Protection Impressed current

Common uses of the impressed current method of protection include long transmission pipelines, complex underground structures, marine structures, ship hulls, and replacement for dissipated galvanic systems, large condenser water boxes, reinforcing steel in concrete, bare or poorly coated structures, unisolated structures and water storage tank interiors. 

This method is especially suitable when large currents are involved, possibly as high as 500 A. The current requirement is calculated as  

Anode materials and their properties along with the recommended uses are noted in Table 1.31. For a number of anodes required for an impressed current ground bed, we calculate the weight required by the equation, 

Platinized tantalum 8 x 1 0-6 Marine environments, potable water, 

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Metal anodes using platinum and precious metal oxide coatings are also incorporated into a variety of designs of impressed current protection for pipeline and deep weU appHcations, as weU as for protection of condenser water boxes in power generating stations (see Pipelines Power generation). 

The low cost, light weight, and exceUent electrical conductivity of graphite anodes have made this impressed current protection system valuable for cathodic protection of pipelines, storage vessels, process equipment, and also for weU casings both on- and offshore. 

The current needed for cathodic protection by impressed current is supplied from rectifier units. In Germany, the public electricity supply grid is so extensive that the CP transformer-rectifier (T-R) can be connected to it in most cases. Solar cells, thermogenerators or, for low protection currents, batteries, are only used as a source of current in exceptional cases (e.g., in sparsely populated areas) where there is no public electricity supply. Figure 8-1 shows the construction of a cathodic impressed current protection station for a pipeline. Housing, design and circuitry of the rectifier are described in this chapter. Chapter 7 gives information on impressed current anodes. 

Magnesium anodes were chosen as the source of the protection current in this old example, because on one hand a sufficient current, including current reserve, could be achieved due to the relatively low soil resistivity, and on the other hand, use of an impressed current protection system would have required much greater expenditure. 

In this case, impressed current protection with several anodes was chosen on the one hand to achieve uniform current distribution with the relatively high protection current density, and on the other hand to avoid large anode voltage cones. A transformer-rectifier with a capacity of 10 V/1 A was chosen. 

Anodes of small impressed current protection installations can be installed close to the cable or the cable duct. They can, however, also be inserted as an... 

Protection with impressed current, with galvanic anodes, and a combination of both processes is used for marine structures and offshore pipelines. Their properties, as well as their advantages and disadvantages, are given in Table 16-1. The protective measures must be optimized for every structure. In the impressed current protection of offshore platforms, for example, the difficulties of maintenance and repair will be of major importance, whereas in harbor installations these problems can be... 

The difficulties of such operations on the research platform Nordsee are described in Ref. 9. The Murchison platform was provided with a combination of impressed current protection and galvanic anodes because there was a limit to the load to be transported [12]. The anodes for platforms are installed and provided with cables at the yard. They are installed with redundancy and excess capacity so that no repairs are necessary if there is a breakdown. The lower part of the platform up to the splash zone is usually placed in position in the designated location at least 1 year before the erection of the deck structure so that impressed current protection cannot initially be put in operation. This requires cathodic protection with galvanic anodes for this period. This also means that the impressed current protection is more expensive than the galvanic anodes. 

The distance between the structure and fixed impressed current anodes is an important factor. The number of anodes has to be small so the anodes need to be relatively large, which will result in too negative a potential if the distance is not sufficiently great. A minimum distance of 1.5 m is prescribed [1-3], but this involves considerable construction effort due to the effects of heavy seas. Besides the so-called restriction on impressed current installations, there is the requirement that the corrosion protection be switched off when diving work is being carried out [14]. This regulation is not justifiable. Work on the underwater region of production platforms takes place continuously, as far as the weather allows if the protection must be switched off each time, the impressed current protection becomes very limited. 

Potential measurements have been carried out at suitable times on platforms with galvanic anodes after the structures have been commissioned. Where impressed current protection was installed, the potential as well as the anode current was measured with fixed, built-in measuring electrodes during the commissioning period. 

The protection current requirement for aluminum ships is considerably less because of the dense adherent oxide films. The necessary protection current requirement is being clarified in current investigations [24] but good results have been obtained by assuming a figure of 10% of that for steel. With aluminum there is only a very narrow permissible potential range [25] (see Section 2.4) so that impressed current protection cannot be used because of the anodic voltage cone and only selected anode materials can be considered. 

Fig, 17-5 Circuit diagram for an impressed current protection installation. 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

Galvanic or impressed current anodes are used to protect these components. The anode material is determined by the electrolyte zinc and aluminum for seawater, magnesium for freshwater circuits. Platinized titanium is used for the anode material in impressed current protection. Potential-regulating systems working independently of each other should be used for the inlet and outlet feeds of heat exchangers on account of the different temperature behavior. The protection current densities depend on the material and the medium. 

The impressed current protection method is used mainly for the internal protection of large objects and particularly where high initial current densities have to be achieved (e.g., in activated charcoal filter tanks and in uncoated steel tanks). There are basically two types of equipment those with potential control, and those with current control. 

Fig. 20-13 Current and potential-time curves for a 500-liter stainless steel water tank. Impressed current protection with an interrupter potentiostat X (20 C) = 2250 IJ.S cm-i c (CF) = 0.02 mol L" 60 C.

A particular advantage of impressed current systems is the ability to control the output voltage of the rectifier. Also, there are the comparatively low installation costs and relatively uniform current distribution. The costs of impressed current protection compared with aluminum anodes are 0.8 1. With ships this ratio depends on the length of the ship with larger ships it is 1 2.5 since the calculation is made in comparison with zinc and aluminum anodes. The order of magnitude of the annual costs depends on the structure and the investment costs. 

The structural costs of impressed current protection of harbor and coastal structures are about 1.5 to 2.5% of the total cost of the object to be protected. As an example, for the installation of a cathodic protection station in a tanker discharge jetty, the construction costs amounted to 2.2% of the total costs. The annual cost of current, maintenance, testing, and repairs amounted to 5% of the construction costs of the cathodic protection [22]. 

Buried structures bonded into traction system in such a way as to receive impressed-current protection... 

If, with impressed-current protection, the groundbed is installed near the unprotected structure, large negative changes may occur on the structure at points close to the anode. The maximum positive potential change is usually found on the unprotected installation at a distance of 270-450 m from the... 

The amount of interaction caused by a protection scheme using galvanic anodes will be much less than that involved in the case of impressed-current protection, because of the low current output obtained from each anode. Significant positive potential changes have, however, been measured on nearby structures in cases where galvanic anodes are closely spaced and the distance between structures is small. 

This method of corrosion protection consists of (i) the sacrificial anode method and (ii) impressed current cathodic protection. The other related impressed current protection method is anodic protection. 

Fig. 8 Circuit and typical reactions for impressed current protection for steel pipeline buried in soil.

Fig. 15.7 Effect of diffusion rate of oxygen on the impressed current protection system.

R. Baboian, P.F. Prew, K. Kawate, Design of platinum clad wire anodes for impressed current protection, Mater. Performance 23 (1984) 31—35. 

IL Under what circumstances can cathodic protection (either sacrificial protection or impressed current protection) be used to protect an automobile from... 

Overactive cathodic protection— which can occur particularly with magnesium anodes—or badly controlled impressed current protection—leads to the reduction of oxygen to form hydroxyl ion (OH ), which can attack paints. Also, the placing of the zinc anodes requires experience in connection with both type and thickness of the paint (system). Since zinc anodes do not allow the production of excessive amounts of alkali, paint failure or the need to use alkali-resistant paints is avoided. 

Low Resistance Current Paths. An indirect use of zinc for corrosion protection is to provide a low resistance path for impressed current protection of concrete bridge structures with uncoated steel rebar. Electrochemical protection of steel rebar in concrete was developed as a repair technique but is now being promoted also for new structures. Current generated by impressed anodes on the outside of the concrete and a positive electrical contact with the rebar stops the latter from rusting. The primary anode is usually brass or copper. 

Zinc spraying of plastics or phenolic-impregnated asbestos is used to provide reflecting surfaces, and the large open-air dishes used in electronic applications are so coated. In impressed current protection of rebar in concrete, the exterior of the concrete is sometimes zinc sprayed (Morrow, 1991) to give a uniform current distribution (see subsection on impressed current systems in Section II. E) in such cases, it is often policy to avoid direct electrical connection between zinc and rebar, since the consequent use of zinc as a sacrificial anode could be counterproductive if only a limited amount of zinc is present to protect large areas of steel. 

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