Cathodic protection with impressed current

Cathodic Protection with Impressed Current Anodes... 

Cathodic protection with impressed current anodes is used predominantly with cables or steel casing in which the cable is inserted, outside built-up areas where it is possible to build large anode installations without damaging interference with other lines. In densely populated areas, protection with impressed current anodes is often only possible with deep anodes, with surface anodes or locally at individual problem points (local cathodic protection, see Chapter 12). 

Today loading piers are mostly cathodically protected with impressed current. At moorings for tankers, cathodic protection rectifiers are installed on extinguisher bridges as far as possible from the hazardous area. Otherwise, they must be of an explosion-proof type. 

Cathodic protection with impressed current, aluminum or magnesium anodes does not lead to any promotion of germs in the water. There is also no multiplication of bacteria and fungi in the anode slime [32,33]. Unhygienic contamination of the water only arises if anaerobic conditions develop in the slurry deposits, giving rise to bacterial reduction of sulfate. If this is the case, HjS can be detected by smell in amounts which cannot be detected analytically or by taste. Remedial measures are dealt with in Section 20.4.2. 

The decision on whether cathodic protection with impressed current or with magnesium anodes is more economical depends on the protection current requirement and the soil resistivity. This estimate only indicates the basic influence of the different variables. In the individual case, installation costs can vary widely so that a specific cost calculation is necessary for every project. 

Examples of cathodic protection with impressed current are, at the present time, protection of steel pipelines in maritime environments or in subsoil. An important example of anodic protection is in the storage of acids in steel tanks—the anodic current passivates the steel (see Fig. 16.1a). 

The high-alloy silicon irons are used in draining pipelines, pumps, valves, other process equipment, and anodes for cathodic protection with impressed current. 

Figure 10.16 Schematic potential-distance diagram for cathodic protection with impressed current. Definition of Ea, E and AE as in Figure 10.15. Pj = electrical potential in the anode material, Pstmcture = electrical potential in the structure (the cathode material), Pj = the terminal voltage of the external current source.

Figure 11.7 Principle of cathodic protection with impressed current (schematic).

Figure 20-13 shows current and potential time curves for a stainless steel 500-liter tank with cathodic protection by impressed current and interrupter potentiostat. 

Three types of anodic protection can be distinguished (1) impressed current, (2) formation of local cathodes on the material surface and (3) application of passivating inhibitors. For impressed current methods, the protection potential ranges must be determined by experiment (see information in Section 2.3). Anodic protection with impressed current has many applications. It fails if there is restricted current access (e.g., in wet gas spaces) with a lack of electrolyte and/or in the... 

Cathodic protection by impressed current involves the use of a rectifier connected to a power line. Contrary to sacrificial anodes, which operate at a fixed potential, the use of a rectifier permits to adjust the voltage (or the current) to the particular requirements of a protection scheme. This not only allows one to optimize the electrochemical conditions for protection, but the method is also well suited to protect large surfaces. On the other hand, protection by impressed current needs more maintenance than the use of sacrificial anodes. In order to protect buried structures by impressed currents one uses consumable anodes such as scrap iron or, more often, non-consumable anodes made of iron-silicon alloy, graphite or of titanium coated with noble-metal oxides. 

Coatings must be considered for aU applications of steel. Cathodic protection should be considered for steel pipe where soil or groundwater resistivity is less than 10,000 t2-cm, and where steel win be in contact with process streams. Cathodic protection of steel is strongly recommended where resistivity is less than 5000 Q-cm. For aU exposures, steel should be electrically isolated from dissimilar metals to prevent the formation of unfavorable galvanic corrosion ceUs. In areas where abrasive materials are hkely to damage coatings, cathodic protection by impressed current or galvanic anodes may be desirable. 

Fig. 10.29 Cathodic protection using impressed current, (a) A circuit showing the principles the signal from the reference electrode is passed to a power-unit control where it is compared with a preset level. The resultant error signal is amplified and used to control semiconductor power devices which allow a controlled current to pass through the anodes, (b) A typical layout of components in various types of vessel, (c) Transformer/rectifier power units for marine use. (d) Platinized titanium or lead-silver alloy anodes being installed on a ship s hull. The anodes are insulated from the hull and have special insulating, backing shields which help to improve potential distribution and prevent over-protection, i.e. too negative a potential (Photographs courtesy Corrintec (UK) Ltd.)...

It is little known that Thomas Alva Edison tried to achieve cathodic protection of ships with impressed current in 1890 however, the sources of current and anodic materials available to him were inadequate. In 1902, K. Cohen achieved practical cathodic protection using impressed direct current. The manager of urban works at... 

Cathodic protection with magnesium anodes can be just as economical as impressed current anode assemblies for pipelines only a few kilometers in length and with protection current densities below 10 xA m" e.g., in isolated stretches of new pipeline in old networks and steel distribution or service pipes. In this case, several anodes would be connected to the pipeline in a group at test points. The distance from the pipeline is about 1 to 3 m. The measurement of the off potential... 

Fig. 11-4 Local cathodic protection of a tank farm with impressed current.

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. 

Fig. 16-9 Cathodic protection of a steel piling with impressed current.

Cathodic protection of reinforcing steel with impressed current is a relatively new protection method. It was used experimentally at the end of the 1950s [21,22] for renovating steel-reinforced concrete structures damaged by corrosion, but not pursued further because of a lack of suitable anode materials so that driving voltages of 15 to 200 V had to be applied. Also, from previous experience [23-26], loss of adhesion between the steel and concrete due to cathodic alkalinity [see Eqs. (2-17) and (2-19)] was feared, which discouraged further technical development. 

Cathodic protection cannot work with prestressed concrete structures that have electrically insulated, coated pipes. There is positive experience in the case of a direct connection without coated pipes this is protection of buried prestressed concrete pipelines by zinc anodes [38], Stability against H-induced stress corrosion in high-strength steels with impressed current has to be tested (see Section 2.3.4). 

Figure 20-9 shows the negative effect of uninsulated heating elements on corrosion protection. In a 250-liter tank, an electric tube heating element with a 0.05-m surface area was screwed into the upper third without electrical separation, and in the lower third a tinned copper tube heat exchanger with a 0.61 -m surface area was built in. The Cu heat exchanger was short-circuited for measurements, as required. For cathodic protection, a potential-controlled protection system with impressed current anodes was installed between the two heating elements. The measurements were carried out with two different samples of water with different conductivities. 

Fig. 20-14 Internal cathodic protection of pressure filter boiler with impressed current.

Besides the use of anodic polarization with impressed current to achieve passivation, raising the cathodic partial current density by special alloying elements and the use of oxidizing inhibitors (and/or passivators) to assist the formation of passive films can be included in the anodic protection method [1-3]. 

---