Callister - An introduction - 8th edition

17.10 Oxidation • 705
Table 17.3
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 © 1959 by John Wiley & Sons,
New York. Reprinted by permission of
John Wiley & Sons, Inc.
where A O is the molecular (or formula) weight of the oxide, A M is the atomic
weight of the metal, and r O and r M are the oxide and metal densities, respectively.
For metals having P–B ratios less than unity, the oxide film tends to be porous
and unprotective because it is insufficient to fully cover the metal surface. If the
ratio is greater than 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 17.3 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 hightemperature
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 instances,
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. A parabolic relationship exists