Corrosion sacrificial anodes

The corrosion, or rusting, of iron is an example of a naturally occurring voltaic cell. To prevent corrosion, sacrificial anodes are sometimes attached to rust-susceptible iron. Sacrificial anodes must... 

Galvanic corrosion can be controlled by the use of sacrificial anodes. This is a common method of controlling corrosion in heat exchangers with Admiralty tube bundles and carbon steel tube sheets and channel heads. The anodes are bolted direcdy to the steel and protect a limited area around the anode. Proper placement of sacrificial anodes is a precise science. 

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. 

Applied current devices as well as sacrificial anodes have frequently been used to decrease corrosion associated with deposition. The effec-... 

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. 

Cathodic protection with a sacrificial anode that is less noble than either member of the couple is frequently used to reduce the severity of bimetallic corrosion, particularly that resulting from the use of bronze propellers in steel ship hulls. 

Corrosion in these areas is sometimes effectively controlled by cathodic protection with zinc- or aluminium-alloy sacrificial anodes in the form of a ring fixed in good electrical contact with the steel adjacent to the non-ferrous component. This often proves only partially successful, however, and it also presents a possible danger since the corrosion of the anode may allow pieces to become detached which can damage the main circulating-pump impeller. Cladding by corrosion-resistant overlays such as cupronickel or nickel-base alloys may be an effective solution in difficult installational circumstances. 

Copper-base alloys will corrode in aerated conditions. It is, therefore, sometimes appropriate to consider cathodic protection. It becomes particularly relevant when the flow rates are high or when the design of an item causes the copper to be an anode in a galvanic cell (e.g. a copper alloy tube plate in a titanium-tubed heat exchanger). Corrosion can be controlled by polarisation to approximately — 0-6V (vs. CU/CUSO4) and may be achieved using soft iron sacrificial anodes. 

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. 

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. 

The proximity of the anodes to structures is also important. For example, if the sacrificial anodes are placed on, or very close to, steel pipework in soil then the output from the face of the anodes next to the steelwork can be severely limited. Alternatively, in high conductivity environments, corrosion products may build up and wedge between the anode and the structure. The resulting stresses can lead to mechanical failure of the anode. On the other hand, when anodes are located at an appreciable distance from the steelwork, part of the potential difference will be consumed in overcoming the environmental resistance between the anode and cathode. 

For use in high resistivity soils, the most common mixture is 75% gypsum, 20% bentonite and 5% sodium sulphate. This has a resistivity of approximately 50 ohm cm when saturated with moisture. It is important to realise that carbonaceous backfills are relevant to impressed current anode systems and must not be used with sacrificial anodes. A carbonaceous backfill is an electronic conductor and noble to both sacrificial anodes and steel. A galvanic cell would therefore be created causing enhanced dissolution of the anode, and eventually corrosion of the structure. 

Klinghoffer, O. and Linder, B., A New High Performance Aluminium Anode Alloy with High Iron Content , Paper No. 59, Corrosion/87, San Francisco, USA, March (1987) Crundwell, R. F., Sacrificial Anodes — Old and New . In Cathodic Protection Theory and Practice, 2nd International Conference, Stratford upon Avon, June (1989)... 

Schreiber, C. F. and Murray, R. W., Effect of Hostile Marine Environment on the Al-Zn-In-Si Sacrificial Anode , Paper 32, Corrosion/88, St. Louis, USA, March (1988)... 

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. 

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. 

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. 

Sacrificial anodes are of limited application due to accumulation of anode corrosion product and also in many cases, to high water resistivity. 

For a hot water environment general guidance can be given for the desirable properties in good enamels. Five factors affecting enamel life are corrosiveness of contact liquor, design, operating conditions, life of sacrificial anode (if any) and the durability of the enamel coat. This implies that... 

Foods such as meat, fish, and some vegetables contain sulfur-bearing amino acids that form volatile sulfur compounds during processing and storage. When these compounds react with iron, a black precipitate forms on the container and in most instances darkens the food. A small piece of aluminum welded to the tinplate can has been used to prevent container corrosion and sulfide staining in commercially canned hams. In this case, the aluminum acts as a sacrificial anode and stops the reaction with tin and iron that otherwise could occur at the small exposed tinplate areas (14). 

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. 

The calculation shows that zinc is oxidized preferentially over iron. Later in this chapter we describe the use of zinc as a sacrificial anode to prevent corrosion of iron. 

Corrosion 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. 

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.). 

Another way of preventing iron corrosion is to use a sacrificial anode, a process which involves placing the iron with another metal that oxidizes even faster. 

The use of sacrificial anodes for protection of the heads of the heat exchanger may not be so effective in brackish water. The water may be very corrosive because of pollution or contamination, even when the salinity is relatively low. More anodes, with closer spacing, normally are needed to give sufficient protection in brackish water than in sea water. 

Table 5.10 lists the metals in order of their position in the galvanic series. The more positive or anodic metals are more active and prone to corrosion. In some installations, to prevent corrosion of a specific metal member, sacrificial anodes are installed in the ground. The sacrificial anodes are more electropositive than the metals they are protecting, so they are sacrificed to protect the structural steel. 

When it is desirable to prevent corrosion of iron, a new anode, such as a zinc (Zn) or magnesium (Mg) strip, is connected to the surface of the iron. These metals are stronger reducing agents than iron and will be more easily oxidized. As oxidation occurs, the zinc or magnesium, rather than the iron, will furnish electrons. These sacrificial anodes will erode instead of the iron. They must be replaced periodically, but the iron will remain intact. (See Figure 6.12.)... 

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