Environmental Engineering Reference
In-Depth Information
pounds even at room temperature due to fulfillment of favorable thermodynamic
conditions. Such reactions can be represented as follows:
b
2
X
2
(g)
a
M(s)
M
a
X
b
(s)
(5.1)
For example:
1
2
O
2
(g)
Ni(s)
NiO(s)
1
2
S
2
(g)
2Cu(s)
Cu
2
S(s)
1
2
Cl
2
(g)
Ag(s)
AgCl(s)
Moreover, this type of reaction can be either reversible or nonreversible de-
pending on the temperature under consideration and the partial pressure of the
oxidant. Normally, the reversible type of uptake of the electronegative component
by the metal occurs at sufficiently low temperatures and low pressures of oxidant
when the film thickness is restricted to a few monolayers only. But with gradual
increase in temperature, this adsorption process leads to the formation of distinct
metallic compounds. From a thermodynamic point of view, the formation of
metal oxide, metal sulfide, metal halide on a metal surface is very feasible at
elevated temperatures and appears to be one of the simplest reactions, but in
reality this is not the case. The formations of such thermodynamically stable
compounds frequently appear as compact and adherent layers on the metal sub-
strate. As a consequence, the two reacting species, metal and oxidant, are spatially
separated from each other and further reaction is only possible if at least one of
the reactants can diffuse through the scaling layer to the reaction partner. In such
cases the course of reaction or the thickening of the product layer is no longer
determined by the equations of the type 5.1 but by diffusional transport of reacting
species and phases boundary reactions, for which the mechanisms can be quite
complex.
It is a well-established fact that the rates of thickening of various oxide scales
or oxidation kinetics are governed by the reaction mechanisms, which in turn
are controlled by the relevant structural, morphological, thermodynamic, and
transport properties of the solid reaction product. Day by day, new experimental
data and novel ideas are pouring into this field of research. Some of these are in
support to the already proposed theories, whereas others do not fit to the existing
multiplicity of kinetic laws and oxidation mechanisms. Therefore, the confronta-
tion between the theory and experimental results for both low- and high-tempera-
ture regimes is not uncommon in recent literature.
