Voltaic cells corrosion

Corrosion An Environmental Voltaic Cell Corrosion of Iron... 

Although corrosion is favored by a large difference of potential between the anodic and cathodic portions of a system, even the smallest of such differences is sufficient to stimulate corrosion in the presence of a depolarizer. In an apparently uniform piece of metal, any portion which has been subjected to strain is less noble than an unstrained portion and small crystals are less noble than large ones further, minute inclusions of noble material are often found in relatively pure metals. These differences permit local voltaic cells to be set up and, in the presence of a depolarizer, corrosion of the baser (anodic) regions will occur. 

Corrosion occurs when air, water, and iron set up a voltaic cell similar to the conditions shown at the surface of this iron I-beam. 

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... 

By now, you may be thinking that spontaneous electrochemical processes are always beneficial, but consider the problem of corrosion, the natural redox process that oxidizes metals to their oxides and sulfides. In chemical teims, coiTOsion is the reverse of isolating a metal from its oxide or sulfide ore in electrochemical terms, the process shares many similarities with the operation of a voltaic cell. Damage from corrosion to cars, ships, buildings, and bridges runs into tens of billions of dollars annually, so it is a major problem in much of the world. We focus here on the corrosion of iron, but many other metals, such as copper and silver, also conode. 

The most common and economically destructive form of corrosion is the rusting of iron. About 25% of the steel produced in the United States is made just to replace steel already in use that has corroded. Contrary to the simplified equation shown earlier in the text, rust is not a direct product of the reaction between iron and oxygen but arises through a complex electrochemical process. Let s look at the facts of iron corrosion and then use the features of a voltaic cell to explain them ... 

The components of the corrosion process resemble those of a voltaic cell ... 

Galvanization protects iron in two ways. As long as the zinc layer is intact, water and oxygen cannot reach the irons surface. Inevitably, the zinc coating cracks. When this happens, zinc protects iron from rapid corrosion by becoming the anode of the voltaic cell set up when water and oxygen contact iron and zinc at the same time. Figure 20.18 illustrates how these two forms of corrosion protection work. 

Rusting of iron requires both oxygen and water, and the process can be accelerated by other factors such as pH, presence of salts, contact with metals more difficult to oxidize than iron, and stress on the iron. The corrosion process involves oxidation and reduction, and the metal conducts electricity. Thus, electrons can move through the metal from a region where oxidation occurs to a region where reduction occurs, as in voltaic cells. Because the standard reduction potential for reduction of e (aq) is less positive than that for reduction of O2, Fe(s) can be oxidized by 02(g) ... 

Because the cathode is generally the area having the largest supply of O2, rust often deposits there. If you look closely at a shovel after it has stood outside in the moist air with wet dirt adhered to its blade, you may notice that pitting has occurred under the dirt but that rust has formed elsewhere, where O2 is more readily available. The enhanced corrosion caused by the presence of salts is usually evident on autos in areas where roads are heavily salted during winter. Like a salt bridge in a voltaic cell, the ions of the salt provide the electrolyte necessary to complete the electrical circuit. 

The corrosion of iron is electrochemical in nature. Not only does the corrosion process involve oxidation and reduction, the metal itself conducts electricity. Thus, electrons can move tiirough the metal from a region where oxidation occurs to another region where reduction occurs, as in voltaic cells. 

The enhanced corrosion caused by the presence of salts is usually evident on autos in areas where roads are heavily salted during winter. Like a salt bridge in a voltaic cell, the ions of the salt provide the electrolyte necessary to complete the electrical circuit. 

Analyze We are asked to describe how the corrosion occurs when Fe is in contact with Al. Plan A voltaic cell can be formed at the point of contact of the two metals. The metal that is more easily oxidized will serve as the anode and thereby undergo corrosion. The other metal will serve as the cathode. To determine which metal is oxidized most readily, we must compare their standard reduction potentials from Table 20.1 or Appendix E. Solve The standard reduction potentials of Fe and Al are as follows ... 

Corrosion is a natural, spontaneous, electrochemical process with similarities to the operation of a voltaic cell. It is a major economic problem because the anode is typically a metal tool or structure. (Section 21.6)... 

Another use of voltaic cells is to control the corrosion of underground pipelines and tanks. Such pipelines and tanks are usually made of steel, an alloy of iron, and their corrosion or rusting is an electrochemical process. 

Corrosion A redox reaction of a metal with its enviromnent that results in the deterioration of the metal is called corrosion. The most common example is the rusting of iron, which occurs when air, water, and iron set up a natural voltaic cell that causes iron atoms to lose electrons. In the overall cell reaction, iron is oxidized and oxygen is reduced. 

Corrosion Unwanted Voltaic Cells 19-7 Electrolysis Causing... 

The reactions occurring in voltaic cells (batteries) are important sources of electricity, but similar reactions also underlie corrosion processes. First, we will consider the electrochemical basis of corrosion, and then we will see how electrochemical principles can be applied to control corrosion. 

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