Cathodic and Anodic Protection

TABLE 4.12 Fraction of Polymers Used for Corrosion Control in 1997  

All the polymers that contain fluorine side groups in the molecular structure have excellent high-temperature stability and chemical resistance. Polytetrafluoroethy- 

In summary. Table 4.12 shows the fraction of polymers used for corrosion control in 1997. 

The cost of cathodic protection (CP) and anodic protection of metallic structures prone to corrosion can be divided into the cost of materials and the costs of installation and operation. Industry data have provided estimates for 1998 sales of various hardware components amounting to a total of 146 million as shown in Table 4.13. 

The largest share of CP market is taken up by sacrificial anodes at 60 million of which magnesium has the greatest market share. The costs of installation of a CP system vary to a significant extent depending on the location and specific details of the construction. The range of cost for labor, materials, and the number of installations for various systems in 1998 are given in Table 4.14. 

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In this section a survey is given of the critical protection potentials as well as the critical potential ranges for a possible application of electrochemical protection. The compilation is divided into four groups for both cathodic and anodic protection with and without a limitation of the protection range to more negative or more positive potentials respectively. 

Potential measurements concerning cathodic and anodic protection are excluded here. 

Two main approaches to this important subject exist. If the object (as with a ship s hull) is in contact with an unlimited amount of aqueous solution, addition of a chemical to the solution is not feasible. For this kind of situation there are two electrochemical approaches cathodic and anodic protection. However, often (as with oil pipelines) if the corroding liquid (e.g., sea water) is at least partially confined,4 then there is great value in developing organic molecules that adsorb on the metal and reduce the velocity of anodic dissolution. 

Ref. [i] Juchniewicz R, Jankowski J, Darowicki K (2000) Cathodic and anodic protection. In Schutze M (ed) Corrosion and environmental degradation, vol. 1. Wiley-VCH, Weinheim, pp 383... 

Both cathodic and anodic protection methods involve modification of a metal s potential. In these methods, the potential of the metal to be protected is shifted, either by the application of a direct current from a power supply or by galvanic action from the connection of dissimilar metals. The potential can be decreased or shifted into a region of passivity for the metal. Shifting the potential of the metal to a lower value is referred to as cathodic protection (CP). Shifting the metal to more oxidi2ing conditions or more positive potentials within a region of passivity is referred to as anodic protection. 

C. E. Locke, Corrosion cathodic and anodic protection. Encyclopedia of Chemical Processing and Design, Marcel Dekker, New York, 1981, pp. 13-59, Vol. 12. 

TABLE 4.13 Total Cost of Components for Cathodic and Anodic Protection... 

R. Juchniewicz, J. Jankowski, K. Darowicki, Cathodic and anodic protection, in M. Schutzee (Ed.), Mater. Sci. Tech., 19, 1 (Corros. Environment. Degradation) Wdey-VCH, Weinheim, Germany, (2000), pp. 383-470. 

Corrosion current density is a particularly suitable measure of corrosion rate when treating corrosion theory and in connection with electrochemical corrosion testing. Current density is also directly applicable for cathodic and anodic protection (Sections 10.4 and 10.5). In corrosion testing the unit pA/cm is most often used. When dealing with cathodic protection the units mA/m and A/m are used for the cathode (sfructure to be protected) and the anode, respectively. 

Figure 15.4 illustrates the origin of the corrosion potential and also the principles of cathodic and anodic protection for a single oxidation reaction (M M ) and a single reduction reaction (H H2) occurring at the metal surface (the dashed lines represent the current-potential behavior of the reverse reactions and are not important to the present discussion). Because charge balance must be maintained, the potential is pinned at a value, Ecom where the cathodic current and the anodic current are equal (i.e., where the two curves intersect). This corrosion potential (Ecorr) is called a mixed potential, as it is determined by a mixture of two (sometimes more) electrochemical reactions. The anodic current (also the cathodic current, as they are equal) at this potential is the corrosion current (torr)- It is important to note that E orr and icorr are influenced by both the thermodynamics of the two reactions, manifested by the equilibrium potentials E(h+/h2) and E(m/m+)> and by the kinetics of the two reactions, manifested by the exchange current densities io(H+/h2) and o(m/m+)> and by the slopes of the two linear curves (the Tafel slopes). 

Corrosion test methods can be divided into electrochemical and non-electrochemical methods. Among the electrochemical techniques that have been used successfully for corrosion prediction are potentiodynamic polarization scans, electrochemical impedance, corrosion current monitoring, controlled potential tests for cathodic and anodic protection, and the rotating cylinder electrode for studies of velocity effects [3i,32]. Though not literally a test, potential-pH (Pourbaix) diagrams have been used as road maps to help understand the results of other tests. 

Fig, 10,15 Schematic tog f - diagrams illustrating the principle of cathodic and anodic protection for a metal e c hi biting passivity. The cathodic process (C) may be reduction of HjO or O], The zones of electrode potential for protection are indicated. 

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