Corrosion of iron

The reaction of iron with acids can be treated in terms of these reactions. Any metal below = 0 V in the series is able to reduce acidic H (aq) to H2. Both of the half-reactions involving Fe metal come into this category and so reaction of Fe with aqueous acids will produce H2. Because E° for the formation of Fe is lower than that for Fe, the formation of Fe + will be favoured. 

The reaction of Fe with water can be treated in a similar way. Consider Reaction [S3.8], involving water in the absence of oxygen. E° is —0.83 V and so Fe metal will not be oxidised by oxygen-free water in the standard conditions implied by the table. (Reaction [S3.8] is below the Fe halfreactions). However, standard conditions refer to an OH concentration of 1 molL, with a pH of 14. At pH 7, which corresponds to normal neutral water, E = —0.42 V. This is just above the Fe /Fe couple, so that an oxidation of Fe to Fe by pure water is just possible. That is, Fe will have only a very slight tendency to be oxidised by water in the absence of acid. 

When we consider damp air, the requisite reaction is Reaction [S3.3]. At pH 7 we see that this reaction will cause Fe to oxidise to Fe or Fe and that Fe will also oxidise to Fe. Thus, as is well known, moist air will cause Fe metal to rust. The range of pH and oxidation conditions over which iron will react with acidic or alkaline water are summarised on a Pourbaix diagram. 

Many metals, including iron, react with air and/or water (corrode). The reactivity of metals with air or water can be predicted using reduction potentials. For a neutral solution  

Therefore pure water has only a slight tendency to oxidize Fe(s) to Fe (aq). 

However, if oxygen is present, the following half-reaction can occur in neutral solution (pH = 7 see page 151) 

by using anticlockwise arrows, that this couple can oxidize Fe to Fe and then from Fei to Fe.  

Both oxygen and water can therefore oxidize Fe to Fe + and oxygen is further capable of oxidizing Fe to Fe +. These reactions are known as rustii.  

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Prinz FI and Strehblow FI-FI 1998 Investigations on pitting corrosion of iron in perchlorate electrolytes Corn Scl. 40 1671-83... 

McCoUum and Logan, Electrolytic Corrosion of Iron in Soils, Technical Paper No. 25, Bureau of Standards, Washiagton, D.C., 1913, p. 7. 

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). 

Fig. 6. Effect of pH on corrosion of iron in aerated water at room temperature (22). Point A is where evolution begins. To convert mm/yr to mils per...

Typical examples of inhibitors used for minimizing corrosion of iron and steel in aqueous solutions are the chromates, phosphates, and silicates. Organic sulfide and amine materials are frequently effective in minimizing corrosion of iron and steel in acid solution. 

The use of inhibitors is not limited to controlling corrosion of iron and steel. They frequently are effective with stainless steel and other aUoy materials. The addition of copper sulfate to dilute sulfuric acid will sometimes control corrosion of stainless steels in hot dilute solutions of this acid, whereas the uninhibited acid causes rapid corrosion. 

Figure 5.4 EfTect of temperature on corrosion of iron in water containing dissolved oxygen. (Courtesy of McGraw-Hill Inc., Corrosion, Causes and Prevention, by F. Speller, p. 168, McGraw-Hill, 1951)...

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. 

Red lead (Pb304) is manufactured on the 20000-tonne scale annually and is used primarily as a surface coating to prevent corrosion of iron and steel (check oxidation-reduction potentials). It is also used in the production of leaded glasses and ceramic glazes and. very substantially, as an activator, vulcanizing agent and pigment in natural and artificial rubbers and plastics. 

Ruther, W. E. and Hart, R. K., Influence of Oxygen on High Temperature Aqueous Corrosion of Iron , Corrosion, 19, 127t (1963)... 

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

Freiman, L. I. and Kolotyrkin, Ya. M., Pitting Corrosion of Iron by Perchlorate Ions , Dokl. 

Tousek, J., Kinetics of the Pitting Corrosion of Iron and Nickel , Czech. Chem. Commun., 37, 1454 (1972) C.A., 77, 55490e... 

Corrosion of Iron and Steel by Industrial Waters and its Prevention, Special Report No. 41, I.S.I., London (1949)... 

Starkey, R. L. and Wight, K. M., Anaerobic Corrosion of Iron in Soil, Amer. Gas Assoc., New York (1945)... 

Corrosion of iron and steel, especially in anaerobic conditions such as waterlogged soils, is usually caused by sulphate-reducing bacteria of which the genus Desulphovibrio is the most commonly occuring. The presence of organic materials such as acetate often stimulates these organisms reducing... 

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... 

Henthorne, M., Intergranular Corrosion of Iron and Nickel Base Alloys, Localised Corrosion-Cause of Metal Failure, ASTM STP 516, ASTM, 66 (1972)... 

The aqueous corrosion of iron under conditions of air access can be written ... 

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... 

Fe-HS )jds has been postulated to lead to easier anodic dissolution than that through (Fe-014)3 5. This effect of hydrogen sulphide is thought to be responsible for the acceleration of corrosion of iron observed with some inhibitive sulphur compounds, e.g. thioureas , at low concentrations, since hydrogen sulphide has been identified as a reduction product. However, the effects of hydrogen sulphide are complex, since in the presence of inhibitors such as amines , quaternary ammonium cations , thioureas ", ... 

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 ... 

Table 19.3 Summary of chemical tests used for the determination of susceptibility to intergranular corrosion of iron-nickel Chromium alloys ...

Several sizes and shapes of specimens have been used in addition to the common ones already mentioned. In the long-time test of bare and zinc-coated steels undertaken by ASTM Committee A-5 on Corrosion of Iron and Steel, full-size sheets were used . This Committee has also exposed specimens in the form of hardware and wire and fencing . 

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