Chemistry
Corrosion
Corrosion refers to the gradual degradation of materials, usually metals, due to chemical or electrochemical reactions with their environment. This process often leads to the formation of rust or other forms of oxide on the surface of the metal, weakening its structural integrity. Corrosion can be caused by factors such as moisture, oxygen, and pollutants, and it is a significant concern in various industries.
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11 Key excerpts on "Corrosion"
eBook - PDF- DENNIS A SNOW(Author)
- 2013(Publication Date)
- Newnes(Publisher)
This page intentionally left blank Corrosion basics 30/3 30.1 Corrosion basics 30.1.1 Definitions of Corrosion Corrosion is generally taken to be the waste of a metal by the action of corrosive agents. However, a wider definition is the degradation of a material through contact with its environment. Thus Corrosion can include non-metallic materials such as concrete and plastics and mechanisms such as cracking in addition to wastage (i.e. loss of material). This chapter is primarily concerned with metallic Corrosion, through a variety of mechanisms. In essence, the Corrosion of metals is an electron transfer reaction. An uncharged metal atom loses one or more electrons and becomes a charged metal ion: M ^ ± M + + electron In an ionizing solvent the metal ion initially goes into solution but may then undergo a secondary reaction, combining with other ions present in the environment to form an insoluble molecular species such as rust or aluminium oxide. In high-temperature oxidation the metal ion becomes part of the lattice of the oxide formed. 30.1.2 Electrochemical Corrosion The most important mechanism involved in the Corrosion of metal is electrochemical dissolution. This is the basis of general metal loss, pitting Corrosion, microbiologically induced Corrosion and some aspects of stress Corrosion cracking. Corrosion in aqueous systems and other circumstances where an electrolyte is present is generally electrochemical in nature. Other mechanisms operate in the absence of electrolyte, and some are discussed in Section 30.1.4. Figure 30.1 depicts a metal such as iron, steel or zinc immersed in electrolyte such as sodium chloride solution. The fundamental driving force of the Corrosion reaction is the difference in the potential energies of the metal atom in the solid state and the product which is formed during Corrosion. Thus Corrosion may be con- sidered to be the reverse of extractive metallurgy. Metals are obtained by the expenditure of energy on their ores.
eBook - PDFElectrochemistry for Technologists
Electrical Engineering Division
- G. R. Palin, N. Hiller(Authors)
- 2016(Publication Date)
- Pergamon(Publisher)
CHAPTER 5 Corrosion 5.1 Introduction The word corrode derives from the Latin corrodere, meaning to eat away or destroy by degrees. The term Corrosion is most commonly applied to metals, and it is in this context that it will be discussed here. Corrosion of a metal involves an attack on it by its environment, leading to a change in its form. Very few metals occur naturally in the uncombined state, and a great deal of energy is expended in the extraction of metals from their ores. In a general sense, therefore, the combined state can be thought of as the stable form of a metal, with the uncombined state a metastable form. In all but a few special cases the change in a metal resulting from environmental attack is a chemical combina-tion. The change is usually slow, because metals are not used in environments in which they are rapidly attacked, but the end result may be the failure of the metal to fulfil its function. To combat Corrosion hundreds of millions of pounds are spent annually in the U.K. on protective coatings for metals, and on the replacement of corroded parts. Most metallic compounds are electrovalent, and all Corrosion processes involve ionisation, and are electrochemical. It is con-venient to discuss the various Corrosion mechanisms separately, but it must be remembered that the Corrosion of a metal in a given environment may not be confined to only one of these processes. Detailed study of Corrosion processes enables the choice of a metal for a given function to be made with greater certainty about its freedom from Corrosion, and also ensures that the best means of protection is employed. E.F.T.—5* 129 130 Corrosion 5.2 Oxidation Metals combine with oxygen giving oxides. A metallic oxide is an electrovalent compound which exists in the solid state as an array of metal ions and oxygen ions, O 2 . All metals, with the exception of gold, react with oxygen even at normal temperatures, and in the absence of moisture.
eBook - PDFEngineering Chemistry
Fundamentals and Applications
- Shikha Agarwal(Author)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
3.1 Introduction We have all seen that when piece of iron or an article made up of iron is left in the open it develops a reddish brown coating. If this is left unattended for a long time then the metal becomes weak and brittle and breaks off. A similar phenomenon is observed in copper that develops a greenish coating; similarly silver loses its luster and so on. Thus, metals degenerate in the presence of moisture and air. This is called Corrosion, which may be defined as the process of spontaneous deterioration or disintegration of metals (except gold and platinum) by direct chemical or indirect electrochemical attack with the environment. The common examples of Corrosion are rusting of iron, formation of a green layer of basic carbonate on the surface of copper. Why does Corrosion occur? Metals exist in nature in combined forms like oxides, sulphides, sulphates, carbonates, etc. These combined states (ores) are thermodynamically stable states of the metal. Energy is supplied to extract them from their ores. The extracted metal is at a higher energy level and hence it is in a thermodynamically unstable state. Metals try to get back to their stable states by combining with other elements, and in this process Corrosion occurs and oxides, sulphides, chlorides, sulphates, etc are formed. Although corroded metals are thermodynamically more stable than pure metals, Corrosion affects the useful properties of the metals like malleability, ductility and electrical conductivity. Metallic ore (mineral or metal in Extraction of metal → ← Pure metal combined form) (High energy) Unstable 3.2 Effects of Corrosion The Corrosion process, although slow, affects the metal drastically. The gravity of the problem can be realized by the fact that the approximate loss of metal because of Corrosion is 2 to 2.5 billion dollars per annum all over the world.
eBook - PDFMaterials Science and Engineering
An Introduction
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
Intergranular Corrosion—occurs preferentially along grain boundaries for specific metals/alloys (e.g., some stainless steels). Selective leaching—the case in which one element/constituent of an alloy is removed selectively by corrosive action. Erosion–Corrosion—the combined action of chemical attack and mechanical wear as a consequence of fluid motion. Stress Corrosion—the formation and propagation of cracks (and possible failure) re- sulting from the combined effects of Corrosion and the application of a tensile stress. Hydrogen embrittlement—a significant reduction in ductility that accompanies the penetration of atomic hydrogen into a metal/alloy. • Several measures may be taken to prevent, or at least reduce, Corrosion. These include material selection, environmental alteration, the use of inhibitors, design changes, ap- plication of coatings, and cathodic protection. • With cathodic protection, the metal to be protected is made a cathode by supplying electrons from an external source. • Oxidation of metallic materials by electrochemical action is also possible in dry, gase- ous atmospheres (Figure 17.25). • An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar, that is, if the Pilling–Bedworth ratio (Equations 17.32 and 17.33) is near unity. • The kinetics of film formation may follow parabolic (Equation 17.34), linear (Equation 17.35), or logarithmic (Equation 17.36) rate laws. • Ceramic materials, being inherently Corrosion resistant, are frequently used at elevated temperatures and/or in extremely corrosive environments. Passivity Forms of Corrosion Corrosion Prevention Oxidation Corrosion of Ceramic Materials 646 • Chapter 17 / Corrosion and Degradation of Materials • Polymeric materials deteriorate by noncorrosive processes. Upon exposure to liquids, they may experience degradation by swelling or dissolution.
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
Intergranular Corrosion—occurs preferentially along grain boundaries for specific metals/alloys (e.g., some stainless steels). Selective leaching—the case in which one element/constituent of an alloy is removed selectively by corrosive action. Erosion–Corrosion—the combined action of chemical attack and mechanical wear as a consequence of fluid motion. Stress Corrosion—the formation and propagation of cracks (and possible failure) re- sulting from the combined effects of Corrosion and the application of a tensile stress. Hydrogen embrittlement—a significant reduction in ductility that accompanies the penetration of atomic hydrogen into a metal/alloy. • Several measures may be taken to prevent, or at least reduce, Corrosion. These include material selection, environmental alteration, the use of inhibitors, design changes, ap- plication of coatings, and cathodic protection. • With cathodic protection, the metal to be protected is made a cathode by supplying electrons from an external source. • Oxidation of metallic materials by electrochemical action is also possible in dry, gase- ous atmospheres (Figure 17.25). • An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar, that is, if the Pilling–Bedworth ratio (Equations 17.32 and 17.33) is near unity. • The kinetics of film formation may follow parabolic (Equation 17.34), linear (Equation 17.35), or logarithmic (Equation 17.36) rate laws. • Ceramic materials, being inherently Corrosion resistant, are frequently used at elevated temperatures and/or in extremely corrosive environments. Passivity Forms of Corrosion Corrosion Prevention Oxidation Corrosion of Ceramic Materials 684 • Chapter 17 / Corrosion and Degradation of Materials • Polymeric materials deteriorate by noncorrosive processes. Upon exposure to liquids, they may experience degradation by swelling or dissolution.
eBook - PDF- Yongchang Huang, Jianqi Zhang, Yongchang Huang, Jianqi Zhang(Authors)
- 2018(Publication Date)
- De Gruyter(Publisher)
Yongchang Huang 2 Thermodynamics of materials Corrosion 2.1 Thermodynamic criteria of Corrosion Corrosion is the destruction of a material subjected to the chemical or physical action of the surrounding environment. Metal Corrosion, which is a process where a metal is converted from an elemental state to a chemical compound state, follows the laws of chemical thermodynamics. Metal Corrosion and most chemical reactions generally occur in an open envi- ronment system of constant temperature and pressure. When the reaction comes to the thermodynamic equilibrium, the chemical thermodynamics points out that the change in the Gibbs free energy of the reaction is zero; i.e., ΔG = � i ν i 𝜇 i = 0, where ν i is the coefficients of reactant and resultant of chemical reaction. It is regu- lated that the reactant coefficient is negative and the resultant coefficient is positive. 𝜇 i is the chemical potential of the corresponding material in a given system. The change in the Gibbs free energy of the Corrosion reaction can be regarded as a thermodynamic criterion of Corrosion process and judge the possibility of Corrosion. If the change in the free energy in an action is negative, it indicates that the Corrosion process is thermodynamically possible. The more negative the change is, the higher the possibility of the reaction is. When the change is positive, the Corrosion process cannot occur. The following are used for the changes in reaction free energy of Mg, Cu, and Au in moist atmosphere to compare their Corrosion tendencies [1]: Mg + H 2 O + 1 2 O 2 → Mg(OH) 2 △G ∘ = −596.64 KJ Cu + H 2 O + 1 2 O 2 → Cu(OH) 2 △G ∘ = −119.96 KJ Au + H 2 O + 1 2 O 2 → Au(OH) 2 △G ∘ = +65.69 KJ According to the above three actions, the Corrosion tendency of Mg is greatest, followed by Cu; Au does not corrode in the moist atmosphere.
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
• Metallic Corrosion is sometimes classified into several different forms: Uniform attack—degree of Corrosion is approximately uniform over the entire exposed surface. Galvanic Corrosion—occurs when two different metals or alloys are electrically coupled while exposed to an electrolyte solution. Corrosion Rates Prediction of Corrosion Rates Passivity Forms of Corrosion Summary • 687 Crevice Corrosion—the situation when Corrosion occurs under crevices or other areas where there is localized depletion of oxygen. Pitting—a type of localized Corrosion in which pits or holes form from the top of horizontal surfaces. Intergranular Corrosion—occurs preferentially along grain boundaries for specific metals/alloys (e.g., some stainless steels). Selective leaching—the case in which one element/constituent of an alloy is removed selectively by corrosive action. Erosion–Corrosion—the combined action of chemical attack and mechanical wear as a consequence of fluid motion. Stress Corrosion—the formation and propagation of cracks (and possible failure) resulting from the combined effects of Corrosion and the application of a tensile stress. Hydrogen embrittlement—a significant reduction in ductility that accompanies the penetration of atomic hydrogen into a metal/alloy. • Several measures may be taken to prevent, or at least reduce, Corrosion. These include material selection, environmental alteration, the use of inhibitors, design changes, application of coatings, and cathodic protection. • With cathodic protection, the metal to be protected is made a cathode by supplying electrons from an external source. • Oxidation of metallic materials by electrochemical action is also possible in dry, gase- ous atmospheres (Figure 16.25). • An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar, that is, if the Pilling–Bedworth ratio (Equations 16.32 and 16.33) is near unity.
eBook - PDFMaterials Science and Engineering, P-eBK
An Introduction
- William D. Callister, Jr., David G. Rethwisch, Aaron Blicblau, Kiara Bruggeman, Michael Cortie, John Long, Judy Hart, Ross Marceau, Ryan Mitchell, Reza Parvizi, David Rubin De Celis Leal, Steven Babaniaris, Subrat Das, Thomas Dorin, Ajay Mahato, Julius Orwa(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
In a microscopic sense, the oxidation and reduction reactions occur randomly over the surface. Familiar examples include general rusting of steel and iron and the tarnishing of silverware. This is probably the most common form of Corrosion. It is also the least objectionable because it can be predicted and designed for with relative ease. Galvanic Corrosion FIGURE 17.14 Photograph showing galvanic Corrosion around the inlet of a single‐cycle bilge pump that is found on fishing vessels. Corrosion occurred at the interface between a magnesium shell and a steel core around which the magnesium was cast. Galvanic Corrosion Steel core Magnesium shell Courtesy of LaQue Center for Corrosion Technology, Inc. Galvanic Corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. This is the type of Corrosion or dissolution that was described in section 17.2. The less noble or more reactive metal in the particular environment experiences Corrosion; the more inert metal, the cathode, is protected from Corrosion. As examples, steel screws corrode when in contact with brass in a marine environment, and if copper and steel tubing are joined in a domestic water heater, the steel corrodes in the vicinity of the junc- tion. Depending on the nature of the solution, one or more of the reduction reactions, equations 17.3 through 17.7, occurs at the surface of the cathode material. Figure 17.14 shows galvanic Corrosion. The galvanic series in table 17.2 indicates the relative reactivities in seawater of a number of metals and alloys. When two alloys are coupled in seawater, the one lower in the series experiences Corrosion. It is also worth noting from this series that some alloys are listed twice (e.g. nickel and the stainless steels), in both active and passive states.
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
These include material selection, environmental alteration, the use of inhibitors, design changes, application of coatings, and cathodic protection. • With cathodic protection, the metal to be protected is made a cathode by supplying electrons from an external source. • Oxidation of metallic materials by electrochemical action is also possible in dry, gase- ous atmospheres (Figure 16.25). • An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar, that is, if the Pilling–Bedworth ratio (Equations 16.32 and 16.33) is near unity. • The kinetics of film formation may follow parabolic (Equation 16.34), linear (Equation 16.35), or logarithmic (Equation 16.36) rate laws. • Ceramic materials, being inherently Corrosion resistant, are frequently used at el- evated temperatures and/or in extremely corrosive environments. • Polymeric materials deteriorate by noncorrosive processes. Upon exposure to liquids, they may experience degradation by swelling or dissolution. With swelling, solute molecules actually fit into the molecular structure. Dissolution may occur when the polymer is completely soluble in the liquid. • Scission, or the severance of molecular chain bonds, may be induced by radiation, chemical reactions, or heat. This results in a reduction of molecular weight and a deterioration of the physical and chemical properties of the polymer. Corrosion Prevention Oxidation Corrosion of Ceramic Materials Degradation of Polymers Equation Summary Equation Number Equation Solving for Page Number 16.18 ΔV 0 = V 0 2 − V 0 1 Electrochemical cell potential for two standard half-cells 717 16.19 ΔV = ( V 0 2 − V 0 1 ) − RT nℱ ln [M n + 1 ] [M 2 n + ] Electrochemical cell potential for two nonstandard half-cells 718 (continued)
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
16.7 | | FORMS OF Corrosion It is convenient to classify Corrosion according to the manner in which it is manifest. Metallic Corrosion is sometimes classified into eight forms: uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion–Corrosion, and stress Corrosion. The causes and means of prevention of each of these forms are discussed briefly. In addition, we have elected to discuss the topic of hydrogen embrittlement in this section. Hydrogen embrittlement is, in a strict sense, a type of failure rather than a form of Corrosion; however, it is often produced by hydrogen that is generated from Corrosion reactions. Uniform Attack Uniform attack is a form of electrochemical Corrosion that occurs with equivalent in- tensity over the entire exposed surface and often leaves behind a scale or deposit. In a microscopic sense, the oxidation and reduction reactions occur randomly over the surface. Familiar examples include general rusting of steel and iron and the tarnishing of silverware. This is probably the most common form of Corrosion. It is also the least objectionable because it can be predicted and designed for with relative ease. Galvanic Corrosion Galvanic Corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. This is the type of Corrosion or dissolution that was described in Section 16.2. The less noble or more reactive metal in the particular environment experiences Corrosion; the more inert metal, the cathode, is protected from Corrosion. As examples, steel screws corrode when in con- tact with brass in a marine environment, and if copper and steel tubing are joined in a domestic water heater, the steel will corrode in the vicinity of the junction. Depending on the nature of the solution, one or more of the reduction reactions, Equations 16.3 through 16.7, occurs at the surface of the cathode material.
eBook - PDFEngineering Chemistry
Fundamentals and Applications
- Shikha Agarwal(Author)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
This causes rapid, continuous and excessive Corrosion. MoO 3 is the volatile Corrosion product formed on the oxidation of Mo. It volatilizes, causing extensive Corrosion of the underlying metal. Nature of the corroding environment Environmental factors have a vital role in deciding the rate of Corrosion. The major environmental factors that affect Corrosion are as follows (i) Temperature As the rate of all chemical reactions increases with the rise in temperature, rate of Corrosion also increases as temperature rises. At higher temperatures, even the passive metal can become active and get corroded. However, differential aeration Corrosion slows down at higher temperatures. This is because of the faster diffusion of O 2 into pits and crevices. Moreover, Corrosion also occurs due to the formation of differential temperature cells. (ii) Humidity Corrosion increases with the increase in humidity of the atmosphere. The effect of humidity on Corrosion of enameled steel is given in the table below. Relative humidity 0–65 65–80 80–90 100 Corrosion (appearance) No Corrosion Thin Corrosion filaments Wide Corrosion filaments Blisters due to Corrosion This is due to the fact that moisture acts as a solvent for O 2 , H 2 S, SO 2 , NaCl, and so on to furnish the electrolyte for setting up a Corrosion cell. 228 Engineering Chemistry: Fundamentals and Applications Common examples, • Atmospheric Corrosion of Fe is slow in dry air compared with moist air. Steel parts left in desert areas remain bright and tarnish free for very long periods of time. • Gases like H 2 S and SO 2 dissolve in water and increase the acidity of medium and hence increase the Corrosion rate. • Solids like NaCl dissolve in water to increase its conductivity and hence increase the rate of Corrosion. (iii) Presence of impurities in atmosphere Gases like H 2 S, SO 2 and CO 2 increase acidity of the liquid closer to the metal surface and hence increases the rate of Corrosion.
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