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

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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
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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
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Chapter 23: Corrosion and Wear
Figure 23-1
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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.
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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.
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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.
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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
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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.
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10. 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.
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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.
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12. 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
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13. 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)
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14. 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.
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15. Table 23.2 - The Galvanic Series in Seawater
Chapter 23: Corrosion and Wear
Table 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.
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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.
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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.
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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.
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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.
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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.
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Chapter 23: Corrosion and Wear
Figure 23.11
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Chapter 23: Corrosion and Wear
Figure 23.12
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23. 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.
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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.
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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.
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Chapter 23: Corrosion and Wear
Figure 23.16
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27. 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
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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.
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29. 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.
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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.
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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.
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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.
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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
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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
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