Environmental Engineering Reference
In-Depth Information
growing scale is a function of the shape of the specimens and their surface-to-
volume ratio. For a metal with an infinite plane surface oxidizing uniformly over
the surface, the overlying scale in principle collapses on the retreating metal core
without any deformation of the scale. But if nonuniform oxidative attack occurs
(more common), stress amd straims are produced in the scales. For specimens
with finite dimensions, constraints are imposed on the system, and this produces
voids and cavities. For metal rods or cylindrical specimens, large deformations
are necessary if the oxide is to remain in contact with the metal and large voids
are likely to form. A classical example of this is given in Fig. 5.27, which exhibits
the cross-section of an oxdized iron wire [56] where decohesion and void forma-
tion has appreciably affected the oxidation rate.
Stresses and strains, void formation, scale detachment, and deformation and
fracturing of scales will be dependent on whether the oxidizing surface is concave
or convex. In these cases also there will be major differences with respect to
predominance of inward oxygen transport or outward metal transport. The vari-
ous limiting cases have been well addressed in detail by Hancock and Hurst [55],
as well as by Evans [51].
5.8.6 Compressive Failure of Scales and Spallation
Oxide scale spallation frequently occurs, particularly under conditions of thermal
cycling and poses a severe threat both to the maintenance of protective layer and,
ultimately, to the endurance of the component. It is only in the past decade or
so that significant progress has been made in identifying crack models for the
spallation process. In early studies, spallation was frequently reported during
cooling cycles because it is then that the oxide is usually subjected to compression
(for flat specimens), as a result of the differences in thermal expansion coefficient
between oxide and metal.
For spallation to take place, it is necessary to generate cracks through the
oxide layer to the interface at which spallation will take place and also to produce
decohesion along that interface. This interface may be one between layers of
oxide of different composition, or, indeed, a plane of weakness within a single
oxide layer. Here in order to establish the principles, it is simply assumed that
the interface is the one existing between the oxide and metal.
Two distinct processes of spallation of a compressively stressed oxide can be
identified. If the oxide-metal interface has a high adhesive strength relative to
the cohesive strength of the oxide, cracking of the oxide will be the consequence
before decohesion. On the other hand, poor interfacial adhesion will lead to de-
cohesion before the occurrence of cracking of the oxide. These two distinct situa-
tions of stronger and weaker interfacial adhesion provide ideas of two entirely
different routes leading to spallation of oxide as schematically shown in Fig.
5.28. When adhesion between the metallic substrate and the oxide is quite strong
