Sacrificial anode, corrosion prevention

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. 

Figure 21.22 The use of sacrificial anodes to prevent iron corrosion. In...

Figure 21.23 The use of a sacrificial anode to prevent iron corrosion, in...

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. 

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). 

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. 

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. 

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.)... 

A sacrificial anode is allowed to corrode. Why is use of a sacrificial anode considered to be a way to prevent corrosion ... 

Because corrosion can cause considerable damage, it is important to investigate ways to prevent rust and deterioration. Paint or another protective coating is one way to protect steel structures from corrosion. Q Sacrificial anodes of magnesium or other active metals are also used to prevent corrosion. 

The steel hulls of ships are constantly in contact with saltwater, so the prevention of corrosion is vital. Although the hull may be painted, another method is used to minimize corrosion. Blocks of metals, such as magnesium, aluminum, or titanium, that oxidize more easily than iron are placed in contact with the steel hull. These blocks rather than the iron in the hull become the anode of the corrosion cell. As a result, these blocks, called sacrificial anodes, are corroded while the iron in the hull is spared. Of course, the sacrificial anodes must be replaced before they corrode away completely, leaving the ship s hull unprotected. A similar technique is used to protect iron pipes that are run underground. Magnesium bars are attached to the pipe by wires, and these bars corrode instead of the pipe, as shown in Figure 21-15b. 

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. 

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... 

I Methods of preventing corrosion are painting, coating with another metal, or using a sacrificial anode. 

MP. Pedeferri, E. Redaelli, Prevention of steel corrosion in concrete exposed to seawater with submerged sacrificial anodes , Corro n Science, 2002, 44,... 

G. Sergi, C. L. Page, Sacrificial anodes for cathodic prevention of reinforcing steel around patch repairs applied to chlorideEuropean Federation of Corrosion, Event No. 227, Aachen, 30 August-2 September 1999, 248. 

CP or other protection systems should be incorporated, if necessary, in the design phase of the ship. CP system is a secondary defense against corrosion when holidays or cracks form in the coating. CP systems use either sacrificial zinc anodes or impressed-current systems to mitigate corrosion. Other corrosion prevention equipment and materials are inert gases to drive out corrosive gases. Corrosion inhibitors are also used. 

Sometimes the need to be environmentally acceptable may lead to new problems. For instance, ozone was suggested to replace biocides with no data available on the performance in the chlorination of water (60). Corrosion control techniques can have both favorable as well as ill effects and hence one has to exert balanced judgment before embarking on a corrosion prevention method. Organotin antifouling coatings on ships were effective, but they polluted the seawater and hence were banned from further use. The use of cadmium as a sacrificial anode is restricted because of its toxicity. Large amounts of zinc are used to protect steel platforms in the sheltered and shallow waters of the sea, and the effects of zinc on the contamination of waters are not known. 

Cathodic protection can also be applied to prevent pitting. Regarding aluminium, strong cathodic polarization should be avoided because this can lead to a large increase of pH close to the metal surface, which can cause so-called alkaline corrosion (compare with the Pourbaix diagram for aluminium in Figure 3.11, Section 3.8). Use of sacrificial anodes of Zn or A1 alloys is therefore safer than impressed current. 

For pipe systems of stainless steel carrying chlorinated seawater, internal localized corrosion can be very efficiently prevented by the application of Resistance-controlled Cathodic Protection (RCP). A resistance is simply inserted between the sacrificial anode and the pipeline, and this makes a system that is particularly suitable when there is a low diffusion-limiting cathodic current in the critical potential range [10.29]. Typical of the method is that the current output from the anode is kept low, which has the consequence that the voltage drops are low and the protected pipe length from each anode is long. 

To prevent corrosion, such as rusting, many engineers will add zinc to metal structures to intentionally act as sacrificial anode. When your bicycle starts to rust, the Fe metal of your bike frame is getting oxidized to iron cations while the oxygen in the air and water around your bike frame gets reduced to OH anions. When these iron cations and hydroxide anions meet, rust is produced. If an engineer installs zinc into the system, zinc will be oxidized instead of the iron, which can prevent the formation of rust. Because the anode gets consumed, it is called a sacrificial anode. 

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