Environmental Engineering Reference
In-Depth Information
given environment and with an expected life of 10 years should have 200 mils.
i.e., 0.2 in. of additional wall thickness over and above the thickness required
for meeting the mechanical requirements of pressure, weight, and stress concen-
trations. However, since in practice no attack is completely uniform and there is
a demand for material conservation, other measures for prevention of corrosion
are used as well. For the prevention of uniform corrosion, measures such as the
application of protective coatings, use of inhibitors, and use of cathodic or anodic
protection are widely adopted.
The selection of a suitable material is very important in minimizing general
corrosion. Special corrosion-resistant alloys have also been developed of which
stainless steels are most important. So far as the atmospheric corrosion of steel
is concerned, the performance of special weathering steels containing small
amounts of copper (0.3-0.4%) or of microalloyed steels containing copper, phos-
phorus, nickel, and chromium, particularly in temperate climates, has been com-
mendable. The rust formed on the steels is adherent and protective.
The corrosion preventive measures have been discussed in Chapter 4.
3.3 GALVANIC CORROSION
When two dissimilar metals are in contact or electrically connected in the same
electrolyte, one of them is preferentially corroded whereas the other remains free
from corrosion. This localized corrosion of one of the constituent members of a
couple is referred to as galvanic corrosion or two-metal corrosion. When placed
singularly in the corrosive, both members may corrode, but at different rates.
The metal showing a higher corrosion rate becomes the corroding member in the
couple, analogous to the anode in a galvanic cell (e.g., in a Daniell cell, Section
2.1.2). The other member becomes cathode. This is essentially a corrosion due
to dissimilar electrode cell formation (Section 2.1.3). While galvanic corrosion
occurs on the anodic member of the couple, the cathode may suffer from hydro-
gen damage (Chapter 8).
Sometimes nonmetallic conductors may act as cathodes in galvanic couples.
Both carbon brick in vessels made of common structural metals and impervious
graphite in heat exchanger applications are examples. Conductive films such as
mill scale (Fe
2
O
3
) or iron sulfide on steel or of lead sulfate on lead are cathodic
to the base metal or to some metallic components in their contact.
3.3.1 Prediction of Galvanic Corrosion
The cell potential, which is the algebraic sum of the single-electrode potentials,
is the driving force for corrosion reaction. The standard single-electrode poten-
tials of metals have been presented in the emf series (Table 2.1). In a galvanic
coupling, the metal having a more noble (i.e., more positive) redox potential acts
