The Corrosion of Iron

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  

Iron does not rust in dry air moisture must be present. 

The loss of iron and the depositing of rust often occur at different places on the same object. 

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Reduction of oxygen is one of the predominant cathodic reactions contributing to corrosion. Awareness of the importance of the role of oxygen was developed in the 1920s (19). In classical drop experiments, the corrosion of iron or steel by drops of electrolytes was shown to depend on electrochemical action between the central relatively unaerated area, which becomes anodic and suffers attack, and the peripheral aerated portion, which becomes cathodic and remains unattacked. In 1945 the linear relationship between rate of iron corrosion and oxygen pressure from 0—2.5 MPa (0—25 atm) was shown (20). 

Sulfur dioxide emissions may affect building stone and ferrous and nonferrous metals. Sulfurous acid, formed from the reaction of sulfur dioxide with moisture, accelerates the corrosion of iron, steel, and zinc. Sulfur oxides react with copper to produce the green patina of copper sulfate on the surface of the copper. Acids in the form of gases, aerosols, or precipitation may chemically erode building materials such as marble, limestone, and dolomite. Of particular concern is the chemical erosion of historical monuments and works of art. Sulfurous and sulfuric acids formed from sulfur dioxide and sulfur trioxide when they react with moisture may also damage paper and leather. 

Joseph, G. and Perret, R., Inhibitor Action on the Corrosion of Iron in Salt-water Solution , Corros. ScL, 7, 553 (1%7)... 

The corrosion of iron and steel in soil is generally electrochemical in character but the conditions are such that the corrosion products usually remain in contact with the metal. Moreover, the rate of oxygen supply is often low in comparison with that in air or in water. This is probably the main reason... 

One advantage of steel as an anode is the low gassing at the electrode during operation, since the predominant reaction is the corrosion of iron. Thus, the problem of resistive polarisation due to gas blocking, as may be the case with more inert materials, does not occur. Iron compounds do, of course, form but these do not appreciably affect the anode/soil resistivity. Furthermore, the introduction of metallic ions, by anode corrosion, into the adjacent high resistivity soil is beneficial in lowering the resistivity. 

Hudson, J. C., The Corrosion of Iron and Steel, Chapman and Hall, London, 66 (1940)... 

Adsorbed species may also accelerate the rate of anodic dissolution of metals, as indicated by a decrease in Tafel slope for the reaction. Thus the presence of hydrogen sulphide in acid solutions stimulates the corrosion of iron, and decreases the Tafel slope The reaction path through... 

The corrosion of iron (or steel) can be inhibited by the anions of most weak acids under suitable conditions " . However, other anions, particularly those of strong acids, tend to prevent the action of inhibitive anions and stimulate breakdown of the protective oxide film. Examples of such aggressive anions are the halides, sulphate, nitrate, etc. Brasher has shown that, in general, most anions exhibit some inhibitive and some aggressive behaviour towards iron. The balance between the inhibitive and aggressive properties of a specific anion depends on the following main factors (which are themselves interdependent). 

Nature of the metal surface The critical concentration of an anion required to inhibit the corrosion of iron may increase with increasing surface roughness. Thus, Brasher and Mercer" showed that the minimum concentration of benzoate required to protect a grit-blasted steel surface was about 100 times greater than that required to protect an abraded surface. However, surface preparation had little effect on the critical inhibitive concentrations for chromate" or nitrite " The time of exposure of the iron surface to air after preparation and before immersion may also affect the ease of inhibition by anions. There is evidence """ that the inhibition by anions occurs more readily as the time of pre-exposure to air increases. Similarly, if an iron specimen is immersed for some time in a protective solution of an inhibitive anion, it may then be transferred without loss of inhibition to a solution of the anion containing much less than the critical inhibitive concentration . ... 

The mechanism of action of inhibitive anions on the corrosion of iron, zinc and aluminium in near-neutral solution involves the following important functions ... 

The corrosion of iron is accelerated by the presence of oxygen, moisture, and salt. Corrosion can be inhibited by coating the surface with paint or zinc or by using cathodic protection. 

The corrosion of iron occurs particularly rapidly when an aqueous solution is present. This is because water that contains ions provides an oxidation pathway with an activation energy that is much lower than the activation energy for the direct reaction of iron with oxygen gas. As illustrated schematically in Figure 19-21. oxidation and reduction occur at different locations on the metal surface. In the absence of dissolved ions to act as charge carriers, a complete electrical circuit is missing, so the redox reaction is slow, hi contrast, when dissolved ions are present, such as in salt water and acidic water, corrosion can be quite rapid. 

The best known way of lowering the corrosion of iron is by its alloying with chrominm, nickel, and other metals. The corrosion resistance of the corresponding stainless steels is dne to the fact that chrominm is readily passivated. This is a quality that is fonnd even in alloys with relatively low chromium contents. Hence, stainless steels are practically always strongly passivated, and their spontaneous dissolution rates are very low. 

Thus, in the case of iron coated with zinc (galvanized sheet), zinc would protect iron by sacrificing itself, i.e., by anodically dissolving in the corroding media. However, in the case of iron coated with tin (tinned sheet), tin would protect iron against corrosion by virtue of its own corrosion-resistance properties however, any flaw in the coating would enhance the corrosion of iron since it is anodically disposed to tin according to their placements in the electrochemical series. 

There is some evidence that nickel slag can be involved in the corrosion of iron and steel in the presence of moisture (probably galvanic corrosion). In Canada, where nickel slag is used in fill applications, it is common practice to provide a layer [typically 150 mm (6 in.) thick] of natural aggregate between ferrous materials and the slag.21... 

Fig. 11. RRHSE data for the corrosion of iron in 10 N NaOH at 60 °C. (a) Polarization curve of iron hemisphere, (b) Ring current vs. hemisphere potential. Curve 1 is for the detection of HFeOj ion, and curve 2 is for detecting FeOj ion. From [62],...

The corrosion of iron represents an electrochemical reaction of huge economic significance, accounting literally for billions of dollars of waste every year. The phenomenon has been investigated since the time of Faraday and still presents many controversial and puzzling aspects which only the arrival of in situ spectroscopic techniques has begun to clarify. 

FIGURE 8.7 A representation of the phases present and the motion of ions in the corrosion of iron. (Adapted from Borg and Dienes, 1988, p. 295.)... 

Since the corrosion of iron is such an expensive problem, why do you think that iron is still used for so many purposes ... 

Bacterial H2 metabolism impacts on human activities in other ways. The corrosion of iron, for example, is accelerated by bacteria, particularly sulfate-reducing bacteria. They do not appear to interact with the iron directly, but with the hydrogen that is produced on the surface of iron in contact with water. For this reason antibacterial agents are used in preservative solutions for heating systems. 

The corrosion of iron is one of the most widespread and technologically important examples of metallic corrosion. In the presence of water and oxygen, the corrosion of iron proceeds to form a complicated mixture of hydrated iron oxides and related species a complete description is beyond the scope of the present discussion, and the interested reader is referred to the previously cited general references on corrosion as well as to the well known descriptions of electrochemical equilibria in aqueous solution given by Pourbaix (8, 9,). Iron is a base metal, subject to corrosion in aqueous solutions. In the presence of oxidizing species, iron surfaces can be passivated by the formation of an oxide layer if the oxide layer formed is Imperfect, rapid corrosion may occur. In simplest form, the reaction of iron to form iron oxide can be written as ... 

What iron oxide phase will form when iron corrodes under a particular set of conditions This question is often asked and the present chapter is concerned with answering it. Before considering the products of corrosion, however, a background to the process is required. The corrosion of iron and steel is, of course, a vast field. Detailed information may be found in the books by Uhlig (1963), Evans (1968) and West (1980). Here, a brief summary of the main aspects is provided. 

Preliminary results showed that these types of compounds are possible inhibitors of the corrosion of iron in acids. This anticorrosion behavior is believed to arise from the fact that a dithiocarbamate-substituted cobalt cyclam can affect the hydrogen evolution reaction within the system. A... 

Corrosion occurs more rapidly when there is a dissolved salt, higher acidity or air pollution, because dissolved CO2 produces and HCO3" ions. One way to prevent the corrosion of iron is to coat it with special paints containing potassium dichromate (K2Cr202) and trilead tetroxide (Pb304). 

In the corrosion of iron by neutral aerated water, in a bimetallic couple with, for example, copper or platinum as the cathode (as in Fig. 16.2), the initial product at the anode is Fe2+(aq) ... 

Sometimes it happens that incursion of oxoanion reduction in place of hydrogen evolution as the cathodic reaction in the corrosion of iron leads, not to an increased rate of corrosion, but to a drastic retardation. This is because strongly oxidizing conditions (e.g., in concentrated nitric acid) can force the immediate oxidation of iron to iron(III), rather than via the persistent iron(II) intermediate (as described in Sections 16.1 and 16.2), so that an insoluble iron (III) oxide layer forms at once on the anodic and cathodic surfaces alike and the iron becomes passivated (Section 16.3). Michael Faraday s demonstration of this phenomenon is instructive ... 

Figure I. Effect of temperature and aeration on the corrosion of iron in 3% sodium chloride solution...

Kielemoes J, DeBoever P. Influence of denitrification on the corrosion of iron and stainless steel powder. Environ Sci Technol 2000 34 663. 

J. O M. Bockris, D. Drazic, and A. Despic, Electrochim Acta 4 325 (1961). First determination of the cathodic Fe2+ deposition current by significantly accurate measurements of the co-evolved H2 mechanism of the corrosion of iron in acid solution. 

A good inhibitor does not have to have all these characteristics, but one at least must be present. Table 12.2 contains examples of excellent inhibitors of the corrosion of iron that were well known in the late 1990s. 

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