Page 730 - Corrosion Engineering Principles and Practice

682 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 683

FIGURE 15.14
Schematic of some Oxide
of the hypothetical
ways the integrity
of film or scale is
impaired. Metal

Blistering

Oxide

Metal

Shear cracking

Oxide

Metal

Flaking

If, on the other hand, the interface is rough, where incipient grain
boundary attack beneath the scale is occurring, a keying action may
prevent rapid crack propagation and thereby improve scale adherence.
It is believed that the major effect of reactive rare earth additions to
heat-resistant alloys is not to enter the scale and lower diffusion rates,
but to accumulate in grain boundaries and thus ensure a small amount
of grain boundary attack to provide the keying action.
A considerable effort is often made in the tubing industry to
produce on the interior wall of the tube an initial scale that has optimal
properties. Centrifugally cast tubing is often bored to remove porosity
near the inner surface, honed to provide a smooth surface profile, and
then steamed heavily to produce a thin, continuous, dense, resistant
oxide layer on the interior with a remarkably increased life of tubing
sections for particular services.

15.4 Practical High-Temperature Corrosion Problems
The oxidation rate laws described earlier are simple models derived
from the behavior of pure metals. In contrast, practical high-temperature
corrosion problems are much more complex and involve the use of