Cathodic protection potential criteria

The terms protection current and protection current densities refer to any values of total cathodic currents that meet the criterion in Eq. (2-40). However, in the field, and for designing cathodic protection stations, another term is of interest, the protection current requirement. This term is concerned with the lowest value of the protection current that fulfills the criteria in Eqs. (2-39) or (2-40). Since with an extended object having a surface S the polarization varies locally, only the current density for the region with the most positive potential has the value J. In other regions 17. 1 > 7. . For this reason, the protection current requirement 4 is given by ... 

Cathodic protection installations must be tested when commissioned and at least annually. The potentials should be measured at several points on the bottom of the tank with special probes under the oil, and the height of the electrolyte solution should be checked. The off potential and the protection potential as in Section 2.4 are the means for checking the protection criterion according to Eq. (2-39). 

Aluminum-sheathed cables should not be connected to other cables because aluminum has the most negative rest potential of all applicable cable sheathing materials. Every defect in the protective sheath is therefore anodically endangered (see Fig. 2-5). The very high surface ratio SJS leads to rapid destruction of the aluminum sheathing according to Eq. (2-44). Aluminum can also suffer cathodic corrosion (see Fig. 2-11). The cathodic protection of aluminum is therefore a problem. Care must be taken that the protection criterion of Eq. (2-48) with the data in Section 2.4 is fulfilled (see also Table 13-1). Aluminum-sheathed cables are used only in exceptional cases. They should not be laid in stray current areas or in soils with a high concentration of salt. 

Since the object to be protected represents a cell consisting of active and passive steel, considerable IR errors in the cell current must be expected in measuring the off potential. The considerations in Section 3.3.1 with reference to Eqs. (3-27) and (3-28) are relevant here. Since upon switching off the protection current, 7, the nearby cathodes lead to anodic polarization of a region at risk from corrosion, the cell currents and 7, have opposite signs. It follows from Eqs. (3-27) and (3-28) that the 77 -free potential must be more negative than the off potential. Therefore, there is greater certainty of the potential criterion in Eq. (2-39). 

Protection current devices with potential control are described in Section 8.6 (see Figs. 8.5 and 8.6) information on potentiostatic internal protection is given in Section 21.4.2.1. In these installations the reference electrode is sited in the most unfavorable location in the protected object. If the protection criterion according to Eq. (2-39) is reached there, it can be assumed that the remainder of the surface of the object to be protected is cathodically protected. 

The region of immunity [Fig. 1.15 (bottom)] illustrates how corrosion may be controlled by lowering the potential of the metal, and this zone provides the thermodynamic explanation of the important practical method of cathodic protection (Section 11.1). In the case of iron in near-neutral solutions the potential E = —0-62 V for immunity corresponds approximately with the practical criterion adopted for cathodically protecting the metal in most environments, i.e. —0-52 to —0-62V (vs. S.H.E.). It should be observed, however, that the diagram provides no information on the rate of charge transfer (the current) required to depress the potential into the region of immunity, which is the same (< —0-62 V) at all values of pH below 9-8. Consideration of curve//for the Hj/HjO equilibrium shows that as the pH... 

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. 

Reference electrodes The generally accepted criterion for the effectiveness of a cathodic-protection system is the structure/electrolyte potential (Section 10.1). In order to determine this potential it is necessary to make a contact on the structure itself and a contact with the electrolyte (soil or water). The problem of connection to the structure normally presents no difficulties, but contact with the electrolyte must be made with a reference electrode. (If for example an ordinary steel prol e were used as a reference electrode, then inaccuracies would result for two main reasons first, electrochemical action between the probe and the soil, and second, polarisatibn of the probe owing to current flow through the measuring circuit.)... 

It is apparent that since the electrode potential of a metal/solution interface can only be evaluated from the e.m.f. of a cell, the reference electrode used for that purpose must be specified precisely, e.g. the criterion for the cathodic protection of steel is —0-85 V (vs. Cu/CuSOg, sat.), but this can be expressed as a potential with respect to the standard hydrogen electrode (S.H.E.), i.e. -0-55 V (vs. S.H.E.) or with respect to any other reference electrode. Potentials of reference electrodes are given in Table 21.7. 

Interaction Testing routine investigation carried out when installing cathodic protection schemes on pipelines. The accepted criterion in the UK is that when the secondary structure potential has been moved more than 0-02 V in a positive direction, steps must be taken to eliminate the interaction. 

Cathodic protection is the process whereby the corrosion rate of a metal is decreased or stopped by decreasing the potential of the metal from Ecorr to some lower value and in the limit to E M, the thermodynamic equilibrium half-cell potential. At this potential, iox M = ired X[ = i() xi- and net transfer of metal ions to the solution no longer occurs. This is the criterion for complete cathodic protection (i.e., E = E m). 

The experience on bridge decks shows that, in the cases in which the cathodic protection path runs according to 4-5 of Figure 20.4, the current required to maintain protection conditions (verified by the so-called four-hour 100 mV potential decay empirical criterion decreases with time, even after months or years from start up. This happens because the cathodic current can bring about repassivation of steel in active zones. When passivity is established on the entire surface of the steel, the current required to maintain passivity is reduced to a few mA/m (e. g. 2-5 mA/m ). If the CP path runs according to 4-6, the current density to fulfil the protection criterion remains high and does not decrease with the time, since passivity is not obtained. 

Figure 20.3c shows the effect of application of cathodic protection on carbonated concrete. The applied cathodic current density, even if it brings about only a small lowering of the steel potential, can produce enough alkalinity to restore the pH to values higher than 12 on the reinforcement surface and thus promote passivation. The effectiveness of cathodic protection in carbonated concrete was studied with specimens with alkaline concrete, carbonated concrete and carbonated concrete with 0.4% chloride by cement mass that were tested at current densities of 10, 5, and 2 mA/m (of steel surface) [45]. Carbonated concrete specimens polarised at 10 mA/m showed that, although initially protection was not achieved since the four-hour decay was slightly lower than 100 mV, after about four months of polarization, the protection criterion was fulfilled and higher values, in the range 200-300 mV of the four-hour potential decay were measured (Figure 20.6). The same results were obtained on carbonated and slightly chloride-contaminated concrete.

Trials. The effectiveness of chloride extraction depends on characteristics of individual structures, such as the concrete composition, the actual chloride-penetration profile and the depth of cover. So, it may be useful to carry out a trial on an area (about 1 to 10 m ), which must be representative of the structure to be treated and should last at least 4 to 8 weeks. The results of such a trial in terms of the chloride profile before, during and after chloride extraction gives an indication of the duration required and can be used to show that chloride-extraction treatment of the particular structure will be effective under field conditions. Trials are most certainly recommended if prestressed structures are to be treated with chloride extraction. Careful monitoring of the potential of the prestressing steel should be carried out to establish the risk of hydrogen embrittlement. As a safe criterion, the potential should not become more negative than -900 mV SCE, as apphes for cathodic protection [13]. 

An important aspect of cathodic protection is the means to monitor the effectiveness and the criteria for protection. Criteria recommended by NACE (RP0169-96) for the CP of steel and cast iron piping are given in Table 4 [24]. Although several criteria are described for CP of steel structures, the most commonly used criterion is that the steel structure to be protected should be maintained at a potential more negative than —0.85 V versus Cu/CuSOr reference electrode. The primary disadvantages of this criterion are no connection of the potential of the steel to the corrosion rate, and a large difference in protective... 

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. 

It would be preferable to implement CP criteria based on the actual corrosion rate of the protected metal - that is, by lowering the corrosion rate using the anodic Tafel constant to some value that is adequate. However, this may be impractical because, in practice, the actual corrosion rate of the structure may not be available. A workable alternative would be to specify the potential change necessary to reduce corrosion by a given percentage. The anodic Tafel constant provides a reasonable guide or criterion for cathodic protection and enables a better understanding of how and why the cathodic protection is effective. However, determination of an accurate anodic Tafel constant for the protected structure is not an easy task. 

The criterion for the cathodic protection of most metals is expressed as a potential of the metal relative to a reference device. A constant potential device is required which should possess the following properties ... 

Structure-to-electrolyte potential measurements are analyzed to determine whether a structure is cathodically protected these measurements are made by the use of cathodic protection criteria. Unfortunately, no one simple criterion has been accepted by all cathodic protection engineers that can be practicably measured in the field under all circumstances. Guidelines for selecting the proper criterion under various circumstances will be provided below. Guidance concerning the criteria of cathodic protection for external corrosion control on underground structures is found in two recommended practices (RPs) published by the National Association of Corrosion Engineers (NACE). These are RP-01-69 and RP-02-85. A summary of the criteria for steel and cast iron structures follows [8]. 

One criterion is a negative (cathodic) potential of at least 850 mV with the cathodic protection applied. This potential is measured with respect to a saturated copper/copper sulfate reference electrode contacting the electrolyte. Voltage drops other than those across the structure-to-electrolyte boundary must be considered for valid interpretation of this voltage measurement. 

In practice, the best control criterion is based on a potential shift. Theory and experiment tell ns that a shift in potential of 100-150 mV will reduce the corrosion rate by at least an order of magnitude. Field evaluations have shown that this stops all further signs of corrosion damage in cathodically protected strnctnres. 

The 100-150 mV criterion is straightforward to apply and is the most universally agreed criterion. Other control criteria such as the plotting of the applied current against the log of the potential (ElogI), absolute potentials, macrocell or null probe current reversals and other potential shifts have been used and are used by some cathodic protection specialists but there is some controversy about their theory and practice. 

A criterion that indicates degree of protection, including overprotection, is obtained through measuring the potential of the protected structure. This measurement is of greatest importance in practice, and it is the criterion generally accepted and used by corrosion engineers. It is based on the fundamental concept that optimum cathodic protection is achieved when the protected structure is... 

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