Protection objective

All organic coatings show varying degrees of solubility and permeability for components of the corrosive medium, which can be described as permeation and ionic conductivity (see Sections 5.2.1 and 5.2.2). An absolute separation of protected object and medium is not possible because of these properties. Certain requirements have to be met for corrosion protection, which must also take account of electrochemical factors [1] (see Section 5.2). 

Enamel coatings are applied for internal protection of storage tanks (see Section 20.4.1). Enamel is impervious to water, i.e., it separates the protected object and corrosive medium. Corrosion protection can fail only at defects in the enamel coating and through corrosion of the enamel (see Section 5.4). 

Galvanic anodes must not be backfilled with coke as with impressed current anodes. A strong corrosion cell would arise from the potential difference between the anode and the coke, which would lead to rapid destruction of the anode. In addition, the driving voltage would immediately collapse and finally the protected object would be seriously damaged by corrosion through the formation of a cell between it and the coke. 

A single or multicored plastic-coated cable of the type NYY or NYY-O is used as the connecting cable between a protected object and an anode in soils and fresh water, and particularly in seawater, medium heavy or heavy rubber-sheathed connections of an NSHou or NSSHou type are used. Heavy welded connections of type NSLFSou are used for severe mechanical loading. In addition to these, for ships, marine cable of type MGCG or watertight cables must be considered. 

In choosing a site for protection installations for steel-water structures, decisive factors are the location in the harbor area and the need to keep the lengths of cable to the protected object and anode as short as possible where very high protection currents are involved. 

Figure 8-5 shows the main circuit diagram of a potential control rectifier provided with magnetic amplifiers (transducers). The chosen potential is set at the nominal value with a potentiometer. The actual potential is compared with this value, which corresponds to the voltage between a reference electrode and the protected object. 

Rectifiers working according to the control diagram in Fig. 8-6 are used for anodic corrosion protection in passivatable systems that go spontaneously from the passive to the active state when the protection current is switched off [12]. The predetermined nominal voltage between reference electrode and protected object is compared with the actual voltage f/j in a differential display unit D. The difference AU = is amplified in a voltage amplifier SV to VqAU. This... 

The current requirement of the protected object basically determines the design of the anode bed. For example, for a pipeline requiring 10 A with horizontal anodes laid in soil with p = 45 H m, according to Fig. 9-14, eight anodes are necessary. The grounding resistance of one anode amounts to Rq = 14 H. From Fig. 9-8, the grounding resistance of the anode bed with an interference factor F= 1.34 for 8 anodes spaced at 5 m comes to R = 2.34 Q.. 

Interference from the Cathodic Voltage Cone of the Protected Object... 

Fig. 10-5 Protection measure by separation of electrical operational equipment that is connected to the cathodically protected object via the housing, with an FI protection circuit leakage current circuit breaker (see Ref. 14) Tj and isolating transformers (see Ref. 15).

Additional individual anodes must be installed at points on the protected object where a sufficiently negative pipe/soil potential cannot be achieved. Since usually only the voltage cone is of interest, the place of installation does not depend on the specific soil resistivity. Coke backfill is not necessary, and the place of installation is determined by the local circumstances. Individual horizontal anodes are conveniently installed parallel to the pipeline at the depth of the pipe axis. The voltage, length and distance of the anodes from the protected object are chosen according to Section 9.1 so that criterion No. 6 or No. 7 in Table 3-3 is fulfilled. 

With local cathodic protection, the off potential measurement cannot be used directly to check the protective action because, due to the mixed type of installation of the protected object and foreign cathodic structures in the soil, there is a considerable flow of cell currents and equalizing currents. The notes to Eq. (3-28) in Section 3.3 are relevant here, where the // -free potentials must be substantially more negative than the off potential of the protected object. If t/ ff is found to be more positive than U, this does not confirm or conclusively indicate insufficient... 

Since cathodic protection of concrete structures in the United States has been very much advanced, protection criteria have been developed [46]. They correspond to the pragmatic criteria Nos. 3 and 4 in Table 3-3 (see Section 3.3.3.1). It is assumed that the protective effect is adequate if, upon switching off the protection current, the potential becomes more than 0.1 V more positive within 4 hours. The measurements are carried out in various parts of the protected object with built-in Ag-AgCl reference electrodes or with any electrodes on the external surface. 

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. 

In Europe, the first internal cathodic protection installation was put into operation in 1965 for 24 water-powered Kaplan turbines with a propeller diameter of 7.6 m. These were in the tidal power station at La Ranee in France. The protected object consisted of plain carbon and high-alloy stainless steels. Each turbine was... 

In contrast to the traditional methods the geotechnical investigations should be aimed at positively solving environmental protection objectives. Only this approach can ensure a successful solution of all the major geotechnical objectives during the design and construction of the pipeline. 

The public authorities stipulate national regulations setting protection objectives and means, and controlling their implementation by the private sector. Governments decide on the criteria to apply for risk assessment and determine which are the expected benefits and drawbacks of the protection measures. The operators of the CII are responsible and accountable by the government for the implementation of the protection measures. 

An assessment of whether a particular standard has met its original protection objectives should always be an integral part of the standard-setting process. This should be conducted along with a review of the standard should there be information that can be used to update it and increase overall confidence in its application. A stringent standard may be more likely to meet its protection goals, but it is also important to consider the social and economic aspects surrounding its implementation as it is also important that standards do not place unnecessary burdens beyond what may be required to achieve objectives. 

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