Environmental Engineering Reference
In-Depth Information
5.2 THERMODYNAMIC ASPECTS OF METAL-SINGLE
OXIDANT SYSTEMS
The primary approach to tackle the degradation of a metallic component exposed
to a certain environment in an industrial installation should be to examine the
extent of affinity of the metal to oxidizing gases, which leads to its degradation
manifested by the formation of some compounds with the oxidant. For judging
the relative stability of different metallic compounds, one has to derive informa-
tion from equilibrium thermodynamics. Therefore, for any reaction of the follow-
ing type involving a metal in an environment of single oxidant:
M(solid metal)
O
2
(g)
MO
2
(solid compound)
(5.2)
one has to know the value of Gibbs' free energy change (
∆
G
) for the total reac-
tion. If
G
is negative, the reaction will be spontaneous in the forward direction,
leading to the formation of metal oxide (MO
2
), whereas if
∆
G
is positive, MO
2
will not be stable at the temperature and pressure of the oxidant, thereby leading
to spontaneous dissociation of the oxide. But thermodynamics deals with the
equilibrium attainment for any metal-compound-oxidant system that needs a
scrutiny of the standard free energy change (
∆
G
0
) of the above reaction. For
ready reference, to judge the relative stability of various metal-oxidant systems,
one has to have a look at the graphical representation of
∆
G
0
vs.
T
plots [1]
for various systems (Ellingham-Richardson diagrams). Such types of graphical
representation are shown in Fig. 5.1(a)-(d) for metal-oxide-oxygen, metal-sul-
fide-sulfur vapor, metal-halide-halogen, and metal-carbide-carbon systems,
respectively. The preferential choice of alloying elements like Cr, Al, Si, etc., in
the development of high-temperature alloys is best judged from such diagrams.
The compounds of these elements are comparatively more stable than the base
metal compounds, as evidenced by more negative
∆
G
0
values in the above-men-
tioned figure parts. It would be appropriate to mention that when a metallic com-
ponent is exposed to an environment containing more than one oxidant, say SO
2
and O
2
, and the
∆
G
0
values are found to be negative for the formation of both
oxide and sulfide; however, preferential growth of either of the compounds will
no longer be guided by thermodynamic considerations alone.
The equilibrium oxidant pressure for the formation of a metal oxide could be
evaluated from the standard free energy change of the reaction by using
∆
∆
G
0
RT
ln
K
(5.3)
where
K
is the equilibrium constant that is related to the activities of species
involved in the reaction. For example,
K
for the reaction (5.2) is given by:
a
MO
2
a
M
K
(
T
)
(5.4)
⋅
a
O
2
