Galvanic anode system

Galvanic anode systems are generally used in well-coated electrically isolated structures, offshore structures, ship hulls, hot-spot pipeline protection, heat exchanger water boxes and other environments of resistivity below 10000 Q cm. 

Galvanic anode systems are typically used where protective current requirements are relatively low, usually in the range of several hundred miUiamperes to perhaps 4 or 5 A. Offshore structures, having current requirements of many hundreds of amperes can also be protected by large galvanic... 

In the design of a galvanic anode system, the following items must be considered [9] ... 

The following are the advantages and the disadvantages of the galvanic anode system ... 

In contrast to the galvanic anode system, the flow of current from the anode to the cathode is forced from a DC source in the impressed current system. Thus, whereas the current is provided by the corrosion of the electrode in the anodic galvanic system, the electrode acts as a conductor and hardly corrodes in the impressed system and the AC input is transformed and rectified to a varying DC voltage. A transformer rectifier is the most important component of the system. 

The above procedure can be used to determine the anode to earth resistance. Design charts can be constructed for vertical anodes and horizontal anode ground-beds for both impressed and galvanic anode systems. The development of design curves is discussed in Section 5.31.2. 

These three passive systems are important in the technique of anodic protection (see Chapter 21). The kinetics of the cathodic partial reaction and therefore curves of type I, II or III depend on the material and the particular medium. Case III can be achieved by alloying additions of cathodically acting elements such as Pt, Pd, Ag, and Cu. In principle, this is a case of galvanic anodic protection by cathodic constituents of the microstructure [50]. 

In poorly conducting media or soils, however, the low driving voltage can limit the use of galvanic anodes. Raising the current delivery, which becomes necessary in service, is only practically possible with the help of an additional external voltage. In special cases this is used if an installed galvanic system is overstretched or if the reaction products take over additional functions (see Section 7.1). 

The required number, n, of anodes can be calculated using Eq. (17-2) from the current requirement, together with the maximum current output 1 of the anodes. The arrangement of the anodes is dealt with in Section 17.3.2.2. Galvanic protection systems are usually designed to give protection for 2-4 years. After this period, a maximum of up to 80% of the anodes should be consumed. 

For use in high resistivity soils, the most common mixture is 75% gypsum, 20% bentonite and 5% sodium sulphate. This has a resistivity of approximately 50 ohm cm when saturated with moisture. It is important to realise that carbonaceous backfills are relevant to impressed current anode systems and must not be used with sacrificial anodes. A carbonaceous backfill is an electronic conductor and noble to both sacrificial anodes and steel. A galvanic cell would therefore be created causing enhanced dissolution of the anode, and eventually corrosion of the structure. 

The sacrificial anode system consists of a galvanic cell system in which the anode is made of a more active metal than the structure. The anode is attached to the structure and the anode output current may be measured. Magnesium and zinc anodes are commonly use in underground operations, zinc and aluminum alloy anodes in salt water. 

Before a galvanic anode CP system is designed, the following information should be obtained [10] ... 

In addition to the well-known application of cathodic corrosion protection to underground pipelines, there has been an increased use for the internal protection of containers and pipes. Initially, galvanic anodes were used to this effect, like the ones currently used, for example, to protect the interiors of tankers and boilers. However, since these anodes are often subject to heavy inherent corrosion, especially with the highly aggressive media often found in the chemical industry, they have largely been replaced by external current systems with insoluble anode material. 

Aluminium and magnesium and their alloys are also used in galvanic anode cathodic protection systems. One advantage of these alloys is that they are lighter than zinc. However, their oxides and other corrosion products are voluminous and could attack the concrete. They are therefore less attractive for concrete applications. 

This has recently been developed further into a small yogurt pot sized anode with a single connecting wire that can be inserted into core holes in the concrete and wired together to produce a galvanic discrete anode system (Figure 7.8). They have been used predominantly on multi-storey car parks and on bridge substructures. 

The principles of galvanic anode cathodic protection were discovered by Sir Humphrey Davy in 1824. His results were used over the next century or so to protect the submerged metallic parts of ships from corrosion. In the early decades of the 20th century the technology was applied to underground pipelines. Impressed current cathodic protection was developed when it was found that the electrolytes like soils had too high electrical resistance for galvanic systems to be effective. 

The system is well established in the United States both as an impressed current and as a galvanic anode (see Figure 7.5). 

Galvanic anodes are extensively used in continuously wetted environments. They need less maintenance than the impressed current systems. One problem with the zinc system is the environmental impact of the during spraying. If enclosure is required during spraying, the costs can be very high. 

Galvanic cathodic protection has its own advantages and disadvantages relative to impressed current cathodic protection and the other electrochemical and conventional rehabilitation techniques. The different anode systems also have their own merits and limitations. 

There are two forms of cathodic protection, impressed current and sacrificial. The impressed current system has been described above and is the system conventionally used for atmospherically exposed reinforced concrete structures. An alternative method is to directly connect the steel to a sacrificial or galvanic anode such as zinc without using a power supply. This anode corrodes preferentially, liberating electrons with the same effect as the impressed current system, e.g. 

Woody, C. L. (1976). Long-term experience with galvanic anodes in a city distribution system. Corrosion 76, Houston, TX, NACE, Paper 160, II pp. 

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