Sacrificial anode galvanic corrosion

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. 

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

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. 

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. 

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-... 

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. 

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. 

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. 

Anode, sacrificial — a rather easily oxidizable metal, e.g., zinc, magnesium, aluminum, electrically connected with a metal construction to be protected from corrosion. Due to the formation of a -> galvanic cell the sacrificial anode is oxidized instead of the metal to be protected. Sacrificial anodes are the oldest and simplest means for electrochemical corrosion protection. 

Cathodic protection can be viewed as a form of galvanic corrosion, put to good use. In this case an active metal (most often zinc, but under special circumstances magnesium or aluminum) is employed as a sacrificial anode. It is attached to the steel structure being protected in one or several locations and does not constitute part of the... 

Galvanizing helps prevent corrosion in two ways. 0 The zinc coating seals the iron from air and water by forming a barrier of zinc oxide that repels water and oxygen. 0 If the zinc coating breaks, the zinc acts as a sacrificial anode. 0 Metal objects that are left outside are often galvanized to prevent rust and corrosion caused by the elements. 

In Chapter 5, we discussed structural and manufacturing aspects of steel, and the fact that galvanized steel possesses a protective Zn coating. Uses of Zn-coated steel include ships hulls, undersea pipelines and oil-rigs, i.e. structures that are in contact with seawater. In the presence of H2O, O2 and an electrolyte (e.g. seawater), steel is subject to corrosion. There is always the possibility that coated steel will be scratched, and that this surface imperfection will permit rusting of the iron beneath it to occur. The Zn coating, however, behaves as a sacrificial anode. The actual process of corrosion is not simple, but can be summarized as follows ... 

In some instances, galvanic corrosion can be helpful in the plant. For example, if pieces of zinc are attached to the bottom of a steel water tank, the zinc will become the anode, and it will corrode. The steel in the tank becomes the cathode, and it will not be effected by the corrosion. This technique is known as cathodic protection. The metal to be protected is forced to become a cathode, and it will corrode at a much slower rate than the other metal, which is used as a sacrificial anode. 

Galvanic corrosion occurs when dissimilar metals exist at different electrical potentials in the presence of an electrolyte. Galvanic corrosion may be reduced by the careful design and selection of materials regarding dissimilar metals and the use of sacrificial anodes. 

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