1. Chapter 22 – Corrosion and Wear
The Science and Engineering of Materials, 4th ed Donald R. Askeland – Pradeep P. Phulé
Chapter 22 – Corrosion and Wear

2. Objectives of Chapter 22
To introduce the principles and mechanisms by which corrosion and wear occur under different conditions. This includes the aqueous corrosion of metals, the oxidation of metals, the corrosion of ceramics, and the degradation of polymers.
To give summary of different technologies that are used to prevent or minimize corrosion and associated problems.

3. Chapter Outline 22.1 Chemical Corrosion 22.2 Electrochemical Corrosion
22.3 The Electrode Potential in Electrochemical Cells
22.4 The Corrosion Current and Polarization
22.5 Types of Electrochemical Corrosion
22.6 Protection Against Electrochemical Corrosion
22.7 Microbial Degradation and Biodegradable Polymers
22.8 Oxidation and Other Gas Reactions
22.9 Wear and Erosion

4. Section 22.1 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.
Dezincification - A special chemical corrosion process by which both zinc and copper atoms are removed from brass, but the copper is replated back onto the metal.
Graphitic corrosion - A special chemical corrosion process by which iron is leached from cast iron, leaving behind a weak, spongy mass of graphite.

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Figure Molten lead is held in thick steel pots during refining. In this case, the molten lead has attacked a weld in a steel plate and cracks have developed. Eventually, the cracks propagate through the steel, and molten lead leaks from the pot.

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Figure Photomicrograph of a copper deposit in brass, showing the effect of dezincification (x50).

7. Section 22.2 Electrochemical Corrosion
Electrochemical corrosion - Corrosion produced by the development of a current in an electrochemical cell that removes ions from the material.
Electrochemical cell - A cell in which electrons and ions can flow by separate paths between two materials, producing a current which, in turn, leads to corrosion or plating.
Oxidation reaction - The anode reaction by which electrons are given up to the electrochemical cell.
Reduction reaction - The cathode reaction by which electrons are accepted from the electrochemical cell.

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Figure 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.

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Figure The anode and cathode reactions in typical electrolytic corrosion cells: (a) the hydrogen electrode, (b) the oxygen electrode, and (c) the water electrode.

10. Section 22.3 The Electrode Potential in Electrochemical Cells
Electrode potential - Related to the tendency of a material to corrode. The potential is the voltage produced between the material and a standard electrode.
emf series - The arrangement of elements according to their electrode potential, or their tendency to corrode.
Nernst equation - The relationship that describes the effect of electrolyte concentration on the electrode potential in an electrochemical cell.
Faraday’s equation - The relationship that describes the rate at which corrosion or plating occurs in an electrochemical cell.

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Figure The half-cell used to measured the electrode potential of copper under standard conditions. The electrode potential of copper is the potential difference between it and the standard hydrogen electrode in an open circuit. Since E0 is great than zero, copper is cathodic compared with the hydrogen electrode.

13. Example 22.1 Half-Cell Potential for Copper
Suppose 1 g of copper as Cu2+ is dissolved in 1000 g of water to produce an electrolyte. Calculate the electrode potential of the copper half-cell in this electrolyte.
Example 22.1 SOLUTION
From chemistry, we know that a standard 1-M solution of Cu2+ is obtained when we add 1 mol of Cu2+ (an amount equal to the atomic mass of copper) to 1000 g of water. The atomic mass of copper is g/mol. The concentration of the solution when only 1 g of copper is added must be:
From the Nernst equation, with n = 2 and E0 = V:

14. Example 22.2 Design of a Copper Plating Process
Design a process to electroplate a 0.1-cm-thick layer of copper onto a 1 cm  1 cm cathode surface.
Example 22.2 SOLUTION
In order for us to produce a 0.1-cm-thick layer on a 1 cm2 surface area, the weight of copper must be:
From Faraday’s equation, where MCu = 63:54 g/mol and n = 2:

15. Example 22.2 SOLUTION
Therefore, we might use several different combinations of current and time to produce the copper plate:
Our choice of the exact combination of current and time might be made on the basis of the rate of production and quality of the copper plate.
A current of ~ 1 A and a time of ~ 45 minutes are not uncommon in electroplating operations.

16. Example 22.3 Corrosion of Iron
An iron container 10 cm  10 cm at its base is filled to a height of 20 cm with a corrosive liquid. A current is produced as a result of an electrolytic cell, and after 4 weeks, the container has decreased in weight by 70 g. Calculate (1) the current and (2) the current density involved in the corrosion of the iron.
Example 22.3 SOLUTION
1. The total exposure time is:
From Faraday’s equation, using n = 2 and M = g/mol:

17. Example 22.3 SOLUTION
2. The total surface area of iron in contact with the corrosive liquid and the current density are:

18. Example 22.4 Copper-Zinc Corrosion Cell
Suppose that in a corrosion cell composed of copper and zinc, the current density at the copper cathode is 0.05 A/cm2. The area of both the copper and zinc electrodes is 100 cm2. Calculate (1) the corrosion current, (2) the current density at the zinc anode, and (3) the zinc loss per hour.
Example 22.4 SOLUTION
1. The corrosion current is:
2. The current in the cell is the same everywhere. Thus:

19. Example 22.4 SOLUTION
3. The atomic mass of zinc is g/mol. From Faraday’s equation:

20. Section 22.4 The Corrosion Current and Polarization
Polarization - Changing the voltage between the anode and cathode to reduce the rate of corrosion.
Activation polarization is related to the energy required to cause the anode or cathode reaction
Concentration polarization is related to changes in the composition of the electrolyte
Resistance polarization is related to the electrical resistivity of the electrolyte.

21. Section 22.5 Types of Electrochemical Corrosion
Intergranular corrosion - Corrosion at grain boundaries because grain boundary segregation or precipitation produces local galvanic cells.
Stress corrosion - Deterioration of a material in which an applied stress accelerates the rate of corrosion.
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.

23. Example 22.5 Corrosion of a Soldered Brass Fitting
A brass fitting used in a marine application is joined by soldering with lead-tin solder. Will the brass or the solder corrode?
Example 22.5 SOLUTION
From the galvanic series, we find that all of the copper-based alloys are more cathodic than a 50% Pb-50% Sn solder. Thus, the solder is the anode and corrodes. In a similar manner, the corrosion of solder can contaminate water in freshwater plumbing systems with lead.

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Figure 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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Figure Photomicrograph of intergranular corrosion in a zinc die casting. Segregation of impurities to the grain boundaries produces microgalvanic corrosion cells (x50).

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Figure Examples of stress cells. (a) Cold work required to bend a steel bar introduces high residual stresses at the bend, which then is anodic and corrodes. (b) Because grain boundaries have a high energy, they are anodic and corrode.

27. Example 22.6 Corrosion of Cold-Drawn Steel
A cold-drawn steel wire is formed into a nail by additional deformation, producing the point at one end and the head at the other. Where will the most severe corrosion of the nail occur?
Example 22.6 SOLUTION
Since the head and point have been cold-worked an additional amount compared with the shank of the nail, the head and point serve as anodes and corrode most rapidly.

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Figure Concentration cells: (a) Corrosion occurs beneath a water droplet on a steel plate due to low oxygen concentration in the water. (b) Corrosion occurs at the tip of a crevice because of limited access to oxygen.

29. Example 22.7 Corrosion of Crimped Steel
Two pieces of steel are joined mechanically by crimping the edges. Why would this be a bad idea if the steel is then exposed to water? If the water contains salt, would corrosion be affected?
Example 22.7 SOLUTION
By crimping the steel edges, we produce a crevice. The region in the crevice is exposed to less air and moisture, so it behaves as the anode in a concentration cell. The steel in the crevice corrodes.
Salt in the water increases the conductivity of the water, permitting electrical charge to be transferred at a more rapid rate. This causes a higher current density and, thus, faster corrosion due to less resistance polarization.

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Figure (a) Bacterial cells growing in a colony (x2700). (b) Formation of a tubercule and a pit under a biological colony.

31. Section 22.6 Protection Against Electrochemical Corrosion
Inhibitors - Additions to the electrolyte that preferentially migrate to the anode or cathode, cause polarization, and reduce the rate of corrosion.
Sacrificial anode - Cathodic protection by which a more anodic material is connected electrically to the material to be protected. The anode corrodes to protect the desired material.
Passivation - Producing strong anodic polarization by causing a protective coating to form on the anode surface and to thereby interrupt the electric circuit.

32. Example 22.8 Effect of Areas on Corrosion Rate for Copper-Zinc Couple
Consider a copper-zinc corrosion couple. If the current density at the copper cathode is 0.05 A/cm2, calculate the weight loss of zinc per hour if (1) the copper cathode area is 100 cm2 and the zinc anode area is 1 cm2 and (2) the copper cathode area is 1 cm2 and the zinc anode area is 100 cm2.

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Figure Alternative methods for joining two pieces of steel: (a) Fasteners may produce a concentration cell, (b) brazing or soldering may produce a composition cell, and (c) welding with a filler metal that matches the base metal may avoid the formation of galvanic cells (for Example 22.8)

34. Example 22.8 SOLUTION
1. For the small zinc anode area:
2. For the large zinc anode area:
The rate of corrosion of the zinc is reduced significantly when the zinc anode is much larger than the cathode.

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Figure Zinc-plated steel and tin-plated steel are protected differently. Zinc protects steel even when the coating is scratched, since zinc is anodic to steel. Tin does not protect steel when the coating is disrupted, since steel is anodic with respect to tin.

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Figure Cathodic protection of a buried steel pipeline: (a) A sacrificial magnesium anode assures that the galvanic cell makes the pipeline the cathode. (b) An impressed voltage between a scrap iron auxiliary anode and the pipeline assures that the pipeline is the cathode.

37. Figure (a) Intergranular corrosion takes place in austenitic stainless steel. (b) Slow cooling permits chromium carbides to precipitate at grain boundaries. (c) A quench anneal to dissolve the carbides may prevent intergranular corrosion.

38. Example 22.9 Design of a Corrosion Protection System
Steel troughs are located in a field to provide drinking water for a herd of cattle. The troughs frequently rust through and must be replaced. Design a system to prevent or delay this problem.
Example 22.9 SOLUTION
We might, for example, fabricate the trough using stainless steel or aluminum. Either would provide better corrosion resistance than the plain carbon steel, but both are considerably more expensive than the current material.
We might suggest using cathodic protection; a small magnesium anode could be attached to the inside of the trough. The anode corrodes sacrificially and prevents corrosion of the steel.

39. Example 22.9 SOLUTION (Continued)
Another approach would be to protect the steel trough using a suitable coating. Painting the steel (that is, introducing a protective polymer coating) and, using a tin-plated steel, provides protection as long as the coating is not disrupted.
The most likely approach is to use a galvanized steel, taking advantage of the protective coating and the sacrificial behavior of the zinc. Corrosion is very slow due to the large anode area, even if the coating is disrupted. Furthermore, the galvanized steel is relatively inexpensive, readily available, and does not require frequent inspection.

40. Example 22.10 Design of a Stainless-Steel Weldment
A piping system used to transport a corrosive liquid is fabricated from 304 stainless steel. Welding of the pipes is required to assemble the system. Unfortunately, corrosion occurs and the corrosive liquid leaks from the pipes near the weld. Identify the problem and design a system to prevent corrosion in the future.

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Figure The peak temperature surrounding a stainless-steel weld and the sensitized structure produced when the weld slowly cools (for Example 22.10)

43. Example SOLUTION
A portion of the pipe in the HAZ heats into the sensitization temperature range, permitting chromium carbides to precipitate. If the cooling rate of the weld is very slow, the fusion zone and other areas of the heat-affected zone may also be affected. Sensitization of the weld area, therefore, is the likely reason for corrosion of the pipe in the region of the weld.
We might use a welding process that provides very rapid rates of heat input, causing the weld to heat and cool very quickly.
We might heat treat the assembly after the weld is made. By performing a quench anneal, any precipitated carbides are re-dissolved during the anneal and do not re-form during quenching.
Perhaps our best design is to use a stainless steel that is not subject to sensitization.

44. Section 22.7 Microbial Degradation and Biodegradable Polymers
Simple polymers (such as polyethylene, polypropylene, and polystyrene), high-molecular-weight polymers, crystalline polymers, and thermosets are relatively immune to attack.
However, certain polymers—including polyesters, polyurethanes, cellulosics, and plasticized polyvinyl chloride (which contains additives that reduce the degree of polymerization)—are particularly vulnerable to microbial degradation.

45. Section 22.8 Oxidation and Other Gas Reactions
Oxidation - Reaction of a metal with oxygen to produce a metallic oxide. This normally occurs most rapidly at high temperatures.
Pilling-Bedworth ratio - Describes the type of oxide film that forms on a metal surface during oxidation.

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Figure The standard free energy of formation of selected oxides as a function of temperature. A large negative free energy indicates a more stable oxide.

47. Example 22.11 Chromium-Based Steel Alloys
Explain why we should not add alloying elements such as chromium to pig iron before the pig iron is converted to steel in a basic oxygen furnace at 1700oC.
Example SOLUTION
In a basic oxygen furnace, we lower the carbon content of the metal from about 4% to much less than 1% by blowing pure oxygen through the molten metal. If chromium were already present before the steel making began, chromium would oxidize before the carbon (Figure 22.16), since chromium oxide has a lower free energy of formation (or is more stable) than carbon dioxide (CO2). Thus, any expensive chromium added would be lost before the carbon was removed from the pig iron.

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Figure Three types of oxides may form, depending on the volume ratio between the metal and the oxide: (a) magnesium produces a porous oxide film, 9b) aluminum forms a protective, adherent, nonporous oxide film, and (c) iron forms an oxide film that spills off the surface and provides poor protection.

49. Example 22.12 Pilling-Bedworth Ratio
The density of aluminum is 2.7 g/cm3 and that of Al2O3 is about 4 g/cm3. Describe the characteristics of the aluminum-oxide film. Compare with the oxide film that forms on tungsten. The density of tungsten is g/cm3 and that of WO3 is 7.3 g/cm3.
Example SOLUTION
For 2Al + 3/2O2  Al2O3, the molecular weight of Al2O3 is and that of aluminum is

50. Example SOLUTION
For tungsten, W+ 3/2O2  WO3. The molecular weight of WO3 is and that of tungsten is :
Since P-B ~ 1 for aluminum, the Al2O3 film is nonporous and adherent, providing protection to the underlying aluminum. However, P-B > 2 for tungsten, so the WO3 should be nonadherent and nonprotective.

51. Example 22.13 Parabolic Oxidation Curve for Nickel
At 1000oC, pure nickel follows a parabolic oxidation curve given by the constant k = 3.9  cm2/s in an oxygen atmosphere. If this relationship is not affected by the thickness of the oxide .lm, calculate the time required for a 0.1-cm nickel sheet to oxidize completely.
Example SOLUTION
Assuming that the sheet oxidizes from both sides:

52. Section 22.9 Wear and Erosion
Adhesive wear - Removal of material from surfaces of moving equipment by momentary local bonding, then bond fracture, at the surfaces.
Abrasive wear - Removal of material from surfaces by the cutting action of particles.
Cavitation - Erosion of a material surface by the pressures produced when a gas bubble collapses within a moving liquid.
Liquid impingement - Erosion of a material caused by the impact of liquid droplets carried by a gas stream.

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Figure The asperities on two rough surfaces may initially be bonded. A sufficient force breaks the bonds and the surfaces slide. As they slide, asperities may be fractured, wearing away the surfaces and producing debris.

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Figure Abrasive wear, caused by either trapped or free-flying abrasives, produces troughs in the material, piling up asperities that may fracture into debris.

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Figure Two steel sheets joined by an aluminum rivet (for Problem 22.25).

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Figure Cross-section through an integrated circuit showing the external lead connection to the chip (for Problem 22.26).