Cathodic protection definition

These terms are derived from various sources, principally from the List of Definitions of Essential Telecommunications Terms (Part I, Corrosion), International Telecommunications Union, Geneva, 1957, 1st Supplement, I960, and the B.S.I. Code of Practice for Cathodic Protection, CP102I 1973 terms taken from the Code of Practice are marked with an asterisk. 

The ohmic drop exerts a sensible influence on the evaluation of the electrochemical parameters as well as on the definition of the reaction scheme that is most suitable for describing the behaviour of a metal in a given environment. It also determines the success of many operations, such as cathodic protection by means of sacrificial anodes or impressed current and corrosion rate monitoring. 

Definition Cathodic protection (CP) is defined as the reduction or elimination of corrosion by making the metal a cathode using an impressed current or attachment to a sacrificial (galvanic) anode. It is a process that reduces the anodic corrosion reaction by creating an electric field at the surface of the metal so that the net flow of current is into the metal. 

Figure 10.16 Schematic potential-distance diagram for cathodic protection with impressed current. Definition of Ea, E and AE as in Figure 10.15. Pj = electrical potential in the anode material, Pstmcture = electrical potential in the structure (the cathode material), Pj = the terminal voltage of the external current source.

Fig. 6.1), so that the current density and accompanying corrosion rate correspond to the low value of (passive- This process is called anodic protection (see Section 13.9) because the current flow is in the direction opposite to that which is used in cathodic protection. Whereas cathodic protection can, in principle, be applied to both passive and nonpassive metals, anodic protection is applicable only to metals that can be passivated when anodically polarized (see Definition 1, Section 6.1). 

One definition of effective cathodic protection is to depress the potential of the cathodes to the level of the anodes, thus stopping current from flowing between anodic and cathodic areas (Mears and Brown, 1938). This works because cathodes are more easily polarized (potential shifted) than anodes. We saw this phenomenon in Section 4.11 where the effect of an external current on the half cell potential allows us to calculate the corrosion rate. 

It was also stated that a theoretical definition of effective cathodic protection is to depress the potential of the mo.st cathodic areas below that of the mo.st anodic areas. However, as time passes and negatively charged chloride ions move away from the negatively charged steel, the most anodic areas will move. Variation. in the resistance of the concrete will mean that current flow from the anode is more likely to reach the anodic areas of steel and polarize them, rather then the passive, cathodic, areas (e.specially if they are new patch repairs with higher resistance concrete). 

It is difficult to give definite cost information as this varies from job to job and country to country. Summaries of costs for cathodic protection are given in Society for Cathodic Protection of Reinforced Concrete (1995) for the UK and in Bennett et sL (1993b) for the USA. These techniques are generally only specified if they offer a co,st saving to the owner of the structure over its lifetime (Unwin and Hall, 1993). 

This definition excludes inhibitor films or other corrosion-protecting layers such as paint and so on, which have the same function as passive films but a different origin. Such films are formed from components in the electrolyte (inhibitors) or are deposited in a technical process (e.g. cathodic deposition of paint). 

Galvanic corrosion and the factors affecting it have been discussed in Chapter 20. However, a few precautionary comments are in order for aluminum, since it is anodic to most common materials of construction, with the exception of magnesium and zinc. In the presence of a good electrolyte, as little as 15 mV difference in corrosion potential of the two metals can have an effect, and if the difference is 30 mV or greater the anodic material will definitely corrode sacrificially to protect the contacting cathodic metal. A recently revised report on galvanic corrosion, with emphasis on automotive applications is available [73]. 

It validates the necessaiy assumption in the definition of the geometiy of the electrode boundaries when a cathodic inhibition is assumed, simulated pH in solution above the coating and the inhibited area of steel is lower. Although SVET measurements give direct information on the surface reactivity and the cathodic inhibition on a part of steel, pH measurements need the numerical model to be interpreted in term of protective inhibition. 

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