Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-16 GTBL042-Callister-v2 September 13, 2007 13:10

Revised Pages

664 • Chapter 16 / Corrosion and Degradation of Materials

0.780 V

Voltmeter

–+

e–

V

e–

Fe

Fe2+ solution,

1.0 M

Membrane

Cu2+ solution,

1.0 M

Fe2+ Cu Cu2+

Figure 16.2 An electrochemical cell consisting

of iron and copper electrodes, each of which is

immersed in a 1Msolution of its ion. Iron

corrodes while copper electrodeposits.

As a consequence of oxidation, the metal ions may either go into the corrod-

ing solution as ions (reaction 16.8), or they may form an insoluble compound with

nonmetallic elements as in reaction 16.12.

Concept Check 16.1

Would you expect iron to corrode in water of high purity? Why or why not?

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

Electrode Potentials

Not all metallic materials oxidize to form ions with the same degree of ease. Consider

the electrochemical cell shown in Figure 16.2. On the left-hand side is a piece of pure

iron immersed in a solution containing Fe^2 +ions of 1Mconcentration.^1 The other

side of the cell consists of a pure copper electrode in a 1Msolution of Cu^2 +ions.

The cell halves are separated by a membrane, which limits the mixing of the two

solutions. If the iron and copper electrodes are connected electrically, reduction will

occur for copper at the expense of the oxidation of iron, as follows:

Cu^2 ++Fe→Cu+Fe^2 + (16.13)

or Cu^2 +ions will deposit (electrodeposit) as metallic copper on the copper electrode,

while iron dissolves (corrodes) on the other side of the cell and goes into solution as

Fe^2 +ions. Thus, the two half-cell reactions are represented by the relations

Fe→Fe^2 ++ 2 e− (16.14a)

Cu^2 ++ 2 e−→Cu (16.14b)

molarity^1 Concentration of liquid solutions is often expressed in terms ofmolarity,M, the number of

moles of solute per million cubic millimeters (10^6 mm^3 , or 1000 cm^3 ) of solution.