Environmental Engineering Reference
In-Depth Information
3.10.1 Microbiological Corrosion
A number of bacteria have been identified that aggravate corrosion of metals
such as carbon steels, cast irons, stainless steels, aluminum alloys, and copper
alloys. These are classified as
aerobic
and
anaerobic
according to their ability
to grow in the presence or absence of air. They survive under a wide range of
temperature (0-50
C) and pH (3-10). The important microorganisms contribut-
ing to corrosion are listed in Table 3.5. The affected areas are characteristically
associated with excessive deposits, tubercles, or biological slimes. Buried gray-
iron structures suffer from graphitic corrosion (Section 3.7.3) where bacteria help
in the process of leaching out of iron.
Anaerobic bacteria grow in waterlogged clay-rich soils, in deaerated waters,
in stagnant cutting fluids, underneath the deposits caused by aerobic bacteria,
and under macroorganisms like mollusks and barnacles. Sulfate-reducing bacte-
ria, e.g.,
D. desulfuricans
, are the most important anaerobic bacteria that influence
the corrosion behavior of steels. These bacteria reduce inorganic sulfates to sul-
fides in the presence of hydrogen according to the schematic equation:
°
SO
4
2
8H
→
S
2
4H
2
O
(3.8)
Hydrogen is provided by the cathodic reaction or by the organic matters present
in soil or in the general environment. For each equivalent of hydrogen atom
consumed by the bacteria, one equivalent of Fe
2
enters the solution to form rust
and FeS. The presence of sulfide in rust is often an indication of the corrosion
having been influenced by sulfate-reducing bacteria. Qualitative detection is
made possible by the treatment of the deposit with dilute hydrochloric acid, which
produces H
2
S with its characteristic smell.
Aerobic bacteria contribute to corrosion by oxidizing or sulfide and iron. Sul-
fur-oxidizing bacteria, like
Thiobacillus thiooxidans
, oxidize elemental sulfur or
sulfides to produce sulfuric acid according to the equation:
2S
3O
2
2H
2
O
→
2H
2
SO
4
(3.9)
Highly corrosive conditions are thereby produced. Also the slime, which is a
polymeric excreta of these bacteria, provides a localized anaerobic environment
for sustaining the growth of sulfate-reducing bacteria. Iron-oxidizing bacteria,
such as
Thiobacillus ferrooxidans
, oxidize soluble ferrous ions to less soluble
ferric ions and thereby accelerate the anodic reaction for dissolution of iron. Hy-
drated ferric oxides along with biological slimes tend to form tubercles often
impairing fluid flow inside the pipe, which has been illustrated in Fig. 3.61. The
deposits also produce areas shielded from oxygen, thus providing localized an-
odic areas leading to crevice corrosion.
In soils, aerobic and anaerobic conditions may alternately prevail depending
on seasonal variation, with the former prevailing during dry seasons and the latter
