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

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

Revised Pages

670 • Chapter 16 / Corrosion and Degradation of Materials

in going from metallic to oxidized states. Consequently, essentially all metals occur in

nature as compounds—for example, oxides, hydroxides, carbonates, silicates, sulfides,

and sulfates. Two notable exceptions are the noble metals gold and platinum. For

them, oxidation in most environments is not favorable, and, therefore, they may exist

in nature in the metallic state.

16.3 CORROSION RATES

The half-cell potentials listed in Table 16.1 are thermodynamic parameters that relate

to systems at equilibrium. For example, for the discussions pertaining to Figures 16.2

and 16.3, it was tacitly assumed that there was no current flow through the external

circuit. Real corroding systems are not at equilibrium; there will be a flow of electrons

from anode to cathode (corresponding to the short-circuiting of the electrochemical

cells in Figures 16.2 and 16.3), which means that the half-cell potential parameters

(Table 16.1) cannot be applied.

Furthermore, these half-cell potentials represent the magnitude of a driving

force, or the tendency for the occurrence of the particular half-cell reaction. How-

ever, it should be noted that although these potentials may be used to determine

spontaneous reaction directions, they provide no information as to corrosion rates.

That is, even though aVpotential computed for a specific corrosion situation using

Equation 16.20 is a relatively large positive number, the reaction may occur at only

an insignificantly slow rate. From an engineering perspective, we are interested in

predicting the rates at which systems corrode; this requires the utilization of other

parameters, as discussed below.

The corrosion rate, or the rate of material removal as a consequence of the

chemical action, is an important corrosion parameter. This may be expressed as the

corrosion penetration rate (CPR),or the thickness loss of material per unit of time.

corrosion

penetration rate

(CPR) The formula for this calculation is

CPR=

KW

ρAt

(16.23)

Corrosion

penetration rate—as

a function of

specimen weight loss,

density, area, and

time of exposure

whereWis the weight loss after exposure timet;ρandArepresent the density and

exposed specimen area, respectively, andKis a constant, its magnitude depending

on the system of units used. The CPR is conveniently expressed in terms of either

mils per year (mpy) or millimeters per year (mm/yr). In the first case,K=534 to give

CPR in mpy (where 1 mil=0.001 in.), andW,ρ,A, andtare specified in units of

milligrams, grams per cubic centimeter, square inches, and hours, respectively. In the

second case,K=87.6 for mm/yr, and units for the other parameters are the same as

for mils per year, except thatAis given in square centimeters. For most applications

a corrosion penetration rate less than about 20 mpy (0.50 mm/yr) is acceptable.

Inasmuch as there is an electric current associated with electrochemical corro-

sion reactions, we can also express corrosion rate in terms of this current, or, more

specifically, current density—that is, the current per unit surface area of material

corroding—which is designatedi. The rater, in units of mol/m^2 -s, is determined

using the expression

r=

i

nf

(16.24)

Expression relating

corrosion rate and

current density

where, again,nis the number of electrons associated with the ionization of each

metal atom, andfis 96,500 C/mol.