Cathodic protection current demand

Initial effective electrical resistance of tapes, as evidenced by the cathodic protection current demand, has been outstanding. There have been reports of increasing current demand with time which indicate a need for investigation. The current demand increase has been found, on occasion, to be due to poor construction practice, but not all tapes are affected in this way. 

T. Foster, V.G. Moores, Cathodic Protection Current Demand of Various Alloys in Sea Water, Paper No. 254, CORROSION/86, March 17-21, 1986, Houston, TX. 

Figure 7.15 Cathodic protection current demand vs. chioride content at rebar depth (based on Bennett and Turk, i994).

It was indicated earlier that the cathodic current was a poor indicator of adequate protection. Whilst, to a first approximation the protection potential is a function of the metal, the current required for protection is a function of the environment and, more particularly, of the cathodic kinetics it entails. From Fig. 10.4 it is apparent that any circumstance that causes the cathodic kinetics to increase will cause both the corrosion rate and the current required for full (/") or partial (1/ — /, ) protection to rise. For example, an increase in the limiting current in Fig. 10.5 produced by an increase in environmental oxygen concentration or in fluid flow rate will increase the corrosion rate and the cathodic protection current. Similarly, if the environment is made more acid the hydrogen evolution reaction is more likely to be involved in the corrosion reaction and it also becomes easier and faster this too produces an increased corrosion rate and cathodic current demand. 

Insulation used on underground steel structures causes a significant decrease in the demand for a cathodic protection current. Structures with no insulation usually require a current of approx. 25 mA m density, structures with bituminous coatings 0.05-0.25 mA m , and structures with polyethylene coatings (1.8-3.0 mm) only approx. 0.01 mA m . ... 

In short, the current demand for cathodic protection varies according to the aggressiveness of the corrosive environment. It is for this reason that cathodic protection finds its greatest application where the pH is close to neutral. The more acid environments entail a current output that rapidly becomes uneconomic. The more alkaline environments prove less aggressive to the structure and therefore often do not justify cathodic protection. Table 10.5 provides some estimated current densities for cathodic protection that illustrate the point. 

Cathodic Current Densities for Protecting Steel Examples of current density requirements for the protection of steel (to achieve a steel potential of —0-8 V vs. Ag/AgCl/seawater) are given in Tables 10.13 and 10.14. It should be realised that the current demand of a structure will be influenced by, inter alia, temperature, degree of aeration, flow rate, protective scales, burial status, presence of bacteria and salinity. 

Obviously, the total weight of the anode material must equal or be greater than the total weight, IF, calculated above. Similarly each anode must be of sufficient size to supply current for the design life of the cathodic protection system. The anodes must also deliver sufficient current to meet the requirements of the structure at the beginning and end of the system life. That is, if current demand increases (as a result of coating breakdown, for example) the output from the anodes should meet the current demands of the structure. 

Cathodic protection and associated instruments have developed in-line with the changing monitoring demands of both the onshore and offshore industries. In particular, for potential and current density measurements, far greater quantities of data are sought and are required to be processed into an easily assimilated form. Thus cathodic protection instrumentation has benefited from an increased association with microprocessor-based data handling and storage systems. 

The criteria for cathodic protection are not free from criticism. It is beheved that all the listed criteria are deficient to some extent and therefore qualitative in practical appKcation. However, one should be optimistic that any level of cathodic polarization is beneficial, and a broad range of ca-thodically applied potentials will yield adequate protection. As a result, the use of any criterion listed in Table 4 [24] will produce adequate cathodic protection if applied judiciously. The amount of cathodic protection should be sufficient to reduce the corrosion rate to an acceptable range. Caution should be exercised to avoid overprotection. Overprotection results in the premature consumption of sacrificial anodes or excessive amounts of impressed current demands. Moreover, the application of too much cathodic protection can result in damage to the structure to be protected as a result of hydrogen embrittlement. 

Knudsen 00, Steinsmo U. Effect of cafliodic disbonding and blistering on current demand for cathodic protection of coated steel. Corrosion,. 56,. 3, 2000 ... 

In principle, cathodic protection can be applied to a bare metal however, the external current demand for snch protection is usually very large and uneconomical. Protection of metals from corrosion may also be achieved by coating the surface. The coating thus applied to the metal surface shields it from the corrosive environment. However, flaws are inherent in coatings, which increase both in size and number with the service life of the coated structure. 

Cathodic protection is usually used in conjunction with coatings. In such instances, the current demand for protection is low. While the coating affords the majority of the protection, current from the cathodic protection system protects flawed and damaged regions of the coatings where the bare metal is exposed to the corrosive environment. 

Examples of current densities for protection of bare steel abound in the literature. However, it is absolutely true that it will not be economically sound to use cathodic protection for steel in hot H2SO4. The current density demand is too high. 

The possibilities of cathodic protection are limited in two respects. The cathodic polarization required for protection can lead to cathode corrosion in some systems. This kind of protective coating is often combined with cathodic protection in order to keep the current demand low. The attack of acidic solutions on equipment parts cannot therefore be prevented in general by a cathodic protection system. Cathodic polarization without evolution of large amounts of hydrogen is at best a possibility with copper alloys in acidic solutions. 

The required output of the transformer/rectifier and of the anode can be estimated from Figure 7.15 taken from Bennett and Turk (1994). Although the work in the report was done to try to develop a simple constant current criterion for cathodic protection, it has never been validated outside the laboratory. It should also be remembered that the areas of highest corrosion and highest chloride will have been repaired so the current demand will be reduced in proportion to the area repaired. 

This Is another vital part of an impressed current system. The T/R must be rugged and reliable with minimal maintenance requirements. It should be easy to maintain with good instruction manuals, circuit diagrams for maintenance and easy access to fuses and other replaceable components. Compared with pipeline or marine cathodic protection applications (steel piles, etc.) the power demand is modest. Steel in concrete needs less than 20 mA m - to provide protection, usually at less than 10 V, The power for a 100 W light bulb will protect 5000 m, This means that a single-phase, air cooled T/R vill usually protect even the largest structure and power consumption is rarely an economic concern. 

---