Environmental Engineering Reference
In-Depth Information
From the 1950s the importance of corrosion to the economy became
increasingly evident. Today, corrosion is one of the major degradations to
overcome in order to extend the lifetime of nuclear power plants in agree-
ment with safety requirements.
2.1.2 Fundamental principles of corrosion
The second law of thermodynamics is an expression of the tendency over
time differences in temperature, pressure and chemical potential will equili-
brate in an isolated physical system. In other words, every material tends to
reach the maximum of disorder, in order to minimize its potential energy.
With regard to corrosion, it means that leaving the crystalline network
under the action of an electric fi eld, metal ions yields energy. According
to thermodynamics, almost all metals have negative free energy, suggesting
their reactivity in environments where they are exposed.
Corrosion reactions are electrochemical in nature, based on mass and
charge transfers. Reactions can be split into partial oxidation and reduction
reactions. The potential is the propensity to exchange electrons: the metal
donating electrons is oxidized, while the metal receiving electrons is reduced.
The stability of elements in a given medium is predicted by the correspond-
ing Pourbaix diagram, where predominant phases are defi ned in agreement
with thermodynamics. However, reaction kinetics play a major role in the
evolution of the system (such as changes in pH, potential or temperature).
For example, if iron is introduced into hydrogenated water at 300°C (with-
out any dissolved oxygen) at pH 7 and a potential of −700 mV SHE , cations
Fe 2 + are dissolved in the water (Reaction [2.1]). This anodic reaction (oxi-
dation of metal) is coupled to the cathodic reaction (reduction of water)
described by Reaction [2.2]. Then, dissolved cations Fe 2 + can join oxidant
ions OH (Reaction [2.3]), to form ferrous hydroxide Fe(OH) 2 .
￿ ￿ ￿ ￿ ￿ ￿
2
Fe
Fe
+
e
[2.1]
F
2
→+
2H O2
H OH
[2.2]
′ →+
H 2
H
e
2
O2
2
2
+
[2.3]
(
)
Fe
2OH
e
OH
+
2OH
2
Cations can be released in the water due to dissolution. When the satura-
tion in ferrous hydroxide Fe(OH) 2 or ferrous cation Fe 2+ is reached, accord-
ing to the Pourbaix diagram, magnetite Fe 3 O 4 forms based on Schikorr
Reaction [2.4].
[2.4]
(
)
3Fe
OH
FeO
H HO
FeO
Fe
+
4
4
H
2
2
3
2
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