interiors is generally limited to the first 4 to 6 in. of tube length. Such systems, however, must be properly designed and maintained to be effective. Corrosion can be intensified if the polarity of the cathodic protection system is inadvertently reversed. 

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. 

Recommended Practice Design, Installation, Operation and Maintenance of Internal Cathodic Protection Systems in Oil Treating Vessels, RP-05-75, NACE, Houston (1975)... 

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. 

Although the principles of cathodic protection are essentially simple and were in fact first outlined by Sir Humphry Davy in 1824, the application of the method to practical problems remains more of an art than a science. A properly designed cathodic protection system will be both economical and effective. On the other hand an incorrectly designed scheme will be inefficient, uneconomical and under certain circumstances may accelerate corrosion instead of controlling it. 

Often it is necessary in designing a cathodic-protection system to know the conductivity of a protective coating (e.g. bitumen enamel) on a structure. This measurement is usually carried out by finding the resistance between an electrode of known area placed in contact with the coating and the structure itself. The electrode placed on the structure can be either of thin metal foil or, preferably, of material such as flannel soaked in weak acidic solution. The resistance between the pad and the metal is measured by means of either a resistivity meter, as previously described, or a battery with a voltmeter and an ammeter or microammeter. Generally speaking, in field work where such measurements have to be made, a resistivity meter is preferable. 

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. 

Care must be taken when designing cathodic protection systems in built-up areas as electrical interference from transmission lines or electrical rail systems may cause stray current corrosion at points in the structure close to the tramway or transmissions line [41]. 

R.A. Adey, J. Baynham. Design and Optimization of Cathodic Protection Systems using Computer Simulation. CORROSION 2000, Paper 723. Houston, Texas. NACE International, 2000. 

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... 

Current-distribution simulations are valuable for the design and analysis of electrochemical processes. For example, such simulations are ubiquitous in the battery and fuel-cell literature. They are used for electrochemical metallization processes not only in reactor design but also in wafer design. " A great deal of effort has also been put into the development of analog solvers for cathodic protection systems. ... 

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. 

Design Aspects of Sacrificial Cathodic Protection System... 

When selecting a cathodic protection system, the designer should consider the size of the structure and the past project experience in operating and maintaining both types of systems. 

Successful application of cathodic protection depends upon the selection, design, installation, and maintenance of the system. Before designing the cathodic protection systems, adequate field data must be collected, analyzed, and evaluated. Nature and conditions of the soil are reflected by field measurements like soil resistance, hydrogen ion activity (pH), and the redox potential. To understand the nature of the pipeline, potential measurements, coating resistance, and meaningful design current requirement tests must be conducted. 

The ideal design for a cathodic protection system is the one that wiU provide desired degree of protection at the minimum total annual cost over the projected Ufe of the structure. Design of cathodic protection systems for pipeUnes has been extensively studied, and several standards have evolved over the decades [56,57,83-96]. A detailed description has been illustrated in the work by Peabody [56]. The basic procedure involved in the design of cathodic protection is summarized below. 

K. Bethime, W.H. Hartt, Applicability of the slope parameter method to the design of cathodic protection systems for marine pipelines. Corrosion 57 (2001) 78—83. 

With intelligent eombinations of these simple models it is possible to make approximate estimations for other geometries. Typieal of the three models dealt with above is that both the anode and the cathode potential are constant over the respective electrode surfaces. Analytical solutions exist also for some other geometries. On the basis of such solutions (and partly on an empirical basis) several formulae for the resistance of the electrolyte volume near the anode - the anode resistance Ra - have been developed. Such formulae for various anode geometries are used in design of cathodic protection systems [10.25-10.34]. 

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. 

DESIGN OF A CATHODIC PROTECTION SYSTEM 19.4.1 Design Basis... 

---