Design of Cathodic Protection

Table 10-3 Form for design of cathodic protection installation...

In all cases partial or total hulls of aluminum or stainless steel must be provided with cathodic protection. This also applies to high-alloy steels with over 20% chromium and 3% molybdenum since they are prone to crevice corrosion underneath the coatings. The design of cathodic protection must involve the particular conditions and is not gone into further here. 

Table 10.25 Steps in design of cathodic-protection installation Sacrificial and impressed-curreni anodes...

Design of cathodic protection for marine structures in both fresh and salt water require special techniques. Galvanic systems usually employ zinc or aluminum alloy anodes. Impressed current systems frequently use high silicon, chromium bearing iron, platinized niobium, or mixed-metal oxide/titanium anodes. The structure being protected affects the design. Stationary facihties such as bulkheads and support piles require different techniques from ship hulls [55]. 

Computer-Aided Design of Cathodic Protection Exercises... 

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]. 

The described property of spherical electrodes permits to use equation (12.41) for the design of cathodic protection systems for structures buried in soil. In this case, the anode is usually much smaller than the structure it protects and often has an approximately spherical shape. Instead of having to calculate the ohmic resistance of the entire system, a more complicated task, it may be enough to estimate the ohmic resistance in the vicinity of the anode by using equation (12.49). A number of expressions for the ohmic resistance of buried anodes of other geometries are available in the literature [34]. 

To provide cathodic protection, the entire surface must remain within a specified potential range of protection. Furthermore, the current density must exceed the protection current density at each point, but it should not become too high in areas close to an anode because this could lead to excessive hydrogen formation, which could damage the coating or create a risk of hydrogen embrittlement (Chapter 11). For the design of cathodic protection systems it is therefore important to consider the various factors that influence the current and potential distributions at an electrode. 

The basic principle in force during the designing of cathodic protection is the necessity of obtaining a uniform protective current distribution, and at the same time a potential distribution on the surface of the protected structure (Morgan, 1987 Nisancio-glu, 1992 Baeckmann et al., 1997). The following relationship is valid for impressed current cathodic protection... 

Solutions of Eqs. (8-38) and (8-39) take the form of well known attenuation equations (Sec. 8.2.2). Current changes, as well as voltage changes in the case of a linear object take an exponential form. The character of changes of current and potential as a function of distance depend mainly on the attenuation constant a given by Eq. (8-10). The derived relationships are of fundamental importance in the design of cathodic protection of linear objects. 

In the presented approach, designing of cathodic protection installations is reduced to analysis of potential change profiles obtained for various initial conditions. The... 

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. 

Detailed design of cathodic protection systems is a highly specialized field of expertise and should be left primarily to a corrosion specialist. However, it will be the designer s task to accommodate, eventually, the diagrammatic detailed design rendered by the corrosion specialist in the functional design of the utility to their mutual satisfaction. 

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