Sacrificial anode number, anodes

Sacrificial anodes Small land based schemes and for avoidance of interaction problems. Marine structures, e.g. offshore platforms High soil/water resistivities and small driving e.m.f. may require a large number of anodes Reasonably uniform Cannot be applied in high-resistivity environments... 

Figures 10.29a and b give an indication of the relative numbers of anodes that may be involved for sacrificial anode and impressed-current systems.

FIGURE 12.20 In the cathodic protection of a buried pipeline or other large metal construction, the artifact is connected to a number of buried blocks of metal, such as magnesium or zinc. The sacrificial anodes (the magnesium block in this illustration) supply electrons to the pipeline (the cathode of the cell), thereby preserving it from oxidation. 

The largest share of CP market is taken up by sacrificial anodes at 60 million of which magnesium has the greatest market share. The costs of installation of a CP system vary to a significant extent depending on the location and specific details of the construction. The range of cost for labor, materials, and the number of installations for various systems in 1998 are given in Table 4.14. 

For the application of cathodic protection to structures to be protected, the initial considerations are best made at the early design and preconstruction phase of the structure. For underground structures, it may be necessary to visit the proposed site, or for pipelines the proposed route, to obtain additional information on low-resistivity areas, availability of electric power, and the existence of stray dc current or other possible interactions. Other considerations will include fundamental design decisions to select the type of system and the most suitable type of anode appropriate to that system. In addition, it will be a requirement to determine the size and number of the power sources or sacrificial anodes and their distribution on the structure. Other factors that must be considered to ensure that cathodic protection is applied in the most economic and reliable manner are given as follows. 

Sacrificial-anode-type cathodic protection systems have a number of advantages ... 

There are a number of elements and their alloys vhich are more active than steel in the electrochemical series. The main practical metals are zinc, aluminium and magnesium. All of these metals are used, often in alloyed form, as sacrificial anodes for steel structures in water. Zinc and aluminium alloys are being used in experimental SACP systems (Whiting et si, 1995). An experimental system using an expanded aluminium mesh anode is shown in Figure 6.4. 

Anode Resistance. The impressed-current system permits variability of the current out and monitor-control instmmentation. However, interactions with nearby stmctures may cause detrimental results. On the other hand, sacrificial anode systems do not require a power source and are easily installed, but large numbers of anodes are needed to protect stmctures. In addition, offshore stmctures are cathodicaUy protected by hybrid systems [13]. 

In the former, a deliberate electrolysis cell is set up between the structure to be protected and a number of strategically positioned anodes of another mital. The essential property of a sacrificial anode is its ability to dissolve freely at a reasonably uniform rate at a potential negative to the corrosion potential of the metal to be protected in order to provide a consistent and sufficiently high protective current to the steel. Figure 10,27(a) illustrates the principle of cathodic protection using sacrificial anodes. The dissolution of the auxiliary metal will cause the equilibrium potential of the metal to be protected to shift to a more negative value. While both metals have almost the same potential (there will be an iR drop between sites) the auxiliary metal will function as an anode while the surface to be protected will become cathodic. It is apparent that the overall rate of metal loss will increase but it is the auxiliary metal that dissolves the rate of metal loss from the protected surface decreases hence, the term sacrificial anode. 

Practice has shown that in systems with high potential (magnesium) sacrificial anodes, the flow of polarizing current is mainly controlled by the number and dimensions of the anodes, while in the case of the application of low potential sacrificial anodes (zinc and aluminum), the type and state of the cathode (structure) is the controlling factor. This means that doubling of... 

Cathodic protection can be applied by connecting sacrificial anodes to a structure. Basically, the principle is to create a galvanic cell, with the anode representing the less noble material that is consumed in the galvanic interaction (Fig. 11.5). Ideally, the structure will be protected as a result of the galvanic current flow. In practical applications a number of anodes usually have to be attached to a structure to ensure overall protection levels. The following advantages are associated with sacrificial anode CP systems ... 

The design of CP systems lies in the domain of experienced speciabsts. Only the basic steps involved in designing a sacrificial anode system are outlined. Prior to any detailed design work a number of fundamental factors such as the protection criteria, the type and integrity of the coating system, the risk of stray current corrosion, and the presence of neighboring structures that could be affected by the CP system have to be defined. 

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