Environmental Engineering Reference
In-Depth Information
Figure 4.4
Relative performance of steel and cast iron as a function of time.
Numerous salts and salt solutions can be handled by cast iron without exces-
sive corrosion. Cast iron is also used extensively in the exhausts of flue gas sys-
tems where the formation of sulfurous acid on surfaces below the dew point of
the flue gases at the cold end is less detrimental for cast iron than steel because
thicker sections can be used.
4.1.2 Carbon and Low-Alloy Steels
Carbon steels have poor corrosion resistance. They exhibit high rates of corrosion
in nearly all aqueous environments including atmospheric exposures. The hy-
drated oxide corrosion product produced on atmospheric exposures is called
rust
.
This being flaky and nonadherent, corrosion continues unabated. Carbon steels,
however, are the most economical choice to handle sulfuric acid above concentra-
tions of 65% because of the formation of the passive ferrous sulfate protective
film. Carbon steels are also passive in alkaline solutions of high pH and are used
to handle such solutions at ambient temperatures. Carbon steels rebars are used
to reinforce concrete and are often used bare as the leaching of concrete provides
an alkaline medium. However, coating or cathodic protection of rebars is necessi-
tated in structures exposed to marine conditions or to aggressive salts, as well
as to prevent stress corrosion cracking of the highly stressed rebars by the accu-
mulated hydroxyl ions.
Carbon steels are nevertheless the most widely used structural material be-
cause of their wide range of mechanical properties developed through composi-
tional variation and heat treatment. Tanks, pipelines, railroad wagons, automo-
biles, ship hulls, and marine structures are a few applications of carbon steels.
