Callister - An introduction - 8th edition

17.4 Prediction of Corrosion Rates • 683
corrosion penetration
rate (CPR)
Corrosion
penetration rate—
as a function of
specimen weight
loss, density, area,
and time of exposure
Expression relating
corrosion rate and
current density
although these potentials may be used to determine spontaneous reaction directions,
they provide no information as to corrosion rates. That is, even though a V potential
computed for a specific corrosion situation using Equation 17.20 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 use of other parameters, as discussed next.
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.
The formula for this calculation is
CPR KW
rAt
(17.23)
where W is the weight loss after exposure time t; and A represent the density and
exposed specimen area, respectively, and K is 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.), and W, , A, and t are 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 that A is 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 corrosion
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 designated i.The rate r, in units of mol/m # 2 s, is determined using
the expression
r
i
nf
(17.24)
where, again, n is the number of electrons associated with the ionization of each
metal atom, and f is 96,500 C/mol.
17.4 PREDICTION OF CORROSION RATES
Polarization
polarization
Consider the standard Zn/H 2 electrochemical cell shown in Figure 17.5, 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 17.1 because the system is now a nonequilibrium
one. The displacement of each electrode potential from its equilibrium
value is termed polarization, and the magnitude of this displacement is the overvoltage,
normally represented by the symbol . 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 17.5 has a potential of 0.621 V
after it has been connected to the platinum electrode. The equilibrium potential is
0.763 V (Table 17.1), and, therefore,
h 0.621 V 10.763 V2 0.142 V
There are two types of polarization—activation and concentration. We will now
discuss their mechanisms because they control the rate of electrochemical reactions.