Environmental Engineering Reference
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of Fe(OH) 2 or Fe(OH) 3 form on iron and further corrosion of iron does not
take place.
Such diagrams can be used for
1.
Predicting the spontaneous direction of reactions
2.
Estimating the stability and composition of corrosion products, and
3.
Predicting environmental changes that will prevent or reduce corrosion.
With reference to Fig. 2.9, corrosion prevention can be achieved by lowering
the electrode potential down to the zone of immunity, raising the electrode poten-
tial up to the region of passivity, or raising the pH or alkalinity of the solution
so that a passive film is formed.
There are, however, a number of limitations in using such diagrams. The most
important of these is that they represent equilibrium conditions and hence cannot
be used for predicting the rate of a reaction. The tacit assumption that corrosion
products, e.g., oxides, hydroxides, and so forth, lead to passivity may not always
be true because they may not precipitate on the metal surface. The possibility of
precipitation of other ions such as chloride, sulfate, and phosphate has been ig-
nored. Finally, the pH at the metal surface may vary drastically because of side
reactions and a prediction of corrosion based on the bulk pH of the solution may
be misleading.
2.3 KINETICS OF AQUEOUS CORROSION
Corrosion reactions are analogous to what happens in a short-circuited cell (Sec-
tion 2.1.2). The system is no longer in equilibrium and the reaction proceeds
either in the forward or backward direction at the electrodes generating a finite
current flow in the circuit. The magnitude of current is a direct measure of the
extent of corrosion and rate of current flow is a measure of the rate of corrosion.
It is important to know the rate of corrosion in practice to determine the applica-
bility of a metallic component in a given environment. Even if the tendency
for a reaction is high, as may be evident from thermodynamic considerations, a
negligibly low rate of corrosion can ensure the reliable use of a metal even in
an apparently corrosive medium. On the other hand, a metal in a given environ-
ment may corrode at a rapid rate in a medium having a relatively low tendency
for reaction (potential difference). The study of the rate of reaction comes under
the purview of kinetics.
Electrochemical dissolution reactions obey the Faraday laws of electrolysis.
When a current of magnitude I ampere is flown through the circuit, the amount
of mass of metal dissolved or deposited in given by:
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