Cathodic protection consumable anodes

Magnesium anodes suspended inside a galvanised hot-water tank and in electrical connection with it afford cathodic protection to the zinc, the alloy layer and the steel, at high temperatures as well as in the cold. The magnesium is eventually consumed but it is probable that in the interim a good protective scale will have formed on the inside of the tank, so that the magnesium anode will then no longer be necessary. One of the difficulties of this method, however, is the maintenance of a sufficiently even current distribution over the inside of a tank to protect the whole surface, especially in waters of low conductivity. The method is therefore unlikely to be applicable to soft waters. 

By contrast, if additional electrons were introduced at the metal surface, the cathodic reaction would speed up (to consume the electrons) and the anodic reaction would be inhibited metal dissolution would be slowed down. This is the basis of cathodic protection. 

It will be seen that the impressed current electrode discharges positive current, i.e. it acts as an anode in the cell. There are three generic types of anode used in cathodic protection, viz, consumable, non-consumable and semi-consumable. The consumable electrodes undergo an anodic reaction that involves their consumption. Thus an anode made of scrap iron produces positive current by the reaction ... 

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. 

This material can be used only in seawater or similar chloride-containing electrolytes. This is because the passivation of the silver at discontinuities in the platinum is dependent upon the formation of a film of silver chloride, the low solubility of which, in seawater, inhibits corrosion of the silver. This anode, consisting of Pt-lOPd on Ag, was tried as a substitute for rapidly consumed aluminium, for use as a trailing wire anode for the cathodic protection of ships hulls, and has been operated at current densities as high as 1 900 AmHowever, the use of trailing anodes has been found inconvenient with regard to ships manoeuvrability. 

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 ... 

Cathodic protection in the negative potential zone where reduction of oxygen or water commences, and where the rate of metal oxidation is low. In this case there has to be an inert auxiliary electrode close to the surface to be protected. The protection process consumes current, the quantity depending on solution resistance between the surface to be protected and the anode. This protection can be expensive in terms of energy consumption, and even more if there is hydrogen release and, consequently, hydrogen embrittlement. 

Anode, low consumable — This term is used for different anodes that show a low dissolution rate (a) in cathodic protection (-> corrosion, and subentry - corrosion protection) that are anodes that form a protective... 

Applying Le Chatelier s Principle to Equations 10.2 and 10.5 it is to be expected that by electrical intervention in the corrosion reactions, that the rates of the anodic and cathodic reactions could be altered. If electrons are removed from the system the rate of the anodic reaction increases to yield more electrons. At the same time the rate of the cathodic reactions decreases so as to consume fewer electrons. Under these circumstances the iron will corrode more rapidly, the hydrogen production rate will decrease and the electrode potential of the metal will rise. On the other hand if electrons are imposed on the metallic iron from an external source, the cathodic reaction rate will increase and that of the anode will decrease. Under the imposition of this external source of current the iron will dissolve more slowly, the rate of hydrogen production will increase and the electrode potential will fall. The use of an external source of electrical current (electrons) in this way, to reduce or prevent metallic corrosion is generally called cathodic protection. 

Aluminum and aluminum-zinc alloy anodes have become the preferred sacrificial anodes for the cathodic protection of offshore platforms. This preference is because aluminum anodes demonstrate reliable long-term performance when compared with magnesium, which might be consumed before the platform has served its useful hfe. Aluminum also has better current/weight characteristics than zinc. Weight can be a major consideration for large offshore platforms. The major disadvantage of aluminum for some applications, for example, the protection of painted ship hulls, is that aluminum is too corrosion resistant in many environments. Aluminum alloys will not corrode reliably onshore or in freshwater [37]. In marine... 

Sacrificial anode cathodic protection A system of cathodic protection that u,ses a more easily corroded metal such as zinc, aluminium or magnesium to protect a steel object from corrosion. No power supply is required, but the anode q.v.) is consumed,... 

Cathodic protection by impressed current involves the use of a rectifier connected to a power line. Contrary to sacrificial anodes, which operate at a fixed potential, the use of a rectifier permits to adjust the voltage (or the current) to the particular requirements of a protection scheme. This not only allows one to optimize the electrochemical conditions for protection, but the method is also well suited to protect large surfaces. On the other hand, protection by impressed current needs more maintenance than the use of sacrificial anodes. In order to protect buried structures by impressed currents one uses consumable anodes such as scrap iron or, more often, non-consumable anodes made of iron-silicon alloy, graphite or of titanium coated with noble-metal oxides. 

Another approach for reducing corrosion is to employ mechanisms that can modify the electrochemical processes that consume materials. Cathodic protection, either through the use of sacrificial anodes or an impressed current system, can convert a material that normally will corrode quite readily into a material that resists corrosion. This approach, which is the topic of Chap. 13, works very well for protecting fixed assets in contact with potentially corrosive environments such as soils, seawater, or any other electrolytically conducting medium. 

The designing of cathodic protection systems is rather complex, however, it is based on simple electrochemical principles described earlier in Chapter 2. Corrosion current flows between the local action anodes and cathodes due to the existence of a potential difference between the two (Fig. 5.1). As shown in Fig. 5.2, electrons released in an anodic reaction are consumed in the cathodic reaction. If we supply additional electrons to a metallic structure, more electrons would be available for a cathodic reaction which would cause the rate of cathodic reaction to increase and that of anodic reaction to decrease, which would eventually minimize or eliminate corrosion. This is basically the objective of cathodic protection. The additional electrons are supplied by direct electric current. On application of direct current, the potential of the cathode shifts to the potential of the anodic area. If sufficient direct current is applied, the potential difference between the anode and cathode is eliminated and corrosion would eventually cease to occur. 

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