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

GTBL042-16 GTBL042-Callister-v2 September 17, 2007 17:32

Revised Pages

694 • Chapter 16 / Corrosion and Degradation of Materials

Time, t

Weight gain per unit area,

W

Linear

Parabolic

Logarithmic

Figure 16.25 Oxidation film

growth curves for linear,

parabolic, and logarithmic rate

laws.

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

islinear;that is,

W=K 3 t (16.35)

Linear rate

expression for metal

oxidation

whereK 3 is a constant. Under these circumstances oxygen is always available for re-

action 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 16.3).

Linear growth rate kinetics is also represented in Figure 16.25.

Still a third reaction rate law has been observed for very thin oxide layers (gen-

erally less than 100 nm) that form at relatively low temperatures. The dependence

of weight gain on time islogarithmicand takes the form

W=K 4 log(K 5 t+K 6 ) (16.36)

Logarithmic rate

expression for metal

oxidation

Again, theK’s are constants. This oxidation behavior, also shown in Figure 16.25,

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 above.

Ceramic materials are frequently utilized because of their resistance to corro-

sion. Glass is often used to contain liquids for this reason. Refractory ceramics must

not only withstand high temperatures and provide thermal insulation but, in many

instances, must also resist high-temperature attack by molten metals, salts, slags,

and glasses. Some of the new technology schemes for converting energy from one

form to another that is more useful require relatively high temperatures, corrosive