Metals sacrificial anode

An alternative synthetic route to silylated thiophene and oligomers has recently been proposed from chloro- and bromothiophene derivatives. This involves an electrochemical technique using a metallic sacrificial anode and provides a selective synthesis of silyl derivatives [15,16]. The bromothiophenes are more easily reduced... 

Two methods of providing cathodic protection for minimizing corrosion of metals are in use today. These are the sacrificial-anode method and the impressed-emf method. Both depend upon making the metal to be protected the cathode in the electrolyte involved. 

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. 

Note that zinc anodes are often used to protect steel and other relatively noble metals cathodically. In this case, the fasteners were acting as unintentional sacrificial anodes, protecting the stainless steel. Simple solutions to the problem would be to insulate the fasteners from the stainless steel electrically or to use stainless steel fasteners. 

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. 

With metals other than Fe, the percent of the ac current leading to corrosion can be considerably different. Cu and Pb behave similarly to Fe [36], whereas A1 [36] and Mg [39] corrode much more severely. This has to be watched with sacrificial anodes of these materials if they are subjected to ac. 

CoiTosion prevention is achieved by correct choice of material of construction, by physical means (e.g. paints or metallic, porcelain, plastic or enamel linings or coatings) or by chemical means (e.g. alloying or coating). Some metals, e.g. aluminium, are rendered passive by the formation of an inert protective film. Alternatively a metal to be protected may be linked electrically to a more easily corroded metal, e.g. magnesium, to serve as a sacrificial anode. 

Sacrificial anode system (reactive metal anode)... 

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. 

This is utilised in the cathodic protection of metals using sacrificial anodes (see Section 10.2). 

The electrochemical properties of zinc also have a large bearing on its corrosion behaviour. Zinc is negative to Eh /h2 and magnesium and aluminium excepted, to most other metals commonly encountered, including those found in the less pure forms of zinc. This means that when zinc is in contact with these metals sacrificial electrochemical action can take place, with zinc forming the anode. Contact with other metals and impurities can... 

In practice pure metals are never used as sacrificial anodes. There are a variety of reasons for this which include the need for ... 

When two different metals are immersed in the same electrolyte solution they will usually exhibit different electrode potentials. If they are then connected by an electronic conductor there will be a tendency for the potentials of the two metals to move towards one another they are said to mutually polarise. The polarisation will be accompanied by a flow of ionic current through the solution from the more negative metal (the anode) to the more positive metal (the cathode), and electrons will be transferred through the conductor from the anode to the cathode. Thus the cathode will benefit from the supply of electrons, in that it will dissolve at a reduced rate. It is said to be cathodically protected . Conversely, in supplying electrons to the cathode the anode will be consumed more rapidly, and thus will act as a sacrificial anode. 

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. 

In practice, with one minor exception (pure zinc), the commercially pure metals are unsuitable as sacrificial anode materials. This is because they fail to meet one or more of the pre-requisites outlined above. In each generic type of material alloying elements are added to ensure more acceptable properties. 

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. 

Reactive (active, sacrificial) Anode a mass of metal (Mg, Zn, Al) which, buried or immersed and connected to a metallic structure which is to be protected, forms a cell with that structure and has the effect of making it more negative with respect to the surrounding environment. 

Environment Reduce kinetics of cathodic reaction Lower potential of metal Cathodic inhibition Reduce a , reduce O2 concentration or concentration of oxidising species lower temperature, velocity agitation Cathodically protect by sacrificial anodes or impressed current sacrificially protect by coatings, e.g. Zn, Al or Cd on steel Formation of calcareous scales in waters due to increase in pH additions of poisons (As, Bi, Sb) and organic inhibitors to acids... 

Aluminum Foil. Studies of various foods wrapped in aluminum foil show that food products to which aluminum offers only fair resistance cause little or no corrosion when the foil is in contact with a nonmetallic object (glass, plastic, ceramic, etc.) The reactions, when found, are essentially chemical, and the effect on the foil is insignificant. However, when the same foods are wrapped or covered with foil that is in contact with another metallic object (steel, tinplate, silver, etc.), an electrochemical or galvanic reaction occurs with aluminum acting as the sacrificial anode. In such cases, there is pitting corrosion of the foil, and the severity of the attack depends primarily on the food composition and the exposure time and temperature. Results obtained with various foods cov-... 

Cathodic protection apparatuses are well proven, widely used devices and are not to be confused with magnetic devices gadgets ) or other similar but generally less than satisfactory items of capital equipment. Cathodic protection devices reverse the tendency of a metal to go into solution at the anode (corrosion) by the application of a counter-potential. This counter-potential or electromotive force (EMF) is provided either from a permanent external source such as a battery or rectifier or from the installation of a sacrificial anode. 

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. 

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 chemical reactivity of metallic Mg has been utilized in several ways. It is employed in the reduction step in the manufacture of Ti, in the deoxidation and desulfurization of steels and in the nodularization of cast iron. It has also been used for the preparation of photoengraving plates, in dry batteries, and as a sacrificial anode for cathodic protection of other metals. 

C19-0132. List all the metals that could be used as sacrificial anodes for iron. Which of these could also be sacrificial anodes for aluminum ... 

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