Cathodic Protection Components

The reference electrode generally used for monitoring cathodic protection in soils is the copper sulfate electrode. This consists of an electrode of electrolytic copper in a solution saturated with copper sulfate crystals. However, the most universal reference electrodes are of the silver chloride type. These consist of a silver wire, plate, or mesh coated with silver chloride so that both silver and silver chloride are in contact with a chloride ion rich electrolyte. The potential of the Ag/AgCl reference electrode will depend upon this chloride ion concentration. 

A commonly used method for preparing the silver chloride electrode is by dipping a clean silver wire spiral or welded mesh into molten silver chloride, which had been heated some 50°C above its melting point in a pure silver or quartz crucible. The silver is removed from the melt and the majority of the molten salt shaken off leaving only a thin silver chloride film. This is then made the cathode in a solution of sodium chloride and some of its surface is reduced to silver. 

The accuracy of the silver chloride reference electrode depends upon the amount of chloride ion that is present in the solution and the accuracy with which it is controlled. In practical applications two concentrations of chloride ion are popular. In the first the electrolyte surrounding the silver chloride element is seawater. The other type of reference electrode uses a saturated sodium or potassium chloride solution. Both silver chloride cells have large temperature coefficients but there is no hysteresis or other effects so that these variations can be calculated. These reference electrodes typically have a higher resistance than the copper sulfate cells and this resistance is a function of the thickness of the silver chloride layer. 

A piece of zinc may also be used as a reference electrode by immersing it in an electrolyte that contains some quantity of chloride or sulfate ions or it can be used in the ground if surrounded by a suitable backfill. The anode backfill for zinc is usually bentonite, sodium sulfate, g5q3sum, and sodium chloride. Zinc electrodes made from high purity material usually provide a stable potential that is constant t within a5 mV bracket. The electrode may slowly polarize and may have marine growth on it in seawater but this is generally prevented if the zinc is allowed to act as an anode at a very low current density. 

Often it is necessary to measure potentials where the electrolyte is not easily accessible as, for example, in the water box of a condenser or on the inside of a steel pipe carrying seawater. Permanent reference electrodes could be placed inside the box or pipe but this is expensive and no maintenance can be carried out on the electrode. 

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Heat Tracing Components Cathodic Protection Components Lighting Protection Equipment... 

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. 

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

Enamel coatings are used for the internal protection of storage tanks that in most cases have built-in components (e.g., fittings with exits, probes, temperature detectors) that usually exhibit cathodic effectivity. These constitute a considerable danger of pitting corrosion at small pores in the enamel. Corrosion protection is achieved by additional cathodic protection which neutralizes the effectiveness of the cathodic objects. 

Only local cathodic protection can be used for large installations and old installations with electrical contact to components with low grounding resistances that cannot be isolated (see Section 12.6). The measures necessary for tank installations are described in Ref. 10. 

In the local cathodic protection of the bottoms of flat-bottomed tanks, cell formation with steel-concrete foundations is of little importance since the surfaces are relatively small, in contrast to the installations in Sections 12.2 to 12.5. On the other hand, connected components of the installation, such as cables and grounds, take up considerable protection current. On account of the large foundations of flat-bottomed tanks, which are often bare or only poorly coated, polarization to the protection potential is only possible with very negative on potentials. In tank foundations with the... 

All these components can be included in the cathodic protection if certain features are considered in the design [e.g., bundles of heating tubes in a square array (see Fig. 20-1)], and the electrodes are arranged so that all the surfaces receive sufficient protection current i.e., so that the criterion in Eq. (2-39) is fulfilled. 

As the measurements show, the small heater without an electrical separation (from the boiler) is not detrimental to cathodic protection. However, with the uninsulated built-in Cu heat exchanger without an electrical separation, cathodic protection was not achieved. As expected, the polarization increased with increasing conductivity of the water. It should be pointed out that the Cu tube was tinned and that the tin could act as a weak cathodic component. Apart from the unknown long-term stability of such a coating, the apparent raising of the cathodic polarization resistance of tin is not sufficient to provide cathodic protection with such a large fixture. This applies also to other metal coatings (e.g., nickel). 

Corrosion in these areas is sometimes effectively controlled by cathodic protection with zinc- or aluminium-alloy sacrificial anodes in the form of a ring fixed in good electrical contact with the steel adjacent to the non-ferrous component. This often proves only partially successful, however, and it also presents a possible danger since the corrosion of the anode may allow pieces to become detached which can damage the main circulating-pump impeller. Cladding by corrosion-resistant overlays such as cupronickel or nickel-base alloys may be an effective solution in difficult installational circumstances. 

Provide effective means of corrosion protection of components consistent with operational circumstances. This may involve cathodic protection, inhibition, coatings or combinations of these. 

Sintered and sprayed ceramic anodes have been developed for cathodic protection applications. The ceramic anodes are composed of a group of materials classified as ferrites with iron oxide as the principal component. The electrochemical properties of divalent metal oxide ferrites in the composition range 0- lA/O-0-9Fe2O3 where M represents a divalent metal, e.g. Mg, Zn, Mn, Co or Ni, have been examined by Wakabayashi and Akoi" . They found that nickel ferrite exhibited the lowest consumption rate in 3% NaCl (of 1 56 g A y at 500 Am and that an increase in the NiO content to 40mol 7o, i.e. O NiO-O-bFejO, reduced the dissolution rate to 0-4gA y at the expense of an increase in the material resistivity from 0-02 to 0-3 ohm cm. 

Potential data loggers are now available to undertake close interval pipeline surveys. These increasingly popular surveys, determine a pipeline s pipe-to-soil potential at nominal intervals, of as little as 1 m. Additional information is gained by the recording at each point, of both the pipe-to-soil potential with the cathodic protection system ON , together with the potential some 100-300 ms after the cathodic protection system is switched OFF . This instantaneous OFF potential being devoid of any IR drop component present in the ON potential measurement. 

One must be wary of the use of anodic protection, in that any area that is not polarized completely into the passive region will dissolve at a high rate. The optimum protection range is shown in Fig. 16. Therefore anodic protection is more susceptible to the presence of crevices, deposits, or poor placement of polarizing electrodes than is cathodic protection. If a component is cathodically under protected, the maximum rate at which the unprotected area corrodes is the normal open circuit corrosion rate in anodic protection, underprotection results in high rate dissolution of the unprotected area and can therefore can lead to unexpected career changes. Understanding the manner in which current from an anodic protection system is distributed across a surface is important in such installations. The issues involved in current distribution are discussed in detail in Chapter 4. 

When two dissimilar metals are electrically connected together, such that electrons can flow from one metal to the other, it is probable that the anodic, metal dissolution reaction will concentrate on one metal, while the cathodic reaction occurs on both. This accelerates the corrosion of the anodic component. The acceleration will be particularly severe if the area of the cathodic metal is much larger than that of the anodic metal (Fig. 1). While the anodic metal will corrode more, the cathodic metal will normally corrode less, and one form of galvanic corrosion provides a method of corrosion control (know as cathodic protection), in which the anodic, corroding metal is provided deliberately in order to restrict the corrosion of the cathodic metal. 

Fig- 4.25 Components used to impose and monitor conditions providing cathodic protection by an impressed external current. Note Power supply may be either a galvanostat or a potentiostat. In the latter, the electrometer provides feedbackto the potentiostatto control to constant potential. Electrometer provides check to show that the metal is at the protection potential. 

Zinc is used for cathodic protection in freshwater and marine water. Zinc is especially well suited for cathodic protection on ships that move between salt water and harbors in brackish rivers or estuaries. Zinc anodes also are used to protect ballast tanks, heat exchanger, and many mechanical components on ships, coastal power plants, and similar structures. 

Blistering due to gas formation or gas inclusion Volahle components leading to bhster formahon can become incorporated in the coating during application, or they can result from inclusions of solvents from phase separation. They can also be generated because of cathodic protection or even to a corrosion process if it generates H2. 

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