Cathodic protection basis

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

For structures in brackish water, harbor water and fresh water, the conditions in each case should be considered and addressed on the basis of experience gained from other installations. Since harbor installations are usually very accessible, the cathodic protection installation can be extended if necessary. 

After measuring the zero profile, AU measurements are carried out with the injection of a cathodic protection current. In contrast to the zero profile measurements, the distance between the individual measurements is 25 to 50 m. Shorter distances between the measuring points are used only at depths where there are unusual AU profiles. Current should be injected at at least three different levels. The protection current density of about 12 mA m obtained from experience should be the basis for determining the maximum required protection current. As shown by the results in Fig. 18-3, the AU profiles are greater with increasing protection current. The action of local cells is suppressed when the AU values no longer decrease in the direction of the well head. This is the case in Fig. 18-3 with a protection current I = 4A. 

As an example. Fig. 20-7 shows potential and protection currents of two parallel-connected 750-liter tanks as a function of service life. The protection equipment consists of a potential-controlled protection current rectifier, a 0.4-m long impressed current anode built into the manhole cover, and an Ag-AgCl electrode built into the same manhole [10,11]. A second reference electrode serves to control the tank potential this is attached separately to the opposite wall of the tank. During the whole of the control period, cathodic protection is ensured on the basis of the potential measurement. The sharp decrease in protection current in the first few months is due to the formation of calcareous deposits. 

This handbook deals mainly with the practice of cathodic protection, but the discussion includes fundamentals and related fields as far as these are necessary for a complete review of the subject. We thought it appropriate to include a historical introduction in order to explain the technological development of corrosion protection. The second chapter explains the theoretical basis of metal corrosion and corrosion protection. We have deliberately given practical examples of combinations of various materials and media in order to exemplify the numerous fields of application of electrochemical protection. 

By contrast, if additional electrons were introduced at the metal surface, the cathodic reaction would speed up (to consume the electrons) and the anodic reaction would be inhibited metal dissolution would be slowed down. This is the basis of cathodic protection. 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

Claims are sometimes made that the use of cathodic protection devices eliminates the need for any type of water treatment chemical, including oxygen scavengers (on the basis that oxygen in the FW increases the rate of zinc anode corrosion, producing both zinc ions and hydroxide ions and resulting in the removal of 02 from the BW electrolyte). Such claims that corrosion protection devices provide a complete program are spurious. 

The more negative the potential, the greater the cathodic reaction and the smaller the anodic reaction the metal is more cathodic, which is the basis of cathodic protection of metals. By applying more positive potentials the system moves into the passive region where the corrosion rate may be reduced. This is particularly the case for some steels in particular environments and other metals, which forms the basis of anodic protection of metals. Thus, it is seen that changing the potential of a system in an environment which cannot be altered leads to effective corrosion control by cathodic or anodic protection as the case may be. 

On the other hand, anything that makes iron behave more like the cathode prevents corrosion. In cathodic protection, iron makes contact with a more active metal (stronger reducing agent), such as zinc. The iron becomes cathodic and remains intact, while the zinc acts as the anode and loses electrons (Figure 21.2IB). Coating steel with a sacrificial layer of zinc is the basis of the... 

These chemical modifications are at the basis of chloride removal and concrete realkalization, but are also a decisive factor for the long-term operation of cathodic protection and prevention. 

Electrochemical Basis for Cathodic Protection Criteria The corrosion rate of a steel structure tends to zero when it is polarized to the equilibrium potential because the rate of forward and reverse reactions becomes equal at this potential. For a neutral electrolyte, the calculated potential for the reaction of Fe is —0.59 V (versus saturated hydrogen electrode, SHE), which corresponds to —0.90 V (versus Cu-saturated CUSO4 electrode), not much varied from —0.85 V... 

The Uhlig methodology was used to determine the corrosion costs on the basis of the cost of corrosion protection products and services such as coatings, inhibitors, corrosion-resistant materials, and cathode protection. The total cost amounted to 2.5 trillion yen (US 9.2 billion). Paint and protective coatings accounted for nearly US 6.1 billion. Surface treatments and corrosion-resistant materials accounted for nearly two-thirds of the corrosion costs. Surface treatments and corrosion-resistant materials amounted to nearly one quarter and one-tenth of the costs, respectively. The remaining 5% of the cost was assigned to other corrosion control methods (Table 2.4). 

The Evans diagram is also very useful in estimating the current required in the external circuit to stop the process of corrosion. If an external current is appHed cathodicaUy (negative current), the potential on the cathodic polarization line crosses the equihbrium potential of the anode and the anodic reaction is not thermodynamically feasible. Thus, the corrosion process stops. This process is the basis of cathodic protection and is discussed in Chapter 15. 

Combined methods are also used to monitor cathodic protection efficiency. In the intensive measurement described by Wessling et at. [68], both CIPS and DCVG are used. The worker walks the pipeline route and records the distance and the switch on and switch off potential changes vs. the portable reference electrode at small intervals. The measurements are supplemented with ON/OFF potential gradients in one or two directions perpendicular to the pipeline. The measured (real) values serve as the basis for calculating the insulation defects (% IR). Other techniques such as the intensive holiday detection method can be used to detect the coating defects in the pipelines. Implementation of the above methods has led to a considerable decrease in the number of breakdowns of underground pipelines. 

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]. 

On the basis of the electrochemical nature of corrosion, it is clear that metallic zinc must be in direct contact with the electrolyte to protect bare steel regions. Paint or other coatings on top of the zinc coating will therefore eliminate or at best reduce the cathodic protection effect. 

By measuring the potential of the protected structure, the degree of protection, including overprotection, can be determined. The basis for this determination is the fundamental concept that cathodic protection is complete when the protected structure is polarized to the open-circuit anodic potential of the local action cells [10]. 

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

We can define corrosion monitoring as collecting corrosion related data on a regular basis. In this chapter we will exclude strain gauge monitoring which is adequately covered elsewhere in the literature, and cathodic protection monitoring which is discussed in Section 7.3.7. 

Observing the polarization diagram for the copper-zinc cell in Fig. 5.2, it is clear that, if polarization of the cathode is continued, using external current, beyond the corrosion potential to the thermodynamic potential of the anode, both electrodes attain the same potential and no corrosion of zinc can occur. This is the basis for cathodic protection of metals. Cathodic protection, discussed further in Chapter 13, is one of the most effective engineering means for reducing the corrosion rate to zero. Cathodic protection is accomplished by supplying an external current to the corroding metal that is to be protected, as shown in Fig. 5.14. Current leaves the auxiliary anode (composed of any metallic or nonmetaUic... 

If a sufficiently negative potential is applied to a metal its corrosion rate becomes negligibly small or zero. This forms the basis of cathodic protection, a method that is widely used to protect heavy steel structures such as drilling platforms, ships, pipelines and bridges against corrosion. 

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