Applications sacrificial anodes

It is interesting that the first large-scale application of cathodic protection by Davy was directed at protecting copper rather than steel. It is also a measure of Davy s grasp of the topic that he was able to consider the use of two techniques of cathodic protection, viz. sacrificial anodes and impressed current, and two types of sacrificial anode, viz. zinc and cast iron. 

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

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. 

The forms of corrosion which can be controlled by cathodic protection include all forms of general corrosion, pitting corrosion, graphitic corrosion, crevice corrosion, stress-corrosion cracking, corrosion fatigue, cavitation corrosion, bacterial corrosion, etc. This section deals exclusively with the practical application of cathodic protection principally using the impressed-current method. The application of cathodic protection using sacrificial anodes is dealt with in Section 10.2. 

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. 

As the general standard of protective coatings has improved (materials, application and construction standards) the use of sacrificial anodes has extended to soil of ca. 10 000 ohm cm or more for buried pipelines. By using anodes in extruded ribbon form and placing them as close as possible to the structure to be protected they can be used in soils of high resistivity. 

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. 

Electrocatalysis with nickel-bpy complexes has been shown useful for synthetic applications,202,211 especially when used in combination with the sacrificial anode process in an undivided cell (Equation (45)).207,211 Under these very simple experimental conditions, efficient nickel catalysts can be also generated in the presence of the cheap pyridine ligand.212... 

If possible, the cell should be undivided to minimize the construction cost and also the energy consumption (see goal 1). The application of a controlled reaction at the auxiliary electrode taking place at low potential allows for the use of undivided cells in many cases. For oxidations, the cathodic process at the auxiliary electrode may be a proton reduction under formation of hydrogen. For reductions, the anodic process may be the oxidation of formate or oxalate under production of carbon dioxide [68] or the dissolution of sacrificial anodes [69] (see also Sec. V.B). 

One example of the application of polarization curves in a predictive manner involves their use in galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in electrical and ionic contact as is schematically shown in Fig. 29. Galvanic corrosion is used to advantage in sacrificial anodes of zinc in seawater and magnesium in home water heaters. It slows corrosion of millions of tons of structural materials. The darker side of galvanic corrosion is that it also causes major failures by the accelerated dissolution of materials that are accidentally linked electrically to more noble materials. 

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. 

Application of zinc as a sacrificial anode often overcomes the difficulties with activation of zinc in the Reformatsky reaction and the problems with uncontrolled exothermic reactions. When zinc or indium is employed for the reaction, the amount of electricity used is less than the theoretical, and this suggests that the anode acts as activated metal and reacts with the bromo compound [175]. 

Selective mono- and disilylation of polychloromethanes such as carbon tetrachloride and chloroform has been achieved using Zn and Mg sacrificial anodes [Eqs. (13) and (14)] [63], The product selectivity seems to depend on the electrode material. The application of this method for the reduction of ClCH2SiMe2Cl leads to the formation of polycarbo-silanes [64]. 

The electrochemical reduction of chlorosilanes has received significant research interest because this reaction provides an efficient route to disilanes and polysilanes having Si-Si bonds [Eq. (46) [162, 166-176]. The electrode material and the supporting electrolyte seem to be important factors for the effective homo coupling. The sacrificial metal anodes have been found to be quite effective for the reductive coupling of chlorosilanes, and extensive work has been devoted to this field. In fact, various sacrificial anodes such as Hg [167,171], Mg [172], Cu [173,174], Ag [171], and A1 [174-176] were found to be effective for the reductive coupling of chlorosilanes to form disilanes. The application of this method to dichlorosilanes gives rise to the formation of polysilanes [Eq. (47)]. 

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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