Ebook Applied chemistry: A textbook for engineers and technologists - Part 12
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Ebook Applied chemistry: A textbook for engineers and technologists - Part 12
Chapter 10Corrosion10.1IntroductionCorrosion is the unwanted reaction or destruction of a metal component by the environment. The annual cost of corro Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 osion to the US economy has been estimated to be over $70 billion. Similar costs arc associated with other industrialized countries. Many of the problems can be avoided if basic precautions and design processes are followed.The mechanism of corrosion is electrochemical and can be induced by the How Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 of current or will cause a current to flow. When a corroding metal is oxidized, the reactionM -» M,n + ne"(10.1)must Ik accompanied by a reduction reaEbook Applied chemistry: A textbook for engineers and technologists - Part 12
ction which is usually the reduction of oxygen whether in the air or dissolved in water.o2 1-411' + 4c"-> 2H2O(10.2)o.v. Roussak and H.D. Gesser, ApplChapter 10Corrosion10.1IntroductionCorrosion is the unwanted reaction or destruction of a metal component by the environment. The annual cost of corro Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 10 CorrosionO2 + 2H2O + 4c~ -> 4OHIn some cases, the reduction of hydrogen occurs.2114 + 2c“ — H2(10.3)(10.4)The usual classification of corrosion is according to the environment to which the metal is exposed or the actual reactions which occur. We have seen that the concentration cell is a simple c Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 ell in which a metal can corrode as dissolution takes place.10.2Factors /Xffecting the Rate of CorrosionIt is convenient to classify the corrosion ofEbook Applied chemistry: A textbook for engineers and technologists - Part 12
metals in tenns of (a) the metals and (b) the environment.The reduction potential is the most important characteristic of a metal that determines its Chapter 10Corrosion10.1IntroductionCorrosion is the unwanted reaction or destruction of a metal component by the environment. The annual cost of corro Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 ly dissolve in strong oxidizing solutions which also contain complexing halides or other ions, for example. (CN ). For metals in seawater, the relative order of the reduction potential of metals and alloys has been established. This is illustrated in Table 10.1 where distinction is made between acti Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 ve and passive surfaces for some metals. Magnesium is a most active metal, whereas platinum and graphite are the least active materials. The voltagesEbook Applied chemistry: A textbook for engineers and technologists - Part 12
arc given with respect to the saturated calomel electrode (SCE).1 The oxidation reaction (10.1) represents corrosion which must be accompanied by a reChapter 10Corrosion10.1IntroductionCorrosion is the unwanted reaction or destruction of a metal component by the environment. The annual cost of corro Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 occurs depends on the solution in which the metal conodes, but in most cases the cathodic reaction involves o2.The corrosion rate will thus depend on the partial pressure of oxygen. This is shown in Table 10.2. Hence, the removal of oxygen from water in steam boilers is one method of reducing corros Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 ion.If hydrogen evolution is the cathodic reaction (10.4), then it can be reduced by increasing the overvoltage. The overvoltage of H2 on mercury is vEbook Applied chemistry: A textbook for engineers and technologists - Part 12
ery high (sec Table 9.2), and reaction (10.4) can be inhibited if mercury is used to coat the metal surface and to form an amalgam (sec the zinc air cChapter 10Corrosion10.1IntroductionCorrosion is the unwanted reaction or destruction of a metal component by the environment. The annual cost of corro Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 , the polarization can be expected to increase resulting in a decrease in rate of corrosion. In the case of iron (anode) on a large copper sheet (cathode), the large cathodc/anode ratio favors corrosion of the iron. This is shown in Fig. 10.1.1 The saturated calomel electrode is a convenient referen Ebook Applied chemistry: A textbook for engineers and technologists - Part 12 ce electrode often used instead of the standard hydrogen electrode: ị Hg,Cl2+e — Hg 4- Cl . E°= 0.2224 (25° C).10.2 Factors Affecting the Rate of CorrEbook Applied chemistry: A textbook for engineers and technologists - Part 12
osion177Table 10.1 Galvanic metal and alloy potential V (vs. SCE) in seawater-V(V)Mg1.6 ±0.02Zn1.00 ± 0.02Be0.99 ± 0.01Chapter 10Corrosion10.1IntroductionCorrosion is the unwanted reaction or destruction of a metal component by the environment. The annual cost of corroChapter 10Corrosion10.1IntroductionCorrosion is the unwanted reaction or destruction of a metal component by the environment. The annual cost of corroGọi ngay
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