GTBL042-16 GTBL042-Callister-v2 September 17, 2007 17:32
Revised Pages16.10 Oxidation • 693Table 16.3 Pilling–Bedworth Ratios for a
Number of Metals
Protective Nonprotective
Ce 1.16 K 0.45
Al 1.28 Li 0.57
Pb 1.40 Na 0.57
Ni 1.52 Cd 1.21
Be 1.59 Ag 1.59
Pd 1.60 Ti 1.95
Cu 1.68 Ta 2.33
Fe 1.77 Sb 2.35
Mn 1.79 Nb 2.61
Co 1.99 U 3.05
Cr 1.99 Mo 3.40
Si 2.27 W 3.40
Source:B. Chalmers,Physical Metallurgy.Copyright
©c1959 by John Wiley & Sons, New York. Reprinted
by permission of John Wiley & Sons, Inc.unity, compressive stresses result in the film as it forms. For a ratio greater than
2–3, the oxide coating may crack and flake off, continually exposing a fresh and
unprotected metal surface. The ideal P–B ratio for the formation of a protective
oxide film is unity. Table 16.3 presents P–B ratios for metals that form protective
coatings and for those that do not. Note from these data that protective coatings
normally form for metals having P–B ratios between 1 and 2, whereas nonprotective
ones usually result when this ratio is less than 1 or greater than about 2. In addition
to the P–B ratio, other factors also influence the oxidation resistance imparted by the
film; these include a high degree of adherence between film and metal, comparable
coefficients of thermal expansion for metal and oxide, and, for the oxide, a relatively
high melting point and good high-temperature plasticity.
Several techniques are available for improving the oxidation resistance of a
metal. One involves application of a protective surface coating of another material
that adheres well to the metal and also is itself resistant to oxidation. In some in-
stances, the addition of alloying elements will form a more adherent and protective
oxide scale by virtue of producing a more favorable Pilling–Bedworth ratio and/or
improving other scale characteristics.Kinetics
One of the primary concerns relative to metal oxidation is the rate at which the
reaction progresses. Inasmuch as the oxide scale reaction product normally remains
on the surface, the rate of reaction may be determined by measuring the weight gain
per unit area as a function of time.
When the oxide that forms is nonporous and adheres to the metal surface, the
rate of layer growth is controlled by ionic diffusion. Aparabolicrelationship exists
between the weight gain per unit areaWand the timetas follows:W^2 =K 1 t+K 2 (16.34)Parabolic rate
expression for metal
oxidation—
dependence of
weight gain (per unit
area) on timewhereK 1 andK 2 are time-independent constants at a given temperature. This weight
gain–time behavior is plotted schematically in Figure 16.25. The oxidation of iron,
copper, and cobalt follows this rate expression.