Sacrificial anode limitations

Galvanic corrosion can be controlled by the use of sacrificial anodes. This is a common method of controlling corrosion in heat exchangers with Admiralty tube bundles and carbon steel tube sheets and channel heads. The anodes are bolted direcdy to the steel and protect a limited area around the anode. Proper placement of sacrificial anodes is a precise science. 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

The proximity of the anodes to structures is also important. For example, if the sacrificial anodes are placed on, or very close to, steel pipework in soil then the output from the face of the anodes next to the steelwork can be severely limited. Alternatively, in high conductivity environments, corrosion products may build up and wedge between the anode and the structure. The resulting stresses can lead to mechanical failure of the anode. On the other hand, when anodes are located at an appreciable distance from the steelwork, part of the potential difference will be consumed in overcoming the environmental resistance between the anode and cathode. 

The reason for the use of zinc as a power-impressed rather than a sacrificial anode is that the high concrete resistivity limits the current output, and a higher driving voltage than that provided by the e.m.f. between zinc and steel in concrete is used to provide the necessary current output. No cementitious overlay is required, although it may be advisable to paint the top surface of the sprayed zinc to prevent atmospheric corrosion of the zinc anode. 

It should be noted that when metals like zinc and aluminium are used as sacrificial anodes the anode reaction will be predominantly 10.18a and 10.186, although self-corrosion may also occur to a greater or lesser extent. Whereas the e.m.f. between magnesium, the most negative sacrificial anode, and iron is =0-7 V, the e.m.f. of power-impressed systems can range from 6 V to 50 V or more, depending on the power source employed. Thus, whereas sacrificial anodes are normally restricted to environments having a resistivity of <6 000 0 cm there is no similar limitation in the use of power-impressed systems. 

A typical soil resistivity survey is shown in Fig. 10.22. Soil resistivities will normally indicate whether a cathodic-protection system is advisable in principle and whether impressed current or sacrificial anode schemes in particular are preferable. It may, as a result of the survey, be considered desirable to apply protection to the whole line or to limit protection to certain areas of low soil resistivity or hot spots . 

The use of sacrificial anodes in circulating water systems is limited to the application of cathodic protection to stop gates, coarse screens and other plant that are readily accessible so that the anodes can be replaced when they are consumed. Such anodes are not normally used in condensers, pumps and auxiliary coolers for the following reasons ... 

Sacrificial anodes are of limited application due to accumulation of anode corrosion product and also in many cases, to high water resistivity. 

The best way to overcome the limitations of cathodic protection related to changes in the environment is to monitor the potential and adjust the cathodic currents accordingly. This cannot be readily done with sacrificial anodes, and the method of impressed-current cathodic protection is sometimes preferred, in spite of its higher cost. 

The most severe limitation of the sacrificial anode CP technique is the small driving force, which restricts its use to conductive environments, short current throws and, marine use apart from, wellpower supply, the danger of overprotection, the difficulty of achieving a satisfactory potential profile over a complex shape and the possibility of improper connection causing corrosion of the structure intended to be protected are the major disadvantages of the impressed current CP technique. 

For pipe systems of stainless steel carrying chlorinated seawater, internal localized corrosion can be very efficiently prevented by the application of Resistance-controlled Cathodic Protection (RCP). A resistance is simply inserted between the sacrificial anode and the pipeline, and this makes a system that is particularly suitable when there is a low diffusion-limiting cathodic current in the critical potential range [10.29]. Typical of the method is that the current output from the anode is kept low, which has the consequence that the voltage drops are low and the protected pipe length from each anode is long. 

The two principal alloys of aluminum usually employed as sacrificial anodes are Al-Zn-Hg and Al-Zn-I. These are exclusively used in seawater, which is a major disadvantage. As well, they spark when struck with rusting iron and therefore may not be very useful in the petroleum industries. In addition, these alloys passivate when operated in the cold. This problem is worse in muddy environments. Another limitation to their usage is that they produce poisonous dissolution products. 

Sacrificial-anode-type systems also have disadvantages that limit their application ... 

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. 

It is possible to prevent the corrosion of a metal by connecting it to a more active metal. This active metal becomes anodic and tends to corrode, whereas the cathodic metal is preserved. Iron pipes in soil or water will not corrode if they are connected to a sacrificial anode such as aluminum, zinc, or magnesium. Steel pipes for water and gas are usually protected in this manner. Galvanized iron pipes for hot water lines have a limited life which can be extended by introducing a magnesium rod to act as a sacrificial anode... 

The basic disadvantage of sacrificial protection is the irreversible loss of anode material and the resulting need of its replacement in addition, corrosion products of the anode can pollute the environment. Also, the range of application of galvanic anodes is limited by the resistivity (the specific resistance) of the environment and relatively small values of the protective current. A schematic diagram of sacrificial anode protection is presented in Fig. 8-23. 

Other methods of preparing tertiary bismuthines have been used only to a limited extent. These methods iaclude the electrolysis of organometaUic compounds at a sacrificial bismuth anode (54), the reaction between a sodium—bismuth or potassium—bismuth alloy and an alkyl or aryl haUde (55), the thermal elimination of sulfur dioxide from tris(arenesulfiaato)bismuthines (56), and the iateraction of ketene and a ttis(dialkylainino)bismuthine (57). 

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