Sacrificial anode requirements

Sacrificial Anode Tedmique (SAT) It can also be used to protect an uncoated (bare) stmctures, seagoing vessels, offshore tanks, offshore platforms, water heater, and the like. In fact, a cathodic protection design using sacrificial anodes requires several anodes. These anodes must be monitored frequently because they dissolve sacrificiaUy (purposely) and eventually are reduced in size and become incapable of delivering the necessary current to the stmcture. 

In addition, cathodic protection based on sacrificial anodes requires a cell potential as the driving force for a self-inposed spontaneous protective current imparted by sacrificial anodes liberating electrons at a specific rate [42]. The driving force for this current is the potential difference (overpotential) between the sacrificial anode and cathode that is, E = Ec + o.In fact, the resultant... 

Examples of the sacrificial-anode method include the use of zinc, magnesium, or aluminum as anodes in electrical contact with the metal to be protected. These may be anodes buried in the ground for protection of underground pipe lines or attachments to the surfaces of equipment such as condenser water boxes or on ship hulls. The current required is generated in this method by corrosion of the sacrificial-anode material. In the case of the impressed emf, the direct current is provided by external sources and is passed through the system by use of essentially nonsacrificial anodes such as carbon, noncor-rodible alloys, or platinum buried in the ground or suspended in the electrolyte in the case of aqueous systems. 

Naturally, because the protection depends on the dissolution of the anodes, these require replacement from time to time (hence the term sacrificial anodes). In order to minimise the loss of anode metal, it is important to have as good a barrier layer around the pipe as possible, even though the pipe would still be protected with no barrier layer at all. 

Sacrificial anode systems operate without external power source. The anodes are reactive metals such as magnesium and zinc or aluminum alloys. The energy for the process is derived from the anode material. Careful design is required to match the output and lifetime of the anodes with the polarization and life-expectancy requirements of the plant. Sacrificial anode CP is used for offshore platforms, sub-sea pipelines and the inside of ballast tanks on tanker ships. 

By contrast a cathodic protection system based on sacrificial anodes is designed from the outset to achieve the required protection potential. If this is not achieved in practice there is no control function that can be exercised to improve the situation. Some remodelling of the system will be required. Moreover, the currents from each current source (the sacrificial anodes) is modest so that field gradients in the environment are not significant. It is at once clear that potential measurements are less significant in this case and instant-off measurements are neither necessary nor possible. 

Metallurgical and Inspection Requirements for Cast Sacrificial Anodes for Offshore Applications, RP-0387-87, NACE, Houston (1987)... 

The fundamental requirements of a sacrificial anode are to impart sufficient cathodic protection to a structure economically and predictably over a defined period, and to eliminate, or reduce to an acceptable level, corrosion that would otherwise take place. 

The application of sacrificial anodes for the protection of structures requires the development of suitable anode materials for the exposure environment. Screening tests enable the rapid selection of materials which show potential as candidates for the given application. These tests may typically use a single parameter (e.g. operating potential at a defined constant current density) as a pass/fail criterion and are normally of short duration (usually hours) with test specimen weights of the order of hundreds of grams. The tests are not intended to simulate field conditions precisely. 

New combined (or binary) alloy sacrificial anodes have been developed . An aluminium anode, for example, might have attached to it a short-life supplementary magnesium anode, or anodes, for quick polarisation of the structure. The overall reduction in structure current requirements is claimed to result in an anode weight saving of 35-50% . 

The latter part of this chapter has dealt with the design considerations for a sacrificial anode cathodic protection system. It has outlined the important parameters and how each contributes to the overall design. This is only an introduction and guide to the basic principles cathodic protection design using sacrificial anodes and should be viewed as such. In practice the design of these systems can be complex and can require experienced personnel. 

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. 

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... 

Cathodic protection equipment has been used very successfully in water tanks and HW and steam boilers as anticorrosion devices for 100 years or more. Such equipment comes in many shapes and sizes, and comprises a sacrificial anode of either zinc or magnesium alloy, either bolted directly to a suitable internal water-wetted (cathodic) metal surface, or self-contained by enclosing the anode with a suitable cathode (such as a silver plated base metal). Usually several devices are required for any boiler, more for larger units and less for smaller ones, and these require replacement every one to two years. 

In an undivided cell, the sacrificial anode is used so that it is oxidized in preference to the silyl and chloride ions. There is thus a requirement for the reduction potential of the sacrificial anode to be more negative than the reduction potential of the silyl chlorides, Eesich that is, E°m < E°sici (Equation (3) and Equation (4)) ... 

Anode Materials General Requirements A major problem and thus a decisive factor for the choice of anode materials is corrosion, except when the dissolution of a metal is the desired reaction ( sacrificial anodes , see Sect. 2.4.1.2.4). The stability of anode materials is extremely dependent on the composition of the anolyte (e.g. pH value, aqueous or non-aqueous medium, temperature, presence of halogenides, etc.). 

This sacrificial anode process compares favorably with Wakselman s classical Barbier procedure which requires activated zinc powder, and CF3Br under pressure, leading to lower yields with benzaldehyde (52% in DMF)21. These good results compared to those obtained from a mixture of CF3ZnBr and (CF3)2Zn allow one to illustrate the formation of a transient organozinc compound of the type CF3—Zn , which would be the really active species. This prompts the following question does the reduction of Zn2+ in Zn... 

If sacrificial anodes are used in condenser water boxes, it is desirable to select a type that will last for several years. Pure zinc or specially formulated zinc anodes have regained favor in recent years with some operators. High-purity zinc is said to be self-regulating and does not require a resistance to control the current. In some cases, the tube sheet and tube ends also are coated to reduce the current drain on the anodes. 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

Cathodic protection. Another way of keeping metal in a cathodic state is to connect it to an external direct current source. For smaller structures such as boats and domestic water water heaters the current is supplied by a sacrificial anode made of aluminum or zinc. For larger extended structures such as piers and buried pipelines, an external line-operated or photovoltaic power supply is commonly used. (All interstate oil piplines in the U.S. are required by law to employ cathodic protections.)... 

Cathodic protection has many applications, e.g. in refineries, power stations, gas, water, and oil utilities on marine structures, e.g. jetties, piers, locks, offshore platforms, pipelines, ships hulls, etc. and on land structures, e.g. buried pipeline, storage tanks, cables, etc. For each use, the cathodic protection system requires careful design, either impressed current, sacrificial anodes, or a combination of both may be chosen. There may also be other protection systems, e.g. paint, the nature of which will affect the design parameters and must be taken into consideration. 

Both approaches as well as the Yajima route require the creation of silicon-silicon bonds involving the polycondensation of (di)chlorosilanes by refluxing in toluene or xylene in the presence of sodium. Since metals different from the alkali ones are not reactive or require the use of special conditions, it was decided to investigate a simple and practical electrochemical way, involving the use of an undivided cell, a sacrificial anode, and a constant current density [5]. 

The sacrificial anode technique appears to be competitive with regard to the other electrochemical methods [19] to create the silicon-silicon bond and is very promising for the replacement of alkali metals in many syntheses, especially when high selectivity is required. 

Impressed-currenl cathodic protection requires a little more sophistication than the use of sacrificial anodes, but it also lends itself to periodic adjustment and provides higher flexibility, particularly when structures having rather intricate shapes are considered. 

Sacrificial anodes Independent of external supply of electric power. Simple to install, simple to augment if initial protection proves inadequate. Cannot be incorrectly attached to the structure, does not require control. Difficult to overprotect. Small driving force may restrict application. 

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