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

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

Revised Pages

16.4 Prediction of Corrosion Rates • 671

16.4 PREDICTION OF CORROSION RATES

Polarization

Consider the standard Zn/H 2 electrochemical cell shown in Figure 16.5, which has

been short-circuited such that oxidation of zinc and reduction of hydrogen will occur

at their respective electrode surfaces. The potentials of the two electrodes will not

be at the values determined from Table 16.1 because the system is now a nonequi-

librium one. The displacement of each electrode potential from its equilibrium value

polarization is termedpolarization,and the magnitude of this displacement is theovervoltage,

normally represented by the symbolη. Overvoltage is expressed in terms of plus or

minus volts (or millivolts) relative to the equilibrium potential. For example, suppose

that the zinc electrode in Figure 16.5 has a potential of−0.621 V after it has been

connected to the platinum electrode. The equilibrium potential is−0.763 V (Table

16.1), and, therefore,

η=− 0 .621 V−(− 0 .763 V)=+ 0 .142 V

There are two types of polarization—activation and concentration. We will now

discuss their mechanisms since they control the rate of electrochemical reactions.

Activation Polarization

All electrochemical reactions consist of a sequence of steps that occur in series at

activation the interface between the metal electrode and the electrolyte solution.Activation

polarization polarizationrefers to the condition wherein the reaction rate is controlled by the one

step in the series that occurs at the slowest rate. The term “activation” is applied to

this type of polarization because an activation energy barrier is associated with this

slowest, rate-limiting step.

To illustrate, let us consider the reduction of hydrogen ions to form bubbles of

hydrogen gas on the surface of a zinc electrode (Figure 16.6). It is conceivable that

this reaction could proceed by the following step sequence:

1.Adsorption of H+ions from the solution onto the zinc surface

2.Electron transfer from the zinc to form a hydrogen atom,

H++e−→H

H 2 Gas,

1 atm

pressure

H 2

e–

Zn

Zn2+ solution,

1.0 M

Membrane

H+ solution,

1.0 M

Zn2+ Pt 2H+

Figure 16.5 Electrochemical

cell consisting of standard zinc

and hydrogen electrodes that has

been short-circuited.