Callister - An introduction - 8th edition

706 • Chapter 17 / Corrosion and Degradation of Materials
Weight gain per unit area, W
Linear
Parabolic
Logarithmic
Figure 17.25 Oxidation film
growth curves for linear,
parabolic, and logarithmic rate
laws.
Time, t
Parabolic rate
expression for metal
oxidation—
dependence of
weight gain (per unit
area) on time
Linear rate
expression for metal
oxidation
Logarithmic rate
expression for metal
oxidation
between the weight gain per unit area W and the time t as follows:
W 2 K 1 t K 2
(17.34)
where K 1 and K 2 are time-independent constants at a given temperature.This weight
gain–time behavior is plotted schematically in Figure 17.25. The oxidation of iron,
copper, and cobalt follows this rate expression.
In the oxidation of metals for which the scale is porous or flakes off (i.e., for
P–B ratios less than about 1 or greater than about 2), the oxidation rate expression
is linear; that is,
W K 3 t
(17.35)
where K 3 is a constant. Under these circumstances oxygen is always available for
reaction with an unprotected metal surface because the oxide does not act as a reaction
barrier. Sodium, potassium, and tantalum oxidize according to this rate expression
and, incidentally, have P–B ratios significantly different from unity (Table
17.3). Linear growth rate kinetics is also represented in Figure 17.25.
Still a third reaction rate law has been observed for very thin oxide layers (generally
less than 100 nm) that form at relatively low temperatures. The dependence
of weight gain on time is logarithmic and takes the form
W K 4 log1K 5 t K 6 2
(17.36)
Again, the Ks are constants. This oxidation behavior, also shown in Figure 17.25,
has been observed for aluminum, iron, and copper at near-ambient temperatures.
Corrosion of Ceramic Materials
Ceramic materials, being compounds between metallic and nonmetallic elements,
may be thought of as having already been corroded. Thus, they are exceedingly immune
to corrosion by almost all environments, especially at room temperature.
Corrosion of ceramic materials generally involves simple chemical dissolution, in
contrast to the electrochemical processes found in metals, as described previously.