© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Chapter 23: Corrosion and Wear © 2011 Cengage Learning Engineering. All Rights Reserved.

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Learning Objectives Chemical corrosion Electrochemical corrosion The electrode potential in electrochemical cells The corrosion current and polarization Types of electrochemical corrosion Protection against electrochemical corrosion Microbial degradation and biodegradable polymers Oxidation and other gas reactions Wear and erosion © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 2

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Chemical Corrosion Chemical corrosion: Removal of atoms from a material by virtue of the solubility or chemical reaction between the material and the surrounding liquid 23 - 3 © 2011 Cengage Learning Engineering. All Rights Reserved.

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23-1 © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 4

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23-2 Dezincification occurs in brass containing more than 15% Zn. Both copper and zinc are dissolved by aqueous solutions at elevated temperatures; the zinc ions remain in solution while the copper ions are replated onto the brass. Graphitic corrosion of gray cast iron occurs when iron is selectively dissolved in water or soil, leaving behind interconnected graphite flakes and a corrosion product. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 5

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Chemical Corrosion Dissolution and oxidation of ceramics Ceramic refractories used to contain molten metal during melting or refining may be dissolved by the slags that are produced on the metal surface. Chemical attack on polymers As the solvent is incorporated into the polymer, the smaller solvent molecules force apart the chains, causing swelling. The strength of the bonds between the chains decreases. This leads to softer, lower-strength polymers with low glass-transition temperatures. 23 - 6 © 2011 Cengage Learning Engineering. All Rights Reserved.

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.3 The components in an electrochemical cell: (a) a simple electrochemical cell and (b) a corrosion cell between a steel water pipe and a copper fitting. - The anode gives up electrons to the circuit and corrodes. - The cathode receives electrons from the circuit by means of a chemical, or cathode, reaction. - A liquid electrolyte must be in contact with both the anode and the cathode. The electrolyte is conductive, thus completing the circuit. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 7

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Chemical Corrosion Anode reaction The anode, which is a metal, undergoes an oxidation reaction by which metal atoms are ionized. Cathode reaction in electroplating In electroplating, a cathodic reduction reaction, which is the reverse of the anode reaction, occurs at the cathode © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 8

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.4 The anode and cathode reactions in typical electrolytic corrosion cells: (a) the hydrogen electrode, (b) the oxygen electrode, and (c) the water electrode. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 9

The Electrode Potential in Electrochemical Chapter 23: Corrosion and Wear The Electrode Potential in Electrochemical Cells Electrode potential When a pure metal is placed in an electrolyte, an electrode potential develops that is related to the tendency of the material to give up its electrons; however, the driving force for the oxidation reaction is offset by an equal but opposite driving force for the reduction reaction. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 10

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.5 The electromotive force (or emf) compares the standard electrode potential E0 for each metal with that of the hydrogen electrode under standard conditions of 25°C and a 1 M solution of ions in the electrolyte. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 11

The Electrode Potential in Electrochemical Chapter 23: Corrosion and Wear The Electrode Potential in Electrochemical Cells Effect of Concentration on the Electrode Potential Nernst equation where E electrode potential in a solution containing a concentration Cion of the metal in molar units n change on the metallic ion E0 standard electrode potential in a 1 M solution © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 12

The Electrode Potential in Electrochemical Chapter 23: Corrosion and Wear The Electrode Potential in Electrochemical Cells Rate of corrosion or plating Faraday’s equation where w weight plated or corroded (g) I current (A) M atomic mass of the metal n charge on the metal ion t time (s) F Faraday’s constant (96,500 C) © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 13

The Corrosion Current and Polarization Chapter 23: Corrosion and Wear The Corrosion Current and Polarization A change in the potential of an anode or cathode, which in turn affects the current in the cell is called polarization. Different types of polarization are as follows: Activation polarization Is related to the energy required to cause the anode or cathode reactions to occur. Concentration polarization After corrosion begins, the concentration of ions at the anode or cathode surface may change. Resistance polarization Is caused by the electrical resistivity of the electrolyte. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 14

Table 23.2 - The Galvanic Series in Seawater Chapter 23: Corrosion and Wear Table 23.2 - The Galvanic Series in Seawater Composition cells, or dissimilar metal corrosion, develop when two metals or alloys, such as copper and iron, form an electrolytic cell. Galvanic series - The arrangement of alloys according to their tendency to corrode in a particular environment. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 15

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.6 Example of microgalvanic cells in two-phase alloys: (a) In steel, ferrite is anodic to cementite. (b) In austenitic stainless steel, precipitation of chromium carbide makes the low Cr austenite in the grain boundaries anodic. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 16

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.7 Intergranular corrosion occurs when the precipitation of a second phase or segregation at grain boundaries produces a galvanic cell. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 17

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.8 Stress cells Electrochemical corrosion cells produced by differences in imposed or residual stresses at different locations in the material. Stress corrosion occurs by galvanic action, but other mechanisms, such as the adsorption of impurities at the tip of an existing crack, may also occur. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 18

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.9 Concentration cells Electrochemical corrosion cells produced between two location on a material at which the composition of the electrolyte is different. Oxygen starvation - In the concentration cell, low-oxygen regions of the electrolyte cause the underlying material to behave as the anode and to corrode. Crevice corrosion - A special concentration cell in which corrosion occurs in crevices because of the low concentration of oxygen. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 19

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.10 Bacterial cells growing in a colony (× 2700). Tubercule - Accumulations of microbial organisms and corrosion byproducts on the surface of a material. 23 - 20 © 2011 Cengage Learning Engineering. All Rights Reserved.

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.11 © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 21

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.12 © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 22

The Corrosion Current and Polarization Chapter 23: Corrosion and Wear The Corrosion Current and Polarization Inhibitors Some chemicals when added to the electrolyte migrate preferentially to the anode or cathode surface and produce concentration or resistance polarization. Passivation or anodic protection Passivation is accomplished by producing strong anodic polarization, preventing the normal anode reaction; thus the term anodic protection. Passivation of aluminum is called anodizing. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 23

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.13 A sacrificial anode is attached to the material to be protected, forming an electrochemical circuit. An impressed voltage is obtained from a direct current source connected between an auxiliary anode and the metal to be protected. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 24

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.14 The steel is sensitized. Because the grain boundary regions are small and highly anodic, rapid corrosion of the austenite at the grain boundaries occurs. Addition of titanium or niobium ties up the carbon as TiC or NbC, preventing the formation of chromium carbide. The steel is said to be stabilized. In a quench anneal heat treatment, the stainless steel is heated above 870°C, causing the chromium carbides to dissolve. The structure, now containing 100% austenite, is rapidly quenched to prevent formation of carbides. 23 - 25 © 2011 Cengage Learning Engineering. All Rights Reserved.

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.16 © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 26

Oxidation and Other Gas Reactions Chapter 23: Corrosion and Wear Oxidation and Other Gas Reactions Oxidation of metals Metals may react with oxygen to produce an oxide at the surface. There are three aspects of this reaction: the ease with which the metal oxidizes, the nature of the oxide film that forms, and the rate at which oxidation occurs. For the oxidation reaction nM + mO2MnO2m Pilling Bedworth ratio © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 27

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.17 A linear rate of oxidation occurs when the oxide is porous (as in magnesium) and oxygen has continued access to the metal surface: y = kt where y is the thickness of the oxide, t is the time, and k is a constant that depends on the metal and temperature. A parabolic relationship is observed when diffusion of ions or electrons through a nonporous oxide layer is the controlling factor. y = (kt)-1/2 Logarithmic relationship is observed for the growth of thin-oxide films that are particularly protective, as for aluminum and possibly chromium: y = k ln (ct + 1) where k and c are constants for a particular temperature, environment, and composition. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 28

Oxidation and Other Gas Reactions Chapter 23: Corrosion and Wear Oxidation and Other Gas Reactions Oxidation and thermal degradation of polymers In rigid thermosets, the macroradicals may instantly recombine (a process called the cage effect), resulting in no net change in the polymer. Polymer chains can also unzip. In cyclization, the two ends of the same chain may be bonded together to form a ring. 23 - 29 © 2011 Cengage Learning Engineering. All Rights Reserved.

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.18 Adhesive wear—also known as scoring, galling, or seizing— occurs when two solid surfaces slide over one another under pressure. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 30

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Figure 23.19 When material is removed from a surface by contact with hard particles, abrasive wear occurs. The particles either may be present at the surface of a second material or may exist as loose particles between two surfaces. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 31

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Wear and Erosion Liquid erosion Cavitation occurs when a liquid containing a dissolved gas enters a low pressure region. Gas bubbles, which precipitate and grow in the liquid in the low pressure environment, collapse when the pressure subsequently increases. Liquid impingement occurs when liquid droplets carried in a rapidly moving gas strike a metal surface. High localized pressures develop because of the initial impact and the rapid lateral movement of the droplets from the impact point along the metal surface. © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 32

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Key Terms Chemical corrosion Dezincification Graphitic corrosion Electrochemical corrosion Electrochemical cell Anode Cathode Electrolyte Reduction reaction Electrode potential Electromotive force (or emf) series Nernst equation Faraday’s equation Polarization Composition cells Stress cells Concentration cells Galvanic series Intergranular corrosion Stress corrosion © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 33

© 2011 Cengage Learning Engineering. All Rights Reserved. Chapter 23: Corrosion and Wear Key Terms Oxygen starvation Crevice corrosion Tubercules Inhibitors Sacrificial anode Impressed voltage Passivation Anodizing Sensitized Stabilized Quench anneal Oxidation Oxidation reaction Pilling-Bedworth ratio Adhesive wear Abrasive wear Cavitation Liquid impingement © 2011 Cengage Learning Engineering. All Rights Reserved. 23 - 34