Cathodic protection currents

In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus falsify the potential measurement. In such cases the anodes should be separated. 

The protection current produced by the usual full-wave rectifier has a 100-Hz alternating component of 48%. There are receivers with selective transmission filters for 100 Hz, which corresponds to the first harmonic of the cathodic protection currents [45]. With such a low-frequency test current, an inductive coupling with neighboring pipelines and cables is avoided, which leads to more exact defect location. 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

When measuring AU profiles, the voltage drop is measured along the interior of the casing [2]. The voltage drop is caused by cell currents as in Section 18.2.1 or by cathodic protection currents. The anodic and cathodic sections of the casing and also the effect of the cathodic protection can be determined from the AU profiles. 

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. 

Cathodes made of steel tube rings 10 m in diameter are used m the internal protection of several 30-m-high caustic soda stirring tanks, each with a surface area of 3100 m, The cathodes were insulated and mounted on supporting brackets. The current supply was via armored parallel connected cables. One-inch bolts were used for the entrance in the tank wall as shown in Fig. 21 -13 for a feed current of 500 A. The size of the cathodes has to be such that they can maintain sufficient cathodic protection current. The surface ratio of the object to be protected and the... 

Fig. 10.6 Polarisation diagram showing the limited role hydrogen evolution plays at the corrosion potential of steel in aerated neutral solution, the larger role in determining cathodic protection currents and the dominant role in contributing to current requirements at very negative potenitals. The dotted line shows the total cathodic current due to oxygen reduction and...

In practice the cathodic protection current will be carried in the corrosive environment by more mobile ions, e.g. OH, Na, etc. 

The proof of protection is more difficult to establish in this case for two reasons. First, the object is to restore passivity to the rebar and not to render it virtually immune to corrosion. Second, it is difficult to measure the true electrode potential of rebars under these conditions. This is because the cathodic-protection current flowing through the concrete produces a voltage error in the measurements made (see below). For this reason it has been found convenient to use a potential decay technique to assess protection rather than a direct potential measurement. Thus a 100 mV decay of polarisation in 4 h once current has been interrupted has been adopted as the criterion for adequate protection. It will be seen that this proposal does not differ substantially from the decay criterion included in Table 10.3 and recommended by NACE for assessing the full protection of steel in other environments. Of course, in this case the cathodic polarisation is intended to inhibit pit growth and restore passivity, not to establish effective immunity. 

Fig. 10.9 Diagram illustrating the source of the IR error in potential measurements on a cathodically protected structure. BA is the absolute electrode potential of the structure CD is the absolute electrode potential of the anode and CB is the field gradient in the environment due to cathodic protection current flux. A reference electrode placed at E will produce an IR error of EFin the potential measurement of the structure potential. If placed at G the error will be reduced to GH. At B there would be no error, but the point is too close to the structure to permit insertion of a reference electrode. If the current is interrupted the field immediately becomes as shown by the dotted line, and no IR is included...

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. 

Brown and Fessler have conducted a laboratory evaluation of conductive mastics that can be brushed or sprayed onto the concrete surface to achieve the necessary thickness. However, the most extensive study on conductive paints for cathodic protection purposes has been undertaken by the Federal Highway AuthorityA total of nine commercially available resins were evaluated in this work. It was shown that neither thermal cycling, freeze thawing nor the application of cathodic protection currents... 

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. 

Corrosion Interaction (or interaction) increase (or decrease) in the rate of corrosion of a buried or immersed structure caused by interception of part of the cathodic protection current applied to another buried or immersed structure. 

Typical Cathodic Protection Current Density Ranges.111... 

Functional relationship between cathodic protection current/potential and duration of system deployment in desert conditions... 

J. J. Chang, A study of bond degradation of rebar due to cathodic protection current . Cement and Concrete Research, 2002, 32,... 

Once the corrosion (mixed) potential is known, the estimation of the cathodic protection current is relatively simple the cathodic Tafel line is extended until the ordinate reaches the anode equifibrium value. The current corresponding to that ordinate value is the minimum value of the external current that must be suppfied to stop the corrosion process. For processes in which there are multiple species undergoing cathodic or anodic reactions, the resultant cathodic and anodic Tafel curves are calculated by adding the individual polarization curves within the respective potential range. 

Using the data given in exercise E15.2, calculate the cathodic protection current required to reduce the corrosion rate to zero. (See Case Study 3.3, Chapter 3.)... 

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. 

If the detached coating does not shield the cathodic protection current, the current density can assume locally very high values, and lead to phenomena of embrittlement due to hydrogen evolution. On the other hand, if the detachment is such that the current is shielded, the anaerobic condition of... 

In most cases, the process of corrosion is inevitable however, in practice its control is possible and practicable. Corrosion control measures include the use of cathodic protection and coatings, severally and jointly. The conjoint use of coatings and cathodic protection is an aninently powerful tool in corrosion protection, for while coatings will offer a first line of protection, the cathodic protection current will protect the substrate at imperfections and/or holidays in the coatings. Thus the cathodic current required for protection will be infinitesimally low. 

Structure isolation. A most desirable attribute is to limit the spread of cathodic protection current. For structures such as pipelines and tanks, this may be achieved by the insertion of electrical isolation joints in the structure, which often require regular maintenance. 

Probe anodes have been most widely used on highway structures to provide cathodic protection current to steel that cannot be reached from a surface anodes. They have been applied to half joints and to provide current to two faces of shear walls, etc. where the inside face is inaccessible (Figures 7.13(b), 7.10). They are also increasingly applied on historic steel framed masonry or brick clad structures where they can be fitted in the mortar joints. 

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

Thus it can bee seen that each prestressed structure must be evaluated to determine the feasibility of cathodic protection current reaching the steel that needs protection, whether galvanic cathodic protection can be applied and if not using qualification criteria such as those in NACE 01102 (2002) to determine if the structure is suitable for impressed current cathodic protection. 

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