Cathodic protection and anodes

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

Box 7.3 Undersea steel structures sacrificial anodes and cathodic protection... 

Corrosion control methods consist of protective coatings, corrosion-resistant metals and alloys, corrosion inhibitors, polymers, anodic and cathode protection, corrosion control services, corrosion research and development, and education and training. The total annual cost of corrosion estimated with this method for the average year of 1998 was 121.41 billion or 1.381% of the 8.79 trillion gross domestic product. Table 4.1 shows the distribution of corrosion control methods and services eosts. 

Figure 9.13 Complete log Ivs E characteristic for a metal to show the potential zones for anodic and cathodic protection.

In this section we briefly summarize the more traditional methods for controlling corrosion, including the use of barrier coatings, the use of inorganic and organic inhibitors, and the use of anodic and cathodic protection. More complete discussions of these methods can be found in Refs. [16,17]. 

Cathodic Protection. Schroeder and Berk [4] found that cathodic polarization of steel stressed in hot sodium hydroxide-sodium silicate solution greatly delayed or prevented cracking. Parkins [17] found similar protection in hot nitrate solution. Bohnenkamp [18] reported maximum susceptibility of various carbon steels (0.003-0.11% C) in 33% NaOH boiling at 120°C at -0.66 to -0.75 V (S.H.E.) with orders-of-magnitude longer life at potentials at 0.1 V either noble or active to this range. Both anodic and cathodic protection were found to be effective (see Fig. 8.7, Section 8.3.3). 

Intentional Anodes and Cathodic Protection. If a bond from if to C in Fig. 12.1 is not feasible, an intentional anode of scrap iron may be buried in the direction of the rails and attached by a copper conductor to point B. Stray currents then cause corrosion only of the intentional anode, which is easily replaced at low cost. If a source of dc current is inserted between the intentional anode and the pipe such that current flows in the soil in a direction opposite to that of the stray current, the arrangement is equivalent to cathodically protecting the pipe. Cathodic protection is installed whenever the intentional anode is not sufficient to overcome all corrosion caused by stray currents. 

See, for example, U.K. Mudali, H.S. Khatak, and B. Raj, Anodic and cathodic protection. Chapter 5, Section 5.1 in Encyclopedia of Electrochemistry Volume 4, Corrosion and Oxide Films (A. Bard, M. Stattman, and G. Frankel, eds.), Wiley-VCH, Weinheun, 2003. 

Activation polarization is usually the controlling factor during corrosion in strong acids since both and iR are relatively small. Concentration polarization usually predominates when the concentration of the active species is low for example, in dilute acids or in aerated waters where the active component, dissolved oxygen, is only present at very low levels. The ohmic drop will become an extremely important factor when studying corrosion phenomena for which there is a clear separation of the anodic and cathodic corrosion sites, for example, crevice corrosion. The ohmic drop is also an important variable in the application of protective methods such as anodic and cathodic protection that forces a potential shift of the protected structure by passing a current in the environment. 

The ohmic overpotential appears in Eq. (5.2) as the simple product of a resistance and a current between the anodic and cathodic sites of a corrosion process. For many corrosion situations these sites are adjacent to each other and the ohmic drop is negligible, particularly so when the environment itself is a good electrolytic conductor, that is, seawater. However, there are special conditions where the separation of the anodic and cathodic sites can be an important factor in the corrosion progress, for example, galvanic corrosion, or even an integral part of a particular protection scheme, for example, anodic and cathodic protection. 

Advises on substitutions, clad metals, weld overlays, metallizing, preservation systems, anodic and cathodic protection, environmental adjustment, etc. 

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