Environmental Engineering Reference
In-Depth Information
This type of pressure dependence matches with experimental results in a certain
temperature range. On the other hand, in the case of a Zn/ZnO/O
2
(g) system,
one can visualize the following defect formation reaction as
1
2
O
2
(g)
ZnO
Zn
i
e
′
(5.52)
for which
[Zn
i
]
nP
1/2
O
2
K
(
T
)
(5.53)
When
n
[Zn
i
], one finds the following relation:
n
σ
e′
P
1/4
O
2
(5.54)
When interstitial Zn ion is doubly ionized, the charge neutrality condition be-
comes
n
2[Zn
•
i
]. Therefore, one can visualize the following defect formation
reaction:
1
2
O
2
(g)
ZnO
Zn
••
i
2e
′
(5.55)
The equilibrium constant for Eq. 5.55 is
[Zn
•
i
]
n
2
P
1/2
O
2
K
(
T
)
(5.56)
Therefore,
n
σ
e′
P
1/6
O
2
,
(5.57)
Measured conductivity dependence is reported to be between
P
1/4
O
2
and
P
1/5
O
2
,
suggesting the presence of some singly charged as well as doubly charged Zn
interstitial ions.
In conclusion, the electrical conductivity of a p-type compound increases with
increase of the oxidant pressure whereas that for an n-type conductor decreases
with increased oxidant pressure in the environment.
5.5 MECHANISMS OF TARNISHING AND
SCALING PROCESSES
In 1920, Tammann [6] for the first time proposed a mathematical relation between
the extent of reaction of a metal with the environment and the corresponding
exposure time of reaction at elevated temperatures for a number of metal-oxidant
systems. This relation was experimentally found to be parabolic in nature. Subse-
quently, in 1923, the important concept of the Pilling-Bedworth [5] ratio had
emerged, which still serves as a rough guideline for the prediction of protective
