Page 727 - Corrosion Engineering Principles and Practice

680 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 681

where k is the rate constant
p
y an empirical constant of the system studied
0
After an initial period of time, the rate of scale growth will
decrease to a low level, that is, a considerable measure of oxidation
resistance will be obtained and, provided the scale remains intact,
will be maintained for very long times.
Under special conditions, particularly at intermediate tempera-
tures, the oxidation process may essentially stop after the initial scale
formation. In such cases, no change in scale thickness or weight
would be observed after very long intervals. Presumably this occurs
because, when the scale reaches some minimum thickness the diffu-
sion rate is so low as to be essentially zero, thus creating a condition
under which the metal should last almost indefinitely without fur-
ther deterioration. Some investigators have chosen to call this type of
behavior tarnishing and thereby to distinguish it from the higher
temperature phenomenon of scaling. The latter will be of greater con-
cern in the remainder of this discussion.
The parabolic rate law, although obviously an oversimplification
of the actual state of affairs during oxidation, nevertheless provides a
relatively accurate approximation of the behavior of the majority of
metals and alloys practically used at high temperatures.

15.3.3 Pilling-Bedworth Ratio
Thus far, principally oxide scale formation was considered, and it
was assumed that once formed, the scale would remain intact and
protective. Scale thickness may be an important factor. In early stages,
the scale is thin and reasonably elastic. It also will have little intrinsic
strength and usually will remain tightly adherent to the base metal.
As it thickens, however, less desirable properties may become
manifest. Heavy scales tend to be brittle and will therefore crack and
spall from the surface.
Figure 15.13 presents data on a stainless steel that reflects this
type of behavior. At the lower temperatures, the scale thickens slowly
and the parabolic rate law is followed. At the highest temperature in
Fig. 15.13 (700°C), the oxidation rate is also initially parabolic, but at
some point the scale breaks away locally and there is an immediate
increase in the scaling rate as a new protective scale is formed.
The volume of the oxide formed, relative to the volume of the
metal consumed, is an important parameter in predicting the degree
of protection provided by the oxide scale. If the oxide volume is
relatively low, tensile stresses will tend to crack the oxide layers.
Oxides, which are essentially brittle ceramics, are particularly
susceptible to fracture and cracking under such tensile stresses. If
the oxide volume is very high, stresses will tend to lead to a break
in the adhesion between the metal and oxide. For a high degree