Page 725 - Corrosion Engineering Principles and Practice

678 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 679

grow according to the parabolic rate law as discussed later. In most
instances, the predominant diffusion is that of metal ions plus
electrons outward to the oxide-air interface shown in Fig. 15.9.
The migration of electrons should not be overlooked. If electron
mobility was low, as in the case of oxides which behave as electrical
insulators (Al O , BeO, Cr O ), the metals developing these
3
2
2
3
compounds as surface films could be expected to have good
corrosion resistance.
Because nickel at high temperatures does not form an oxide
layer with a good diffusion-resistant coating, alloying elements are
generally added to nickel-based alloys, in order to provide added
imperviousness to the oxide layer. Chromium is an excellent
alloying element, and the 80 Ni-20 Cr alloy is one of several
compositions commercially used for heating elements in high-
quality domestic electric toasters or electric irons. This alloy
generally develops a suitable NiO-Cr O spinel (oxide) on the
2
3
surface, but under suitable conditions, Cr O forms an even more
2
3
diffusion-resistant barrier, the spinel.
15.3.2 Basic Kinetic Models
Three basic kinetic laws have been used to describe the oxidation
rates of pure metals. It is important to bear in mind that these laws are
based on relatively simple oxidation models. Practical oxidation
problems usually involve alloys and considerably more complicated
oxidation mechanisms and scale properties than considered in these
simple models.

Linear Behavior
If the oxide film or scale cracks or is porous, that is, if the corrosive
gas can continue to penetrate readily and react with the base metal in
a catastrophic manner, no protection will be afforded and attack will
proceed at a rate determined essentially by the availability of the
corrosive gas. In this case, the rate will not sensibly change with time,
and, as is apparent from Fig. 15.12, the weight change or depth of
penetration from oxidation is a straight line or linear function of time
and may be expressed as
y = k t (15.14)
L
where y is scale thickness or weight change
t is time
k is a constant that is dependent on temperature, the material
L
being tested, and other conditions of the high temperature test
Logarithmic Behavior
The logarithmic rate behavior follows an empirical relationship,
which has no fundamental underlying mechanism. This behavior,
also shown in Fig. 15.12, is mainly applicable to thin oxide films